- Email: [email protected]
High-energy collisions of protonated enantiopure amino acids with a chiral target gas
International Journal of Mass Spectrometry 388 (2015) 59–64
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
High-energy collisions of protonated enantiopure amino acids with a chiral target gas Kostiantyn Kulyk ∗ , Oleksii Rebrov, Mark H. Stockett, John D. Alexander, Henning Zettergren, Henning T. Schmidt, Richard D. Thomas, Henrik Cederquist, Mats Larsson Stockholm University, Department of Physics, SE-106 91 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 6 August 2015 Accepted 10 August 2015 Available online 20 August 2015 Keywords: High-energy collisional activation Tandem mass spectrometry Protonated amino acids Chiral collision gas 2-Butanol
a b s t r a c t We have studied the fragmentation of the singly protonated l- and d-forms of enantiomerically pure phenylalanine (Phe), tryptophan (Trp), and methionine (Met) in high-energy collisions with chiral and achiral gas targets. (S)-(+)-2-butanol, racemic (±)-2-butanol, and argon were used as target gases. At center-of-mass frame collision energy of 1 keV, it was found that all of the ions exhibit common fragmentation pathways which are independent of target chirality. For all projectile ions, the elimination of NH3 and H2 O + CO were found to be the main reaction channels. The observed fragmentation patterns were dominated by statistically driven processes. The energy deposited into the ions was found to be sufficient to yield multiple fragment ions, which arise from decomposition via various competitive reaction pathways. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The current drive in pharmaceutical research has shifted toward the development of single enantiomer-based drugs rather than racemic mixture of both enantiomeric forms [1–3]. The fundamental reason for this is the highly specific nature of drug/target interactions which often depend on stereochemistry. The enantiomers of the same drug can produce different biological responses [4]. A shocking but demonstrative example of this phenomenon is the tragedy of birth defects linked with the active use of racemic thalidomide for the treatment of morning sickness in pregnant women in the late 1950s and early 1960s [5,6]. The increasing demand for optically pure pharmaceuticals calls for efficient enantioselective analytical methods to be developed. The fast and sensitive performance of mass spectrometry (MS) places it among the most promising techniques in drug analysis. However, despite significant success in the development of MS-based chiral recognition methods [7–15], many gas-phase processes which lie behind this recognition are still poorly understood. Furthermore, these methods of chiral recognition often involve specifically modified instruments, require enantiomerically pure reference compounds, involve time-consuming building of
∗ Corresponding author. Tel.: +46 8 5537 8612. E-mail address: [email protected] (K. Kulyk). http://dx.doi.org/10.1016/j.ijms.2015.08.010 1387-3806/© 2015 Elsevier B.V. All rights reserved.
calibration curves, and may depend on complex data analysis of fragmentation patterns. Further research efforts as well as an understanding of the ion physics and chemistry involved are urgently needed in order to overcome these limitations. In the late 1960s and early 1970s, McLafferty and co-workers performed in-depth studies of keV collisional activation fragmentation as a function of various parameters [16]. From these studies, it was concluded that the nature of the collision gas has little influence on the product fragment intensity ratios [16,17]. Though, in recent studies with other molecules, the nature of the collision gas was observed to influence the fragmentation [18–20]. Chiral projectile ions, however, were not studied in high-energy collision experiments with chiral target gases. Here we report the results of high-energy collisional activation of ions of enantiomerically pure amino acids with chiral and achiral target gases. Due to its suitable vapor pressure and commercial availability in enantiopure form, 2-butanol was selected to be used as a volatile chiral target molecule in our measurements. Among the additional reasons for choosing this particular gas was its successful utilization as a volatile chiral dopant for enantioselective separations by ion mobility spectrometry [21,22]. The mechanism of enantioselectivity that lay behind those gas-phase chiral discriminations is still not fully understood. This study represents, therefore, the first examination performed by our group directed to the understanding of the ion chemistry and physics behind gas-phase chiral separations based
60
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
Scheme 2. Main fragmentation pathways for protonated amino acids (R-side chain group). Fig. 1. Sketch of the experimental apparatus used in the experiments.
on the use of a chiral target gas. The possibility of stereochemically dependent interactions with keV activation under single collision conditions was addressed in this particular study. A separate manuscript devoted to eV collisions of enantiomeric ions and molecules is under preparation. 2. Experimental 2.1. Electrospray ion source platform An electrospray ion source platform, developed and constructed at the Department of Physics at Stockholm University [18], was used in this investigation. A sketch of the experimental apparatus is shown in Fig. 1. In every measurement, a continuous molecular ion beam of protonated amino acids was generated in an electrospray ion source. After ionization, the ions entered the vacuum system through a heated capillary and were focused in a radiofrequency ion funnel, mass selected in a quadrupole mass filter, and accelerated to the intended collision before entering a collision cell containing the target cell. The typical pressure-range in the 4-cmlong collision cell during the experiments was 0.5–1 mTorr. After passing through the cell, horizontal deflector plates and a cylindrical lens were used to analyze the primary molecular ion beam and the fragments according to their kinetic energy to charge ratio by guiding them to a position-sensitive microchannel plate detector. The position on the detector and the corresponding voltages on the deflector plates were used to obtain the fragment mass distribution. 2.2. Collision gases and reagents The l- and d-forms of enantiomerically pure tryptophan (Trp), phenylalanine (Phe), and methionine (Met) were supplied by Sigma–Aldrich; their structures are shown in Scheme 1. Purity of all the amino acids was ≥98%. The chiral target (S)-(+)-2-butanol (99%) and the racemate of (±)-2-butanol (99.5%) were purchased from Sigma–Aldrich. Methanol (>99.8%) and acetic acid (>99.8%) were obtained from Fluka. Finally, argon (99.998%) was purchased from Air Liquide. Stock solutions of all of the amino acids were prepared at a concentration of 0.05 M in a solvent mixture – methanol:water:acetic acid (49:49:2). Just prior to the experiments, the stock solutions were diluted to a concentration of 0.005 M using the same solvent system. The sample solutions were introduced into electrospray source by using a syringe pump (Harvard Apparatus) at a typical flow rate 1.5 L/min. A stable electrospray of protonated amino acid ions was achieved in the positive ion mode under the following instrument settings: needle voltage, 3.5–4.8 kV; capillary voltage,
200–300 V; capillary temperature, 110 ◦ C. Each new measurement was preceded by a thorough cleaning of the injection system using the above mentioned solvent mixture. 3. Results and discussion 3.1. High-energy collisional activation In this work, we examined the influence of the nature of the collision gas (chiral, achiral) on the dissociation yield of different fragmentation products resulting from the dissociation of enantiopure amino acid ions following 1 keV collisions (center-of-mass frame). The collision induced dissociation (CID) spectra measured after collisions of the protonated amino acids with argon, S-(+)-2butanol and (±)-2-butanol are discussed in Sections 3.2–3.4. In all spectra, the intensity of the parent ion is normalized to 1.0 and the intensity ratios of individual fragments are determined with respect to the parent ion. At present, two major concepts are used to explain the fragmentation mechanisms of protonated amino acid ions: the mobile proton model [23,24] and the model involving side chain groups for facilitation of cleavage reactions [24,25] (Scheme 2). Nearly all our CID results are consistent with these models and could be rationalized by them. 3.2. PheH+ in collisions with Ar, (S)-(+)-2-butanol, and (±)-2-butanol The CID pattern of protonated phenylalanine colliding with Ar is shown in Fig. 2a. Our results agree with previous reports [26–29]: the most common fragment appears at m/z = 120 and corresponds to the iminium ion, formed by the loss of H2 O and CO from the protonated parent. Following the work by Shoeib et al. [26], it is currently believed that this fragment is due to H2 O and CO loss (Scheme 2, Pathway 1) rather than the loss of dihydroxycarbene, as was suggested earlier. The iminium ion is typically observed in the fragmentation of singly protonated amino acids [26–29]. The significant abundances of product ions other than iminium indicate that a collision energy of 1 keV (center-of-mass frame) is high enough for the initiation of competing fragmentation channels. The reaction competing with proton transfer and the combined release of H2 O + CO is the loss of NH3 (Fig. 2a, m/z = 149; Scheme 2, Pathway 2). Decay of the fragment ions formed after NH3 elimination was observed to proceed via the loss of H2 O (m/z = 131), H2 O + CO (m/z = 103), etc. The fragmentation patterns of l- and d-Phe after collisions with racemic and enantiomerically pure 2-butanol are plotted in Fig. 2b and c, respectively. Comparison among the spectra plotted
Scheme 1. Structures of the amino acids used in the collision induced dissociation study.
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
Fig. 2. Comparison of CID mass spectra obtained from collisions of l- and d-Phe with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV.
in Fig. 2a–c shows that there are no significant differences either in the fragmentation patterns or in the fragment ion ratios when changing between the achiral and chiral targets in the case of 1 keV collisions. Assignments of the individual fragment peaks are the same as for the l- and d-Phe CID spectra with Ar. 3.3. TrpH+ in collisions with Ar, (S)-(+)-2-butanol and (±)-2-butanol The CID spectra obtained from fragmentation of the protonated ions of l- and d-Trp after interacting with Ar at a center-of-mass collision energy of 1 keV are shown in Fig. 3a. Unlike phenylalanine, and all other aromatic amino acids, the loss of CO + H2 O
61
Fig. 3. Comparison of CID mass spectra obtained from collisions of l- and d-Trp with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV. The ion with m/z = 130 is assigned as * because various possibilities exist for the interpretation.
and subsequent formation of the iminium ion is not the most prominent fragmentation channel for protonated tryptophan. Here, the dominant fragmentation pathway involves the loss of NH3 resulting in a charged fragment at m/z = 188 (Fig. 3a; Scheme 2, Pathway 2). It is known that the indole substituent facilitates the NH3 loss reaction by acting as an intramolecular nuclephile [24,30]. Following the initial loss of NH3 , the remaining energy deposited in the ion is sufficient to lead to further, thermally driven, fragmentation. Eliminations of CH2 CO (m/z = 146), CO2 + H• (m/z = 144), and CH2 CO + CO (m/z = 118) were observed to occur after NH3 loss (m/z = 188), see Fig. 3a.
62
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
However, it is clear that at 1 keV collisions, the MH+ parent ions receive enough energy to overcome the dissociation threshold needed to initiate other competing fragmentation pathways. The fragment ion at m/z = 159 corresponding to the combined loss of H2 O and CO, and formation of the iminium ion, is abundant (Fig. 3a; Scheme 2, Pathway 1). The formation of this product ion is consistent with the proton transfer model for the fragmentation of protonated amino acids [23–25]. The observation of the iminium ion is in good agreement with earlier reports [31–34]. However, it is in contrast to the suggested theoretical explanation for the absence of the iminium ion, which was reported for Trp fragmentation by Rogalewicz and co-workers [35]. The intensity of ions with m/z = 159 is a factor of five smaller than that for the m/z = 188 peak. It is one of the highest relative abundances of the iminium ion fragment under TrpH+ CID that has been reported this far [24,29,31–34]. El Aribi and co-workers proposed that both key fragmentation channels mentioned above could lead to the same radical cations at m/z = 117 and m/z = 115 (by elimination of the radicals H• and CH3 • ) [31]. We observe strong peaks that could be due to these radical cationic fragments. However, it is difficult to make a definite assignment due to the finite kinetic energy distributions of the fragments leading to substantial peak broadening. In Fig. 3b and c, we show the CID spectra for l- and d-Trp colliding with racemic and (S)-(+)-2-butanol, respectively. The fragmentation patterns and the ratios between the different fragment ions are, within the present experimental uncertainties, identical for the collisions with the chiral and achiral targets. No changes associated with stereospecific interactions between chiral projectile ions and chiral target gas were observed. However, there is one significant exception that does not follow this general trend. The relative abundance of the peak at m/z = 130 to the main fragment peak at m/z = 188 [M-NH3 +H]+ changes dramatically with the target. The intensity ratio m/z = 130: m/z = 188 is measured to be 0.67 for the Ar target (Fig. 3a), 0.81 for racemic 2-butanol (Fig. 3b) and 1.02 for (S)-(+)-2-butanol (Fig. 3c). Nevertheless, this trend is observed for both l- and d-forms of Trp and thus we do not think that this behavior is associated with any chiral effect. This is most likely an experimental artifact due to ion source conditions which were slightly changed when changing the collision gas (but not between experiments with l- and d-amino acid enantiomers, which were performed back-to-back). It could be either a different internal temperature of the ions in the beam and consequently different decay patterns in CID. Or it could even be a contamination of the primary beam with the Trp cation radical, which could have been generated as collisional focusing conditions in the funnel were harsher. The ion with m/z = 130 is a well-established unique fingerprint of the Trp cation radical fragmentation – it fragments exclusively by the release of the benzylic ion with m/z = 130 [30]. This is also consistent with electron ionization and fragmentation experiments with neutral Trp molecules – where Trp converts into the radical cation that immediately fragments to yield m/z = 130 [36]. The presence of this fragment peak is not characteristic of protonated Trp. Different contaminations of the mass selected ion beam of protonated Trp (m/z = 205) with the Trp+• (m/z = 204) cation radical could be the reason for the growth of m/z = 130 contribution. This is a likely explanation since the mass resolution used in the quadrupole mass filter was not sufficient to separate m/z = 205 and 204. 3.4. MetH+ in collisions with Ar, (S)-(+)-2-butanol and (±)-2-butanol The CID spectra obtained from fragmentation of the protonated ions of l- and d-Met after interaction with Ar at the center-of-mass collision energy of 1 keV are shown in Fig. 4a. The key fragment
Fig. 4. Comparison of CID mass spectra obtained from collisions of l- and d-Met with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV.
ions observed are those corresponding to the loss of NH3 (m/z = 133, Scheme 2, Pathway 2) and the loss of both H2 O and CO (m/z = 104, Scheme 2, Pathway 1). The ion with m/z = 115 corresponds to H2 O loss following loss of NH3 . The loss of CH3 SH gives m/z = 102 from the side chain of methionine and is also a possible fragmentation pathway. Unfortunately, limitations in the mass resolution (caused by the kinetic energy distribution of the fragments) do not allow us to distinguish between this ion (at m/z = 102) and the product ion due to H2 O and CO loss at m/z = 104. In Fig. 4b and c, we show the CID spectra for l- or d-Met colliding with racemic and (S)-(+)-2-butanol, respectively. The
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
fragmentation patterns and the ratios between the different fragment ions are, within experimental uncertainties, identical for the collisions of l- and d-Met. However, the relative contributions of different fragmentation pathway are different for measurements with different targets (Fig. 4a–c). The reason is probably the same as in the case of Trp, i.e. slightly changed ion source conditions leading to differing internal energies and/or contamination by Met radical cation. The Met cation radical fragments differently to the protonated Met and mostly yields ions with m/z = 61 [36]. 3.5. Search for chiral effects In interactions of protonated chiral amino acids with a neutral chiral target gas at center-of-mass collision energy of 1 keV we do not observe stereospecific reactions. The energy deposited in the projectile ion was sufficient to initiate various competing fragmentation pathways, and the corresponding fragment ions were observed. All of the amino acids studied show correlated losses of H2 O + CO and NH3 . The formation of key fragment ions can be explained by typical protonated amino acid fragmentation mechanisms – proton migration and participation of side chain groups [23–25]. Fragmentations of Trp and Met, however, showed some deviation that may be linked to our specific experimental conditions rather than to chiral effects. The statistical nature of the fragmentation observed here is most likely due to the dominance of electronic excitation processes and also the reason why no chiral effects are observed in the present velocity regime. The possibility of chemistry (a precondition of stereochemistry) to occur during the activation process was discussed earlier by McLuckey [37]. Chemical reactions (and consequently stereochemically dependent interactions) can occur if activation proceeds in a stepwise fashion, as in multiple collisions. The probability for forming long-lived projectile-target complexes is high under single collision conditions only at collision energies of a few eV or less and with targets of relatively high polarizabilities. We have observed that such complexes formed following the low-energy collisions of the same enantiopure projectiles and target gases in a separate experiment performed at Oslo University. Under such conditions, chemistry, and consequently stereochemistry may occur. However, that is not the case for collisions at center-of-mass collision energies as high as 1 keV. 4. Conclusions We have tested the possibility of chiral recognition of enantiomeric amino acids via high-energy (1 keV center-of-mass energy) CID with chiral target gas. The observed dissociation channels are consistent with well-established fragmentation models for protonated amino acids [23–25]. The relative intensities of fragment ions resulting from collisions with chiral and achiral targets at 1 keV are found to be the same. We conclude, therefore, that there is no dependence on the chiral or achiral nature of the target gas and there are no observed stereochemically dependent interactions at these high-energy collisions. Acknowledgements The authors acknowledge financial support from the Swedish Research Council (Contracts Nos. 821-2013-1642 and 621-20114047). K.K. acknowledges the Swedish Institute for post-doctoral research scholarship within SI Visby Programme.
63
References [1] B.S. Sekhon, Enantioseparation of chiral drugs – an overview, Int. J. PharmTech Res. 2 (2010) 1584–1594. [2] G.-Q. Lin, Q.-D. You, J.-F. Cheng (Eds.), Chiral Drugs: Chemistry and Biological Action, Wiley, 2011. [3] H.Y. Aboul-Enein, I.W. Wainer, The Impact of Stereochemistry on Drug Development and Use, Wiley-Interscience, 1997. [4] J. McConathy, M.J. Owens, Stereochemistry in drug action, J. Clin. Psychiatry 5 (2003) 70–73. [5] J.H. Kim, A.R. Scialli, Thalidomide: the tragedy of birth defects and the effective treatment of disease, Toxicol. Sci. 122 (2011) 1–6. [6] S.W. Smith, Chiral toxicology: it’s the same thing. . .only different, Toxicol. Sci. 110 (2009) 4–30. [7] M. Sawada, Y. Takai, H. Yamada, S. Hirayama, T. Kaneda, T. Tanaka, Chiral recognition in host–guest complexation determined by the enantiomer-labeled guest method using fast atom bombardment mass spectrometry, J. Am. Chem. Soc. 117 (1995) 7726–7736. [8] M. Sawada, Y. Takai, H. Yamada, J. Nishida, T. Kaneda, R. Arakawa, M. Okamoto, K. Hirose, T. Tanaka, K. Naemura, Chiral amino acid recognition detected by electrospray ionization (ESI) and fast atom bombardment (FAB) mass spectrometry (MS) coupled with the enantiomer-labelled (EL) guest method, J. Chem. Soc. Perkin Trans. 2 (3) (1998) 701–710. [9] M.S. Lee (Ed.), Mass Spectrometry Handbook, first ed., John Wiley & Sons, 2012. [10] G. Grigorean, C.B. Lebrilla, Enantiomeric analysis of pharmaceutical compounds by ion/molecule reactions, Anal. Chem. 73 (2001) 1684–1691. [11] J. Ramirez, F. He, C.B. Lebrilla, Gas-phase chiral differentiation of amino acid guests in cyclodextrin hosts, J. Am. Chem. Soc. 120 (1998) 7387–7388. [12] A.R. Hyyrylainen, J.M. Pakarinen, E. Forro, F. Fulop, P. Vainiotalo, Chiral differentiation of some cyclopentane and cyclohexane beta-amino acid enantiomers through ion/molecule reactions, J. Am. Soc. Mass Spectrom. 20 (2009) 1235–1241. [13] W.A. Tao, R.G. Cooks, Chiral analysis by MS, Anal. Chem. 75 (2003) 25A–31A. [14] B.L. Young, R.G. Cooks, Improvements in quantitative chiral determinations using the mass spectrometric kinetic method, Int. J. Mass Spectrom. 267 (2007) 199–204. [15] A.R. Hyyrylainen, J.M. Pakarinen, E. Forro, F. Fulop, P. Vainiotalo, Chiral differentiation of some cyclic beta-amino acids by kinetic and fixed ligand methods, J. Mass Spectrom. 45 (2010) 198–204. [16] F.W. McLafferty, P.F. Bente, R. Kornfeld, S.-C. Tsai, I. Howe, Collisional activation spectra of organic ions, J. Am. Chem. Soc. 95 (1973) 2120–2129. [17] H.H. Budzikiewicz (Ed.), Progress in Mass Spectrometry, vol. 4: Fundamental Aspects of Organic Mass Spectrometry, K. Levsen, Verlag Chemie, Weinheim, New York, 1978. ¯ [18] M.H. Stockett, M. Gatchell, J.D. Alexander, U. Berzin ¸ sˇ , T. Chen, K. Farid, A. Johansson, K. Kulyk, P. Rousseau, K. Støchkel, L. Adoui, P. Hvelplund, B.A. Huber, H.T. Schmidt, H. Zettergren, H. Cederquist, Fragmentation of anthracene C14 H10 , acridine C13 H9 N and phenazine C12 H8 N2 ions in collisions with atoms, Phys. Chem. Chem. Phys. 16 (2014) 21980–21987. [19] T. Chen, M. Gatchell, M.H. Stockett, J.D. Alexander, Y. Zhang, P. Rousseau, A. Domaracka, S. Maclot, R. Delaunay, L. Adoui, B.A. Huber, T. Schlathölter, H.T. Schmidt, H. Cederquist, H. Zettergren, Absolute fragmentation cross sections in atom-molecule collisions: scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules, J. Chem. Phys. 140 (2014) 224306. [20] M. Gatchell, M.H. Stockett, P. Rousseau, T. Chen, K. Kulyk, H.T. Schmidt, J.Y. Chesnel, A. Domaracka, A. Mery, S. Maclot, L. Adoui, K. Støchkel, P. Hvelplund, Y. Wang, M. Alcami, B.A. Huber, F. Martin, H. Zettergren, H. Cederquist, Nonstatistical fragmentation of PAHs and fullerenes in collisions with atoms, Int. J. Mass Spectrom. 365/366 (2014) 260–265. [21] M. Karas, Separation of components of an analysis sample in an ion mobility spectrometer using a supply of selectively interactive gaseous particles, Int. patent #WO02096805, December 5, 2002. [22] P. Dwivedi, C. Wu, L.M. Matz, B.H. Clowers, W.F. Siems, H.H. Hill Jr., Gasphase chiral separations by ion mobility spectrometry, Anal. Chem. 78 (2006) 8200–8206. [23] A.R. Dongre, J.L. Jones, A. Somogyi, V.H. Wysocki, Influence of peptide composition, gas-phase basicity, and chemical modification on fragmentation efficiency: evidence for the mobile proton model, J. Am. Chem. Soc. 118 (1996) 8365–8374. [24] H. Lioe, R.A. O’Hair, G.E. Reid, Gas-phase reactions of protonated tryptophan, J. Am. Soc. Mass Spectrom. 15 (2004) 65–76. [25] R.A. O’Hair, The role of nucleophile–electrophile interactions in the unimolecular and bimolecular gas-phase ion chemistry of peptides and related systems, J. Mass Spectrom. 35 (2000) 1377–1381. [26] T. Shoeib, A. Cunje, A.C. Hopkinson, K.W.M. Siu, Gas-phase fragmentation of the Ag+ -phenylalanine complex: cation– interactions and radical cation formation, J. Am. Soc. Mass Spectrom. 13 (2002) 408–416. [27] S. Bouchonnet, Y. Hoppilliard, G. Ohanessian, Formation and fragmentations of organometallic complexes involving aliphatic ␣-amino acids and transition metal cations—a plasma desorption mass spectrometry study, J. Mass Spectrom. 30 (1995) 172–179. [28] C.W. Tsang, A.G. Harrison, Chemical ionization of amino acids, J. Am. Chem. Soc. 98 (1976) 1301–1308. [29] N.N. Dookeran, T. Yalcin, A.G. Harrison, Fragmentation reactions of protonated ␣-amino acids, J. Mass Spectrom. 31 (1996) 500–508.
64
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
[30] H. Lioe, R.A. O’Hair, Comparison of collision-induced dissociation and electroninduced dissociation of singly protonated aromatic amino acids, cystine and related simple peptides using a hybrid linear ion trap-FT-ICR mass spectrometer, Anal. Bioanal. Chem. 389 (2007) 1429–1437. [31] H. El Aribi, G. Orlova, A.C. Hopkinson, K.W.M. Siu, Gas-phase fragmentation reactions of protonated aromatic amino acids: concomitant and consecutive neutral eliminations and radical cation formations, J. Phys. Chem. A 108 (2004) 3844–3853. [32] W. Kulik, W. Heerma, A study of the positive and negative ion fast atom bombardment mass spectra of alpha-amino acids, Biomed. Environ. Mass Spectrom. 15 (1988) 419–427. [33] C.D. Parker, D.M. Hercules, Laser mass spectra of simple aliphatic and aromatic amino acids, Anal. Chem. 57 (1985) 698–704.
[34] S. Bouchonnet, J.-P. Denhez, Y. Hoppilliard, C. Mauriac, Is plasma desorption mass spectrometry useful for small-molecule analysis? Fragmentations of the natural ␣-amino acids, Anal. Chem. 64 (1992) 743–754. [35] F. Rogalewicz, Y. Hoppilliard, G. Ohanessian, Fragmentation mechanisms of ␣-amino acids protonated under electrospray ionization: a collisional activation and ab initio theoretical study, Int. J. Mass spectrom. 195/196 (2000) 565–590. [36] P.J. Linstrom, W.G. Mallard (Eds.), NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg, MD, http://webbook.nist.gov [37] S.A. McLuckey, Principles of collisional activation in analytical mass spectrometry, J. Am. Soc. Mass Spectrom. 3 (1992) 599–614.
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
High-energy collisions of protonated enantiopure amino acids with a chiral target gas Kostiantyn Kulyk ∗ , Oleksii Rebrov, Mark H. Stockett, John D. Alexander, Henning Zettergren, Henning T. Schmidt, Richard D. Thomas, Henrik Cederquist, Mats Larsson Stockholm University, Department of Physics, SE-106 91 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 16 March 2015 Received in revised form 6 August 2015 Accepted 10 August 2015 Available online 20 August 2015 Keywords: High-energy collisional activation Tandem mass spectrometry Protonated amino acids Chiral collision gas 2-Butanol
a b s t r a c t We have studied the fragmentation of the singly protonated l- and d-forms of enantiomerically pure phenylalanine (Phe), tryptophan (Trp), and methionine (Met) in high-energy collisions with chiral and achiral gas targets. (S)-(+)-2-butanol, racemic (±)-2-butanol, and argon were used as target gases. At center-of-mass frame collision energy of 1 keV, it was found that all of the ions exhibit common fragmentation pathways which are independent of target chirality. For all projectile ions, the elimination of NH3 and H2 O + CO were found to be the main reaction channels. The observed fragmentation patterns were dominated by statistically driven processes. The energy deposited into the ions was found to be sufficient to yield multiple fragment ions, which arise from decomposition via various competitive reaction pathways. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The current drive in pharmaceutical research has shifted toward the development of single enantiomer-based drugs rather than racemic mixture of both enantiomeric forms [1–3]. The fundamental reason for this is the highly specific nature of drug/target interactions which often depend on stereochemistry. The enantiomers of the same drug can produce different biological responses [4]. A shocking but demonstrative example of this phenomenon is the tragedy of birth defects linked with the active use of racemic thalidomide for the treatment of morning sickness in pregnant women in the late 1950s and early 1960s [5,6]. The increasing demand for optically pure pharmaceuticals calls for efficient enantioselective analytical methods to be developed. The fast and sensitive performance of mass spectrometry (MS) places it among the most promising techniques in drug analysis. However, despite significant success in the development of MS-based chiral recognition methods [7–15], many gas-phase processes which lie behind this recognition are still poorly understood. Furthermore, these methods of chiral recognition often involve specifically modified instruments, require enantiomerically pure reference compounds, involve time-consuming building of
∗ Corresponding author. Tel.: +46 8 5537 8612. E-mail address: [email protected] (K. Kulyk). http://dx.doi.org/10.1016/j.ijms.2015.08.010 1387-3806/© 2015 Elsevier B.V. All rights reserved.
calibration curves, and may depend on complex data analysis of fragmentation patterns. Further research efforts as well as an understanding of the ion physics and chemistry involved are urgently needed in order to overcome these limitations. In the late 1960s and early 1970s, McLafferty and co-workers performed in-depth studies of keV collisional activation fragmentation as a function of various parameters [16]. From these studies, it was concluded that the nature of the collision gas has little influence on the product fragment intensity ratios [16,17]. Though, in recent studies with other molecules, the nature of the collision gas was observed to influence the fragmentation [18–20]. Chiral projectile ions, however, were not studied in high-energy collision experiments with chiral target gases. Here we report the results of high-energy collisional activation of ions of enantiomerically pure amino acids with chiral and achiral target gases. Due to its suitable vapor pressure and commercial availability in enantiopure form, 2-butanol was selected to be used as a volatile chiral target molecule in our measurements. Among the additional reasons for choosing this particular gas was its successful utilization as a volatile chiral dopant for enantioselective separations by ion mobility spectrometry [21,22]. The mechanism of enantioselectivity that lay behind those gas-phase chiral discriminations is still not fully understood. This study represents, therefore, the first examination performed by our group directed to the understanding of the ion chemistry and physics behind gas-phase chiral separations based
60
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
Scheme 2. Main fragmentation pathways for protonated amino acids (R-side chain group). Fig. 1. Sketch of the experimental apparatus used in the experiments.
on the use of a chiral target gas. The possibility of stereochemically dependent interactions with keV activation under single collision conditions was addressed in this particular study. A separate manuscript devoted to eV collisions of enantiomeric ions and molecules is under preparation. 2. Experimental 2.1. Electrospray ion source platform An electrospray ion source platform, developed and constructed at the Department of Physics at Stockholm University [18], was used in this investigation. A sketch of the experimental apparatus is shown in Fig. 1. In every measurement, a continuous molecular ion beam of protonated amino acids was generated in an electrospray ion source. After ionization, the ions entered the vacuum system through a heated capillary and were focused in a radiofrequency ion funnel, mass selected in a quadrupole mass filter, and accelerated to the intended collision before entering a collision cell containing the target cell. The typical pressure-range in the 4-cmlong collision cell during the experiments was 0.5–1 mTorr. After passing through the cell, horizontal deflector plates and a cylindrical lens were used to analyze the primary molecular ion beam and the fragments according to their kinetic energy to charge ratio by guiding them to a position-sensitive microchannel plate detector. The position on the detector and the corresponding voltages on the deflector plates were used to obtain the fragment mass distribution. 2.2. Collision gases and reagents The l- and d-forms of enantiomerically pure tryptophan (Trp), phenylalanine (Phe), and methionine (Met) were supplied by Sigma–Aldrich; their structures are shown in Scheme 1. Purity of all the amino acids was ≥98%. The chiral target (S)-(+)-2-butanol (99%) and the racemate of (±)-2-butanol (99.5%) were purchased from Sigma–Aldrich. Methanol (>99.8%) and acetic acid (>99.8%) were obtained from Fluka. Finally, argon (99.998%) was purchased from Air Liquide. Stock solutions of all of the amino acids were prepared at a concentration of 0.05 M in a solvent mixture – methanol:water:acetic acid (49:49:2). Just prior to the experiments, the stock solutions were diluted to a concentration of 0.005 M using the same solvent system. The sample solutions were introduced into electrospray source by using a syringe pump (Harvard Apparatus) at a typical flow rate 1.5 L/min. A stable electrospray of protonated amino acid ions was achieved in the positive ion mode under the following instrument settings: needle voltage, 3.5–4.8 kV; capillary voltage,
200–300 V; capillary temperature, 110 ◦ C. Each new measurement was preceded by a thorough cleaning of the injection system using the above mentioned solvent mixture. 3. Results and discussion 3.1. High-energy collisional activation In this work, we examined the influence of the nature of the collision gas (chiral, achiral) on the dissociation yield of different fragmentation products resulting from the dissociation of enantiopure amino acid ions following 1 keV collisions (center-of-mass frame). The collision induced dissociation (CID) spectra measured after collisions of the protonated amino acids with argon, S-(+)-2butanol and (±)-2-butanol are discussed in Sections 3.2–3.4. In all spectra, the intensity of the parent ion is normalized to 1.0 and the intensity ratios of individual fragments are determined with respect to the parent ion. At present, two major concepts are used to explain the fragmentation mechanisms of protonated amino acid ions: the mobile proton model [23,24] and the model involving side chain groups for facilitation of cleavage reactions [24,25] (Scheme 2). Nearly all our CID results are consistent with these models and could be rationalized by them. 3.2. PheH+ in collisions with Ar, (S)-(+)-2-butanol, and (±)-2-butanol The CID pattern of protonated phenylalanine colliding with Ar is shown in Fig. 2a. Our results agree with previous reports [26–29]: the most common fragment appears at m/z = 120 and corresponds to the iminium ion, formed by the loss of H2 O and CO from the protonated parent. Following the work by Shoeib et al. [26], it is currently believed that this fragment is due to H2 O and CO loss (Scheme 2, Pathway 1) rather than the loss of dihydroxycarbene, as was suggested earlier. The iminium ion is typically observed in the fragmentation of singly protonated amino acids [26–29]. The significant abundances of product ions other than iminium indicate that a collision energy of 1 keV (center-of-mass frame) is high enough for the initiation of competing fragmentation channels. The reaction competing with proton transfer and the combined release of H2 O + CO is the loss of NH3 (Fig. 2a, m/z = 149; Scheme 2, Pathway 2). Decay of the fragment ions formed after NH3 elimination was observed to proceed via the loss of H2 O (m/z = 131), H2 O + CO (m/z = 103), etc. The fragmentation patterns of l- and d-Phe after collisions with racemic and enantiomerically pure 2-butanol are plotted in Fig. 2b and c, respectively. Comparison among the spectra plotted
Scheme 1. Structures of the amino acids used in the collision induced dissociation study.
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
Fig. 2. Comparison of CID mass spectra obtained from collisions of l- and d-Phe with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV.
in Fig. 2a–c shows that there are no significant differences either in the fragmentation patterns or in the fragment ion ratios when changing between the achiral and chiral targets in the case of 1 keV collisions. Assignments of the individual fragment peaks are the same as for the l- and d-Phe CID spectra with Ar. 3.3. TrpH+ in collisions with Ar, (S)-(+)-2-butanol and (±)-2-butanol The CID spectra obtained from fragmentation of the protonated ions of l- and d-Trp after interacting with Ar at a center-of-mass collision energy of 1 keV are shown in Fig. 3a. Unlike phenylalanine, and all other aromatic amino acids, the loss of CO + H2 O
61
Fig. 3. Comparison of CID mass spectra obtained from collisions of l- and d-Trp with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV. The ion with m/z = 130 is assigned as * because various possibilities exist for the interpretation.
and subsequent formation of the iminium ion is not the most prominent fragmentation channel for protonated tryptophan. Here, the dominant fragmentation pathway involves the loss of NH3 resulting in a charged fragment at m/z = 188 (Fig. 3a; Scheme 2, Pathway 2). It is known that the indole substituent facilitates the NH3 loss reaction by acting as an intramolecular nuclephile [24,30]. Following the initial loss of NH3 , the remaining energy deposited in the ion is sufficient to lead to further, thermally driven, fragmentation. Eliminations of CH2 CO (m/z = 146), CO2 + H• (m/z = 144), and CH2 CO + CO (m/z = 118) were observed to occur after NH3 loss (m/z = 188), see Fig. 3a.
62
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
However, it is clear that at 1 keV collisions, the MH+ parent ions receive enough energy to overcome the dissociation threshold needed to initiate other competing fragmentation pathways. The fragment ion at m/z = 159 corresponding to the combined loss of H2 O and CO, and formation of the iminium ion, is abundant (Fig. 3a; Scheme 2, Pathway 1). The formation of this product ion is consistent with the proton transfer model for the fragmentation of protonated amino acids [23–25]. The observation of the iminium ion is in good agreement with earlier reports [31–34]. However, it is in contrast to the suggested theoretical explanation for the absence of the iminium ion, which was reported for Trp fragmentation by Rogalewicz and co-workers [35]. The intensity of ions with m/z = 159 is a factor of five smaller than that for the m/z = 188 peak. It is one of the highest relative abundances of the iminium ion fragment under TrpH+ CID that has been reported this far [24,29,31–34]. El Aribi and co-workers proposed that both key fragmentation channels mentioned above could lead to the same radical cations at m/z = 117 and m/z = 115 (by elimination of the radicals H• and CH3 • ) [31]. We observe strong peaks that could be due to these radical cationic fragments. However, it is difficult to make a definite assignment due to the finite kinetic energy distributions of the fragments leading to substantial peak broadening. In Fig. 3b and c, we show the CID spectra for l- and d-Trp colliding with racemic and (S)-(+)-2-butanol, respectively. The fragmentation patterns and the ratios between the different fragment ions are, within the present experimental uncertainties, identical for the collisions with the chiral and achiral targets. No changes associated with stereospecific interactions between chiral projectile ions and chiral target gas were observed. However, there is one significant exception that does not follow this general trend. The relative abundance of the peak at m/z = 130 to the main fragment peak at m/z = 188 [M-NH3 +H]+ changes dramatically with the target. The intensity ratio m/z = 130: m/z = 188 is measured to be 0.67 for the Ar target (Fig. 3a), 0.81 for racemic 2-butanol (Fig. 3b) and 1.02 for (S)-(+)-2-butanol (Fig. 3c). Nevertheless, this trend is observed for both l- and d-forms of Trp and thus we do not think that this behavior is associated with any chiral effect. This is most likely an experimental artifact due to ion source conditions which were slightly changed when changing the collision gas (but not between experiments with l- and d-amino acid enantiomers, which were performed back-to-back). It could be either a different internal temperature of the ions in the beam and consequently different decay patterns in CID. Or it could even be a contamination of the primary beam with the Trp cation radical, which could have been generated as collisional focusing conditions in the funnel were harsher. The ion with m/z = 130 is a well-established unique fingerprint of the Trp cation radical fragmentation – it fragments exclusively by the release of the benzylic ion with m/z = 130 [30]. This is also consistent with electron ionization and fragmentation experiments with neutral Trp molecules – where Trp converts into the radical cation that immediately fragments to yield m/z = 130 [36]. The presence of this fragment peak is not characteristic of protonated Trp. Different contaminations of the mass selected ion beam of protonated Trp (m/z = 205) with the Trp+• (m/z = 204) cation radical could be the reason for the growth of m/z = 130 contribution. This is a likely explanation since the mass resolution used in the quadrupole mass filter was not sufficient to separate m/z = 205 and 204. 3.4. MetH+ in collisions with Ar, (S)-(+)-2-butanol and (±)-2-butanol The CID spectra obtained from fragmentation of the protonated ions of l- and d-Met after interaction with Ar at the center-of-mass collision energy of 1 keV are shown in Fig. 4a. The key fragment
Fig. 4. Comparison of CID mass spectra obtained from collisions of l- and d-Met with (a) Ar, (b) racemic 2-butanol and (c) (S)-(+)-2-butanol at center-of-mass collision energies of 1 keV.
ions observed are those corresponding to the loss of NH3 (m/z = 133, Scheme 2, Pathway 2) and the loss of both H2 O and CO (m/z = 104, Scheme 2, Pathway 1). The ion with m/z = 115 corresponds to H2 O loss following loss of NH3 . The loss of CH3 SH gives m/z = 102 from the side chain of methionine and is also a possible fragmentation pathway. Unfortunately, limitations in the mass resolution (caused by the kinetic energy distribution of the fragments) do not allow us to distinguish between this ion (at m/z = 102) and the product ion due to H2 O and CO loss at m/z = 104. In Fig. 4b and c, we show the CID spectra for l- or d-Met colliding with racemic and (S)-(+)-2-butanol, respectively. The
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
fragmentation patterns and the ratios between the different fragment ions are, within experimental uncertainties, identical for the collisions of l- and d-Met. However, the relative contributions of different fragmentation pathway are different for measurements with different targets (Fig. 4a–c). The reason is probably the same as in the case of Trp, i.e. slightly changed ion source conditions leading to differing internal energies and/or contamination by Met radical cation. The Met cation radical fragments differently to the protonated Met and mostly yields ions with m/z = 61 [36]. 3.5. Search for chiral effects In interactions of protonated chiral amino acids with a neutral chiral target gas at center-of-mass collision energy of 1 keV we do not observe stereospecific reactions. The energy deposited in the projectile ion was sufficient to initiate various competing fragmentation pathways, and the corresponding fragment ions were observed. All of the amino acids studied show correlated losses of H2 O + CO and NH3 . The formation of key fragment ions can be explained by typical protonated amino acid fragmentation mechanisms – proton migration and participation of side chain groups [23–25]. Fragmentations of Trp and Met, however, showed some deviation that may be linked to our specific experimental conditions rather than to chiral effects. The statistical nature of the fragmentation observed here is most likely due to the dominance of electronic excitation processes and also the reason why no chiral effects are observed in the present velocity regime. The possibility of chemistry (a precondition of stereochemistry) to occur during the activation process was discussed earlier by McLuckey [37]. Chemical reactions (and consequently stereochemically dependent interactions) can occur if activation proceeds in a stepwise fashion, as in multiple collisions. The probability for forming long-lived projectile-target complexes is high under single collision conditions only at collision energies of a few eV or less and with targets of relatively high polarizabilities. We have observed that such complexes formed following the low-energy collisions of the same enantiopure projectiles and target gases in a separate experiment performed at Oslo University. Under such conditions, chemistry, and consequently stereochemistry may occur. However, that is not the case for collisions at center-of-mass collision energies as high as 1 keV. 4. Conclusions We have tested the possibility of chiral recognition of enantiomeric amino acids via high-energy (1 keV center-of-mass energy) CID with chiral target gas. The observed dissociation channels are consistent with well-established fragmentation models for protonated amino acids [23–25]. The relative intensities of fragment ions resulting from collisions with chiral and achiral targets at 1 keV are found to be the same. We conclude, therefore, that there is no dependence on the chiral or achiral nature of the target gas and there are no observed stereochemically dependent interactions at these high-energy collisions. Acknowledgements The authors acknowledge financial support from the Swedish Research Council (Contracts Nos. 821-2013-1642 and 621-20114047). K.K. acknowledges the Swedish Institute for post-doctoral research scholarship within SI Visby Programme.
63
References [1] B.S. Sekhon, Enantioseparation of chiral drugs – an overview, Int. J. PharmTech Res. 2 (2010) 1584–1594. [2] G.-Q. Lin, Q.-D. You, J.-F. Cheng (Eds.), Chiral Drugs: Chemistry and Biological Action, Wiley, 2011. [3] H.Y. Aboul-Enein, I.W. Wainer, The Impact of Stereochemistry on Drug Development and Use, Wiley-Interscience, 1997. [4] J. McConathy, M.J. Owens, Stereochemistry in drug action, J. Clin. Psychiatry 5 (2003) 70–73. [5] J.H. Kim, A.R. Scialli, Thalidomide: the tragedy of birth defects and the effective treatment of disease, Toxicol. Sci. 122 (2011) 1–6. [6] S.W. Smith, Chiral toxicology: it’s the same thing. . .only different, Toxicol. Sci. 110 (2009) 4–30. [7] M. Sawada, Y. Takai, H. Yamada, S. Hirayama, T. Kaneda, T. Tanaka, Chiral recognition in host–guest complexation determined by the enantiomer-labeled guest method using fast atom bombardment mass spectrometry, J. Am. Chem. Soc. 117 (1995) 7726–7736. [8] M. Sawada, Y. Takai, H. Yamada, J. Nishida, T. Kaneda, R. Arakawa, M. Okamoto, K. Hirose, T. Tanaka, K. Naemura, Chiral amino acid recognition detected by electrospray ionization (ESI) and fast atom bombardment (FAB) mass spectrometry (MS) coupled with the enantiomer-labelled (EL) guest method, J. Chem. Soc. Perkin Trans. 2 (3) (1998) 701–710. [9] M.S. Lee (Ed.), Mass Spectrometry Handbook, first ed., John Wiley & Sons, 2012. [10] G. Grigorean, C.B. Lebrilla, Enantiomeric analysis of pharmaceutical compounds by ion/molecule reactions, Anal. Chem. 73 (2001) 1684–1691. [11] J. Ramirez, F. He, C.B. Lebrilla, Gas-phase chiral differentiation of amino acid guests in cyclodextrin hosts, J. Am. Chem. Soc. 120 (1998) 7387–7388. [12] A.R. Hyyrylainen, J.M. Pakarinen, E. Forro, F. Fulop, P. Vainiotalo, Chiral differentiation of some cyclopentane and cyclohexane beta-amino acid enantiomers through ion/molecule reactions, J. Am. Soc. Mass Spectrom. 20 (2009) 1235–1241. [13] W.A. Tao, R.G. Cooks, Chiral analysis by MS, Anal. Chem. 75 (2003) 25A–31A. [14] B.L. Young, R.G. Cooks, Improvements in quantitative chiral determinations using the mass spectrometric kinetic method, Int. J. Mass Spectrom. 267 (2007) 199–204. [15] A.R. Hyyrylainen, J.M. Pakarinen, E. Forro, F. Fulop, P. Vainiotalo, Chiral differentiation of some cyclic beta-amino acids by kinetic and fixed ligand methods, J. Mass Spectrom. 45 (2010) 198–204. [16] F.W. McLafferty, P.F. Bente, R. Kornfeld, S.-C. Tsai, I. Howe, Collisional activation spectra of organic ions, J. Am. Chem. Soc. 95 (1973) 2120–2129. [17] H.H. Budzikiewicz (Ed.), Progress in Mass Spectrometry, vol. 4: Fundamental Aspects of Organic Mass Spectrometry, K. Levsen, Verlag Chemie, Weinheim, New York, 1978. ¯ [18] M.H. Stockett, M. Gatchell, J.D. Alexander, U. Berzin ¸ sˇ , T. Chen, K. Farid, A. Johansson, K. Kulyk, P. Rousseau, K. Støchkel, L. Adoui, P. Hvelplund, B.A. Huber, H.T. Schmidt, H. Zettergren, H. Cederquist, Fragmentation of anthracene C14 H10 , acridine C13 H9 N and phenazine C12 H8 N2 ions in collisions with atoms, Phys. Chem. Chem. Phys. 16 (2014) 21980–21987. [19] T. Chen, M. Gatchell, M.H. Stockett, J.D. Alexander, Y. Zhang, P. Rousseau, A. Domaracka, S. Maclot, R. Delaunay, L. Adoui, B.A. Huber, T. Schlathölter, H.T. Schmidt, H. Cederquist, H. Zettergren, Absolute fragmentation cross sections in atom-molecule collisions: scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules, J. Chem. Phys. 140 (2014) 224306. [20] M. Gatchell, M.H. Stockett, P. Rousseau, T. Chen, K. Kulyk, H.T. Schmidt, J.Y. Chesnel, A. Domaracka, A. Mery, S. Maclot, L. Adoui, K. Støchkel, P. Hvelplund, Y. Wang, M. Alcami, B.A. Huber, F. Martin, H. Zettergren, H. Cederquist, Nonstatistical fragmentation of PAHs and fullerenes in collisions with atoms, Int. J. Mass Spectrom. 365/366 (2014) 260–265. [21] M. Karas, Separation of components of an analysis sample in an ion mobility spectrometer using a supply of selectively interactive gaseous particles, Int. patent #WO02096805, December 5, 2002. [22] P. Dwivedi, C. Wu, L.M. Matz, B.H. Clowers, W.F. Siems, H.H. Hill Jr., Gasphase chiral separations by ion mobility spectrometry, Anal. Chem. 78 (2006) 8200–8206. [23] A.R. Dongre, J.L. Jones, A. Somogyi, V.H. Wysocki, Influence of peptide composition, gas-phase basicity, and chemical modification on fragmentation efficiency: evidence for the mobile proton model, J. Am. Chem. Soc. 118 (1996) 8365–8374. [24] H. Lioe, R.A. O’Hair, G.E. Reid, Gas-phase reactions of protonated tryptophan, J. Am. Soc. Mass Spectrom. 15 (2004) 65–76. [25] R.A. O’Hair, The role of nucleophile–electrophile interactions in the unimolecular and bimolecular gas-phase ion chemistry of peptides and related systems, J. Mass Spectrom. 35 (2000) 1377–1381. [26] T. Shoeib, A. Cunje, A.C. Hopkinson, K.W.M. Siu, Gas-phase fragmentation of the Ag+ -phenylalanine complex: cation– interactions and radical cation formation, J. Am. Soc. Mass Spectrom. 13 (2002) 408–416. [27] S. Bouchonnet, Y. Hoppilliard, G. Ohanessian, Formation and fragmentations of organometallic complexes involving aliphatic ␣-amino acids and transition metal cations—a plasma desorption mass spectrometry study, J. Mass Spectrom. 30 (1995) 172–179. [28] C.W. Tsang, A.G. Harrison, Chemical ionization of amino acids, J. Am. Chem. Soc. 98 (1976) 1301–1308. [29] N.N. Dookeran, T. Yalcin, A.G. Harrison, Fragmentation reactions of protonated ␣-amino acids, J. Mass Spectrom. 31 (1996) 500–508.
64
K. Kulyk et al. / International Journal of Mass Spectrometry 388 (2015) 59–64
[30] H. Lioe, R.A. O’Hair, Comparison of collision-induced dissociation and electroninduced dissociation of singly protonated aromatic amino acids, cystine and related simple peptides using a hybrid linear ion trap-FT-ICR mass spectrometer, Anal. Bioanal. Chem. 389 (2007) 1429–1437. [31] H. El Aribi, G. Orlova, A.C. Hopkinson, K.W.M. Siu, Gas-phase fragmentation reactions of protonated aromatic amino acids: concomitant and consecutive neutral eliminations and radical cation formations, J. Phys. Chem. A 108 (2004) 3844–3853. [32] W. Kulik, W. Heerma, A study of the positive and negative ion fast atom bombardment mass spectra of alpha-amino acids, Biomed. Environ. Mass Spectrom. 15 (1988) 419–427. [33] C.D. Parker, D.M. Hercules, Laser mass spectra of simple aliphatic and aromatic amino acids, Anal. Chem. 57 (1985) 698–704.
[34] S. Bouchonnet, J.-P. Denhez, Y. Hoppilliard, C. Mauriac, Is plasma desorption mass spectrometry useful for small-molecule analysis? Fragmentations of the natural ␣-amino acids, Anal. Chem. 64 (1992) 743–754. [35] F. Rogalewicz, Y. Hoppilliard, G. Ohanessian, Fragmentation mechanisms of ␣-amino acids protonated under electrospray ionization: a collisional activation and ab initio theoretical study, Int. J. Mass spectrom. 195/196 (2000) 565–590. [36] P.J. Linstrom, W.G. Mallard (Eds.), NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg, MD, http://webbook.nist.gov [37] S.A. McLuckey, Principles of collisional activation in analytical mass spectrometry, J. Am. Soc. Mass Spectrom. 3 (1992) 599–614.