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Methanol controllable chromophoric supramolecular switch investigated by electrospray mass spectrometry
Analytica Chimica Acta 495 (2003) 1–10
Methanol controllable chromophoric supramolecular switch investigated by electrospray mass spectrometry Hao-Jie Lu, Yin-Long Guo∗ Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China Received 19 February 2003; accepted 4 August 2003
Abstract The supramolecular switch which is controlled by methanol, with color turned on or off, is prepared by a cyclodextrinincluded dianion of phenolphthalein. This phenomenon is studied by electrospray time of flight mass spectrometry (TOFMS) and electrospray Fourier transform mass spectrometry (FTMS). The influence of the host cavity size and the nozzle potential on the supramolecular switch system is investigated in detail. The dianion phenolphthalein in the supramolecular switch system is also studied by tandem mass spectrometry. The relevant mechanism is discussed, and the result is contrary to those of papers reported previously. © 2003 Elsevier B.V. All rights reserved. Keywords: Supramolecular switch; Mass spectrometry; Methanol
1. Introduction Molecular information processing, which exploits the switching of molecular and supramolecular systems between clearly identifiable states, has received much attention in recent years [1–9]. Aside from the continuing attention being paid to electrochromic and photochemically active compounds, along with chemical sensors, much focus is centered on molecular and supramolecular systems in which the relative positions of their component parts can be altered by an external stimulus [1–3]. Host–guest complexes are supramolecular associations of current scientific and technological interest, whose existence has been demonstrated either in solution or in the solid state by spectroscopic and other physical chemistry methods. ∗ Corresponding author. Tel.: +86-21-64163300-3106; fax: +86-21-64166128. E-mail address: [email protected] (Y.-L. Guo).
In recent years, there has been much interest in the potential of mass spectrometry (MS) for the study of host–guest chemistry in the gas phase [10–23]. These studies by MS have rapidly gained momentum since the introduction of electrospray ionization (ESI) as one of the softest ionization methods [10–21]. Therefore, a large number of reports have dealt with the MS in various types of host–guest systems. Almost all these papers relate to the evaluation of molecular and/or chiral recognition capability, the study of association/dissociation equilibria in solution, the characterization of the complexes and the study of the complex formation processes in the gas phase. Given this huge body of knowledge, it seems surprising that MS has scarcely been used to the field of molecular and supramolecular switches. Herein we developed a ESI–time of flight mass spectrometry (TOFMS) and ESI–Fourier transform mass spectrometry (FTMS) method to a cyclodextrin-based supramolecular switch that can be turned on or off
0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2003.08.017
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H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10
chemically by a color change. It is common knowledge that phenolphthalein is purple in solution at pH >10. When -cyclodextrin is added, phenolphthalein would be included in the cavity of -cyclodextrin, with an accompanying color change from purple to colorless. The solution becomes purple again when a suitable volume of methanol is added. However the color disappears again when an additional volume of methanol is added. This type of supramolecular switch maybe useful as a sensor for detecting organic molecules by color change. This visible phenomenon, color on or off, and the relevant mechanism is investigated by ESI–TOFMS, ESI–FTMS and tandem MS in this paper. And we reach a conclusion which is contrary to those of previous papers [24,25].
2. Experimental 2.1. Mass spectrometer The Mariner electrospray time-of-flight mass spectrometer (Perkin-Elmer Corp.) was utilized in all single stage MS experiments. The used ionspray source was a micro-electrospray source (SCIEX). The ionspray voltage was 3800 V and the nozzle potential was 90 V (unless otherwise stated in the text). The data were acquired by a Mariner Instrument Control Panel for positive ion mode from 100 to 2000 amu, and dealt with by Data Explorer. All tandem MS experiments were carried out at Bruker Daltonics using an APEX IIITM electrospray
Fig. 1. The ESI–TOF mass spectrum of phenolphthalein with ␣-CD (A), -CD (B), and ␥-CD (C) as host.
H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10
3
Fig. 2. The expanded mass spectrum of the peak clusters of singly charged (A) and doubly charged (B) 1:1 -CD–phenolphthalein adduct.
FTMS instrument equipped with a 7.0 T shielded superconducting magnet. The capillary and end plate voltages were −4635 and −4393 V, respectively. The isolation sweep attenuation and activation attenuation were 25 and 31 dB, respectively. The instrument calibrations were both made with a PEG-600 solution. The samples were injected at a flow rate of 7 l min−1 using a syringe pump.
3. Results and discussion 3.1. Formation of supramolecular switch system The phenolphthalein dissolved in deionized water containing 0.8% ammonia was purple and was in the quinonoid form. The color would faded when -CD 200
2.2. Preparation of the samples Intensity
The samples of ␣-cyclodextrin (␣-CD), -cyclodextrin (-CD), and ␥-cyclodextrin (␥-CD) (purchased from Sigma and used without further purification), as well as phenolphthalein were dissolved in deionized water containing 0.8% ammonia. The supramolecular switch was prepared by mixing appropriate quantities of cyclodextrin and phenolphthalein solutions. The supramolecular switch was controned by adding different volumes of methanol.
150 100 50 0 0
0. 5
1
1. 5
2
XA /X CD Fig. 3. Effect of molar ratio of -CD–phenolphthalein on the intensity of the 1:1 adduct.
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was added. This fading reaction might be the reason why the state of phenolphthalein was simultaneously present in two forms in alkaline solution, one a quinonoid form (colored), the other a benzenoid form (colorless) [26]. When -CD was added, the equilibrium was displaced to the benzenoid form (colorless) because -CD included the benzenoid form but not the quinonoid form. This selective inclusion selection fitted the principle of molecular size and shape selectivity. The supramolecular switch was prepared by mixing stock solutions of -CD and phenolphthalein with concentrations of 1.73 × 10−6 mol l−1 in molar ratios of 0.29:1, 0.57:1, 1.15:1, and 1.72:1. The ESI–TOF mass spectrum of the mixed solution with molar ratio 1.15:1 is shown in Fig. 1B. The peaks observed at m/z 319.1, 744.2, 1152.6, and 1470.9 correspond to
[A+H]+ , [A+-CD+2NH4 ]2+ , [-CD+NH4 ]+ , and [A+-CD+NH4 ]+ , respectively (here “A” represents phenolphthalein). The expanded mass spectrum of the peak of the singly charged and doubly changed 1:1 -CD–phenolphthalein adduct is shown in Fig. 2. The peaks observed at m/z 1453.8 and 735.2 corresponded to [A + -CD + H]+ and [A + -CD + H + NH4 ]2+ , respectively. It is clear that only a 1:1 complex was formed between -CD and phenolphthalein, which was in accord with the literature [27]. The peak intensity of the 1:1 -CD–phenolphthalein adduct was strongest when the solution had -CD and phenolphthalein in equimolar ratio (the relationship between the peak intensity of the 1:1 -CD–phenolphthalein adduct and the molar ratios is shown in Fig. 3). Moreover the 1:1 -CD–phenolphthalein complex was not dissociated even when the nozzle
Fig. 4. The expanded mass spectrum of the peak clusters of singly charged (A) and doubly charged (B) 1:1 -CD–phenolphthalein adduct.
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potential was increased to 200 V (this is discussed below). 3.2. Effect of cavity size on supramolecular switch The effect of cavity size on the supramolecular switch was investigated by mixing phenolphthalein with equimolar ␣-CD, -CD, and ␥-CD, respectively. The adduct of ␣-CD and phenolphthalein was not found in the ESI–TOF mass spectrum (as in Fig. 1A). This might be because the cavity of ␣-CD was smaller than the size of benzenoid form of phenolphthalein. So phenolphthalein could not enter the cavity of ␣-CD, so that the color of the mixed solution did not fade [28]. The peaks observed at m/z 973.5 and 990.5 cor-
5
respond to [␣-CD + H]+ and [␣-CD + NH4 ]+ , respectively. The adduct of ␥-CD and phenolphthalein is shown in the ESI–TOF mass spectrum (as in Fig. 1C). The color of the mixed solution also did not fade too. This might be arise by because the cavity of ␥-CD was big enough to include the quinonoid form of phenolphthalein. If only the quinonoid form was included by ␥-CD, the color of the mixed solution would be increased. However this phenomenon was not observed. So we supposed that ␥-CD could include both the quinonoid form and the benzenoid form. The peaks observed at m/z 816.6, 1314.7, and 1615.5 corresponded to [␥-CD + A + H + NH4 ]2+ , [␥-CD + NH4 ]+ , and [␥-CD + A + H]+ , respectively. The expanded mass spectrum of the singly charged and doubly changed 1:1 ␥-CD–phenolphthalein
Fig. 5. The ESI–TOF mass spectra of the 1:1 -CD–phenolphthalein adduct at nozzle potential of 150 V (A) and 200 V (B).
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adduct is shown in Fig. 4. The peak observed at m/z 825.4 corresponded to [A + ␥-CD + 2NH4 ]2+ . The cavity of -CD just fitted the benzenoid form, so the color of the mixed solution faded. This hypothesis would be tested by the following MSn experiments carried out at the ESI–FTMS instrument. 3.3. Effect of nozzle potential on supramolecular switch To investigate effect of nozzle potential on the supramolecular switch, nozzle potentials of at 90, 150, and 200 V were studied. The ESI–TOF mass spectra of the supramolecular complexes of -CD and ␥-CD at a nozzle potential at 150 and 200 V are
presented in Figs. 5 and 6, respectively. Generally a non-covalent compound would dissociate when the nozzle potential increased. However, the supramolecular 1:1 CD–phenolphthalein survived from start to finish whenever the nozzle potential was increased from 90 to 200 V. New peaks at m/z 833 and 975, corresponding to the fragmentation of -CD by the loss of two and one glucose group, respectively, emerged in Fig. 5. In addition, when the nozzle potential increased, the intensity of the doubly charged complex peak (at m/z 744) decreased, and the intensity of the singly charged complex peak (at m/z 1453) increased. As to the supramolecular switch of ␥-CD and phenolphthalein similar results were observed in Fig. 6 except for the appearance of another new peak, at m/z
Fig. 6. The ESI–TOF mass spectra of the 1:1 -CD–phenolphthalein adduct at nozzle potential 150 V (A) and 200 V (B).
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1135.4, corresponding to the fragmentation of ␥-CD losing of one glucose group. These results indicated that the supramolecular species was formed not only by hydrogen bond and hydrophobic interaction but also by the stereospecific inclusion interaction. 3.4. Effect of methanol on the supramolecular switch The color would increase marked by when one volume of methanol was added to eight volumes of -CD and phenolphthalein with a molar concentration of 1.73 × 10−6 mol l−1 in the ratios of 1.15:1. Many research groups explained this phenomenon as substitution of solvent for the guest the cavity of -CD [24,25]. If this hypothesis was correct, two conclusions as follow. One was that the quantity of supramolecular of -CD–phenolphthalein would decrease. The other was that the complex of -CD and methanol was more stable than the complex of -CD and phenolphthalein. Unfortunately we could not find the peak of the adduct of -CD and methanol in the ESI–TOF mass spectrum
7
(as in Fig. 7). On the other hand we also did not find the peak intensity of the 1:1 -CD–phenolphthalein adduct weakened (data not shown). When methanol was added the polarity of the solution was changed. And the size and shape of -CD would be changed accordingly. The macrocycle of -CD would be more pliable and become able to include the quinonoid form of phenolphthalein. So the equilibrium would be displaced to the quinonoid form and the solution would turn colored. The color increased with one volume of methanol added, but then the color would disappear suddenly with extra 11 volumes of methanol added. When the same experiment was demonstrated in the absence of -CD the color would disappear too. So the color disappearance should occur because the pH of the mixed solutions changed from >10 to 8.5 (determined by pH indicator) and the dianion of phenolphthalein changed to the ␥-lactone form (colorless). If this hypothesis based on the equilibrium principle is correct, then both the form of phenolphthalein dianion in the cavity of -CD as
Fig. 7. The ESI–TOF mass spectrum of the 1:1 -CD–phenolphthalein adduct in the solution with methanol added.
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Fig. 8. The ESI–FTMSn spectra of the parent ion phenolphthalein in the solution (A) without methanol and with methanol (B) colored; (C) color decreased; (D) colorless.
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Fig. 9. ESI–FTMSn spectrum of the parent ion of 1:1 -CD–phenolphthalein adduct: (A) isolation sweep attenuation and activation attenuation were 25 and 31 dB, respectively; (B) only isolation of the parent ion with isolation sweep attenuation as 25 dB; (C) isolation sweep attenuation and activation attenuation were 25 and 30 dB, respectively.
well as in the free solution was both changed with methanol addition. As shown in Fig. 8 and Scheme 1, only the quinonoid form could produce fragmentation at m/z 197. Expectedly the peak intensity was changed as the volume of methanol changed that was inconsistent with our hypothesis. Without methanol added the supramolecular species was colorless, and the phenolphthalein dianion existed as the benzenoid form. So no peak at m/z 197 was observed (as in Fig. 8A). The intensity of peak 197 was strongest when methanol was added to give a color (as in Fig. 8B), and the intensity of peak 197 was decreased when more methanol was added so that the color decreased (as in Fig. 8C). Finally the peak at m/z 197 disappeared
when sufficient methanol was added to remove all the color. It is well known that the host–guest complex would release the guest at a much high energy threshold so that the ion of phenolphthalein could be selected as the second step parent ion. Consequently the form of phenolphthalein dianion in the cavity of -CD could be identified. Unexpectedly, even at the highest possible collision energies, the parent ion of -CD–phenolphthalein did not yield any appreciable peaks corresponding to the components (as in Fig. 9). It was presumed that this complex was stable enough to survive in the gas phase under the selected conditions.
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H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10 O
HO C
+
C
O
+
OH C
O
O
m/z 225
m/z 225
1
2
HO
OH
OH
O
OH
O
C +
O H+
OH
m/z 319
m/z 319
4 3 OH
O +
C
m/z 197 Scheme 1.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20175034), and by Natural Science Foundation of Shanghai (No. 00ZA14077). References [1] L. Gobbi, P. Seiler, F. Diederich, C. Angew, Chem. Int. Ed. 38 (5) (1999) 674–678. [2] O.A. Matthews, F.M. Raymo, J.F. Stoddart, A.J.P. White, D.J. Williams, New J. Chem. (1998) 1131–1134. [3] P.R. Ashton, R. Ballardini, V. Balzani, I. Baxter, A. Credi, M.C.T. Fyfe, J. Am. Chem. Soc. 120 (1998) 11932– 11942. [4] U. Pischel, W. Abraham, W. Schnabel, U. Müller, Chem. Commum. (1997) 1383–1384.
[5] S.I. Jun, W. Lee, S. Sakamoto, K. Yamaguchi, K. Kim, Tetrahedron Lett. 41 (2000) 471–475. [6] H. Sugimoto, T. Kimura, S. Inoue, J. Am. Chem. Soc. 121 (1999) 2325–2326. [7] P.R. Ashton, V. Balzani, J. Becher, A. Credi, M.C.T. Fyfe, G. Mattersteig, J. Am. Chem. Soc. 121 (1999) 3951–3957. [8] A. Beyeler, P. Belser, L.D. Cola, Angew. Chem. Int. Ed. Engl. 36 (24) (1997) 2779–2781. [9] B. Bodenant, F. Fages, M.H. Deleville, J. Am. Chem. Soc. 120 (1998) 7511–7519. [10] Z.P. Yao, T.S.M. Wan, K.P. Kwong, C.T. Che, Anal. Chem. 72 (21) (2000) 5383–5393. [11] Z.P. Yao, T.S.M. Wan, K.P. Kwong, C.T. Che, Anal. Chem. 72 (21) (2000) 5394–5401. [12] C.A. Schalley, R.K. Castellano, M.S. Brody, D.M. Rudkevich, G. Siuzdak Jr., J.R., J. Am. Chem. Soc. 121 (1999) 4568–4579. [13] T.J.D. Jorgensen, T. Staroske, P. Roepstorff, D.H. Williams, J.R. Heck, J. Chem. Soc., Perkin Trans. 2 (1999) 1859–1863. [14] G. Grigorean, J. Ramirez, S.H. Ahn, C.B. Lebrilla, Anal. Chem. 72 (18) (2000) 4275–4281. [15] G. Grigorean, C.B. Lebrilla, Anal. Chem. 73 (8) (2001) 1684– 1691. [16] R. Bakhtiar, H. Chen, S. Ogo, R.H. Fish, Chem. Commun. (1997) 2135–2136. [17] M.M. Siegel, K. Tabei, M.Z. Kagan, I.R. Vlahov, R.E. Hileman, R.J. Linhardt, J. Mass Spectrom. 32 (1997) 760– 772. [18] P. Cescutti, D. Garozzo, R. Rizzo, Carbohydr. Res. 290 (1996) 105–115. [19] P. Cescutti, D. Garozzo, R. Rizzo, Carbohydr. Res. 302 (1997) 1–6. [20] A. Selva, E. Redenti, P. Ventura, M. Zanol, B. Casetta, J. Mass Spectrom. 33 (1998) 729–734. [21] A. Selva, E. Redenti, P. Ventura, M. Zanol, B.J. Casetta, J. Mass Spectrom. 31 (1996) 1364–1370. [22] A. Mele, L. Malpezzi, J. Am. Soc. Mass Spectrom. 11 (2000) 228–236. [23] K.P. Madhusudanan, S.B. Katti, A.K. Dwivedi, J. Mass Spectrom. 33 (1998) 1017–1022. [24] T. Kuwabara, M. Takamura, A. Matsushita, H. Ikeda, A. Nakamura, A. Ueno, F. Toda, J. Org. Chem. 63 (1998) 8729– 8735. [25] K.H. Fromming, Cyclodextrins in Pharmacy, Kluwer Academic Publishers, Dordrecht, 1994 (Chapter 4). [26] K.K. Kunimoto, H. Sugiura, T. Kato, H. Senda, A. Kuwae, K. Hanai, Spectrochim. Acta A 57 (2001) 265–271. [27] A. Bruvöri, L. Barcza, J. Chem. Soc., Perkin Trans. II (1988) 1687–1690. [28] N. Yoshida, T. Shirai, M. Fujimoto, Carbohydr. Res. 192 (1989) 291–304.
Methanol controllable chromophoric supramolecular switch investigated by electrospray mass spectrometry Hao-Jie Lu, Yin-Long Guo∗ Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China Received 19 February 2003; accepted 4 August 2003
Abstract The supramolecular switch which is controlled by methanol, with color turned on or off, is prepared by a cyclodextrinincluded dianion of phenolphthalein. This phenomenon is studied by electrospray time of flight mass spectrometry (TOFMS) and electrospray Fourier transform mass spectrometry (FTMS). The influence of the host cavity size and the nozzle potential on the supramolecular switch system is investigated in detail. The dianion phenolphthalein in the supramolecular switch system is also studied by tandem mass spectrometry. The relevant mechanism is discussed, and the result is contrary to those of papers reported previously. © 2003 Elsevier B.V. All rights reserved. Keywords: Supramolecular switch; Mass spectrometry; Methanol
1. Introduction Molecular information processing, which exploits the switching of molecular and supramolecular systems between clearly identifiable states, has received much attention in recent years [1–9]. Aside from the continuing attention being paid to electrochromic and photochemically active compounds, along with chemical sensors, much focus is centered on molecular and supramolecular systems in which the relative positions of their component parts can be altered by an external stimulus [1–3]. Host–guest complexes are supramolecular associations of current scientific and technological interest, whose existence has been demonstrated either in solution or in the solid state by spectroscopic and other physical chemistry methods. ∗ Corresponding author. Tel.: +86-21-64163300-3106; fax: +86-21-64166128. E-mail address: [email protected] (Y.-L. Guo).
In recent years, there has been much interest in the potential of mass spectrometry (MS) for the study of host–guest chemistry in the gas phase [10–23]. These studies by MS have rapidly gained momentum since the introduction of electrospray ionization (ESI) as one of the softest ionization methods [10–21]. Therefore, a large number of reports have dealt with the MS in various types of host–guest systems. Almost all these papers relate to the evaluation of molecular and/or chiral recognition capability, the study of association/dissociation equilibria in solution, the characterization of the complexes and the study of the complex formation processes in the gas phase. Given this huge body of knowledge, it seems surprising that MS has scarcely been used to the field of molecular and supramolecular switches. Herein we developed a ESI–time of flight mass spectrometry (TOFMS) and ESI–Fourier transform mass spectrometry (FTMS) method to a cyclodextrin-based supramolecular switch that can be turned on or off
0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2003.08.017
2
H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10
chemically by a color change. It is common knowledge that phenolphthalein is purple in solution at pH >10. When -cyclodextrin is added, phenolphthalein would be included in the cavity of -cyclodextrin, with an accompanying color change from purple to colorless. The solution becomes purple again when a suitable volume of methanol is added. However the color disappears again when an additional volume of methanol is added. This type of supramolecular switch maybe useful as a sensor for detecting organic molecules by color change. This visible phenomenon, color on or off, and the relevant mechanism is investigated by ESI–TOFMS, ESI–FTMS and tandem MS in this paper. And we reach a conclusion which is contrary to those of previous papers [24,25].
2. Experimental 2.1. Mass spectrometer The Mariner electrospray time-of-flight mass spectrometer (Perkin-Elmer Corp.) was utilized in all single stage MS experiments. The used ionspray source was a micro-electrospray source (SCIEX). The ionspray voltage was 3800 V and the nozzle potential was 90 V (unless otherwise stated in the text). The data were acquired by a Mariner Instrument Control Panel for positive ion mode from 100 to 2000 amu, and dealt with by Data Explorer. All tandem MS experiments were carried out at Bruker Daltonics using an APEX IIITM electrospray
Fig. 1. The ESI–TOF mass spectrum of phenolphthalein with ␣-CD (A), -CD (B), and ␥-CD (C) as host.
H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10
3
Fig. 2. The expanded mass spectrum of the peak clusters of singly charged (A) and doubly charged (B) 1:1 -CD–phenolphthalein adduct.
FTMS instrument equipped with a 7.0 T shielded superconducting magnet. The capillary and end plate voltages were −4635 and −4393 V, respectively. The isolation sweep attenuation and activation attenuation were 25 and 31 dB, respectively. The instrument calibrations were both made with a PEG-600 solution. The samples were injected at a flow rate of 7 l min−1 using a syringe pump.
3. Results and discussion 3.1. Formation of supramolecular switch system The phenolphthalein dissolved in deionized water containing 0.8% ammonia was purple and was in the quinonoid form. The color would faded when -CD 200
2.2. Preparation of the samples Intensity
The samples of ␣-cyclodextrin (␣-CD), -cyclodextrin (-CD), and ␥-cyclodextrin (␥-CD) (purchased from Sigma and used without further purification), as well as phenolphthalein were dissolved in deionized water containing 0.8% ammonia. The supramolecular switch was prepared by mixing appropriate quantities of cyclodextrin and phenolphthalein solutions. The supramolecular switch was controned by adding different volumes of methanol.
150 100 50 0 0
0. 5
1
1. 5
2
XA /X CD Fig. 3. Effect of molar ratio of -CD–phenolphthalein on the intensity of the 1:1 adduct.
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H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10
was added. This fading reaction might be the reason why the state of phenolphthalein was simultaneously present in two forms in alkaline solution, one a quinonoid form (colored), the other a benzenoid form (colorless) [26]. When -CD was added, the equilibrium was displaced to the benzenoid form (colorless) because -CD included the benzenoid form but not the quinonoid form. This selective inclusion selection fitted the principle of molecular size and shape selectivity. The supramolecular switch was prepared by mixing stock solutions of -CD and phenolphthalein with concentrations of 1.73 × 10−6 mol l−1 in molar ratios of 0.29:1, 0.57:1, 1.15:1, and 1.72:1. The ESI–TOF mass spectrum of the mixed solution with molar ratio 1.15:1 is shown in Fig. 1B. The peaks observed at m/z 319.1, 744.2, 1152.6, and 1470.9 correspond to
[A+H]+ , [A+-CD+2NH4 ]2+ , [-CD+NH4 ]+ , and [A+-CD+NH4 ]+ , respectively (here “A” represents phenolphthalein). The expanded mass spectrum of the peak of the singly charged and doubly changed 1:1 -CD–phenolphthalein adduct is shown in Fig. 2. The peaks observed at m/z 1453.8 and 735.2 corresponded to [A + -CD + H]+ and [A + -CD + H + NH4 ]2+ , respectively. It is clear that only a 1:1 complex was formed between -CD and phenolphthalein, which was in accord with the literature [27]. The peak intensity of the 1:1 -CD–phenolphthalein adduct was strongest when the solution had -CD and phenolphthalein in equimolar ratio (the relationship between the peak intensity of the 1:1 -CD–phenolphthalein adduct and the molar ratios is shown in Fig. 3). Moreover the 1:1 -CD–phenolphthalein complex was not dissociated even when the nozzle
Fig. 4. The expanded mass spectrum of the peak clusters of singly charged (A) and doubly charged (B) 1:1 -CD–phenolphthalein adduct.
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potential was increased to 200 V (this is discussed below). 3.2. Effect of cavity size on supramolecular switch The effect of cavity size on the supramolecular switch was investigated by mixing phenolphthalein with equimolar ␣-CD, -CD, and ␥-CD, respectively. The adduct of ␣-CD and phenolphthalein was not found in the ESI–TOF mass spectrum (as in Fig. 1A). This might be because the cavity of ␣-CD was smaller than the size of benzenoid form of phenolphthalein. So phenolphthalein could not enter the cavity of ␣-CD, so that the color of the mixed solution did not fade [28]. The peaks observed at m/z 973.5 and 990.5 cor-
5
respond to [␣-CD + H]+ and [␣-CD + NH4 ]+ , respectively. The adduct of ␥-CD and phenolphthalein is shown in the ESI–TOF mass spectrum (as in Fig. 1C). The color of the mixed solution also did not fade too. This might be arise by because the cavity of ␥-CD was big enough to include the quinonoid form of phenolphthalein. If only the quinonoid form was included by ␥-CD, the color of the mixed solution would be increased. However this phenomenon was not observed. So we supposed that ␥-CD could include both the quinonoid form and the benzenoid form. The peaks observed at m/z 816.6, 1314.7, and 1615.5 corresponded to [␥-CD + A + H + NH4 ]2+ , [␥-CD + NH4 ]+ , and [␥-CD + A + H]+ , respectively. The expanded mass spectrum of the singly charged and doubly changed 1:1 ␥-CD–phenolphthalein
Fig. 5. The ESI–TOF mass spectra of the 1:1 -CD–phenolphthalein adduct at nozzle potential of 150 V (A) and 200 V (B).
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adduct is shown in Fig. 4. The peak observed at m/z 825.4 corresponded to [A + ␥-CD + 2NH4 ]2+ . The cavity of -CD just fitted the benzenoid form, so the color of the mixed solution faded. This hypothesis would be tested by the following MSn experiments carried out at the ESI–FTMS instrument. 3.3. Effect of nozzle potential on supramolecular switch To investigate effect of nozzle potential on the supramolecular switch, nozzle potentials of at 90, 150, and 200 V were studied. The ESI–TOF mass spectra of the supramolecular complexes of -CD and ␥-CD at a nozzle potential at 150 and 200 V are
presented in Figs. 5 and 6, respectively. Generally a non-covalent compound would dissociate when the nozzle potential increased. However, the supramolecular 1:1 CD–phenolphthalein survived from start to finish whenever the nozzle potential was increased from 90 to 200 V. New peaks at m/z 833 and 975, corresponding to the fragmentation of -CD by the loss of two and one glucose group, respectively, emerged in Fig. 5. In addition, when the nozzle potential increased, the intensity of the doubly charged complex peak (at m/z 744) decreased, and the intensity of the singly charged complex peak (at m/z 1453) increased. As to the supramolecular switch of ␥-CD and phenolphthalein similar results were observed in Fig. 6 except for the appearance of another new peak, at m/z
Fig. 6. The ESI–TOF mass spectra of the 1:1 -CD–phenolphthalein adduct at nozzle potential 150 V (A) and 200 V (B).
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1135.4, corresponding to the fragmentation of ␥-CD losing of one glucose group. These results indicated that the supramolecular species was formed not only by hydrogen bond and hydrophobic interaction but also by the stereospecific inclusion interaction. 3.4. Effect of methanol on the supramolecular switch The color would increase marked by when one volume of methanol was added to eight volumes of -CD and phenolphthalein with a molar concentration of 1.73 × 10−6 mol l−1 in the ratios of 1.15:1. Many research groups explained this phenomenon as substitution of solvent for the guest the cavity of -CD [24,25]. If this hypothesis was correct, two conclusions as follow. One was that the quantity of supramolecular of -CD–phenolphthalein would decrease. The other was that the complex of -CD and methanol was more stable than the complex of -CD and phenolphthalein. Unfortunately we could not find the peak of the adduct of -CD and methanol in the ESI–TOF mass spectrum
7
(as in Fig. 7). On the other hand we also did not find the peak intensity of the 1:1 -CD–phenolphthalein adduct weakened (data not shown). When methanol was added the polarity of the solution was changed. And the size and shape of -CD would be changed accordingly. The macrocycle of -CD would be more pliable and become able to include the quinonoid form of phenolphthalein. So the equilibrium would be displaced to the quinonoid form and the solution would turn colored. The color increased with one volume of methanol added, but then the color would disappear suddenly with extra 11 volumes of methanol added. When the same experiment was demonstrated in the absence of -CD the color would disappear too. So the color disappearance should occur because the pH of the mixed solutions changed from >10 to 8.5 (determined by pH indicator) and the dianion of phenolphthalein changed to the ␥-lactone form (colorless). If this hypothesis based on the equilibrium principle is correct, then both the form of phenolphthalein dianion in the cavity of -CD as
Fig. 7. The ESI–TOF mass spectrum of the 1:1 -CD–phenolphthalein adduct in the solution with methanol added.
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Fig. 8. The ESI–FTMSn spectra of the parent ion phenolphthalein in the solution (A) without methanol and with methanol (B) colored; (C) color decreased; (D) colorless.
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Fig. 9. ESI–FTMSn spectrum of the parent ion of 1:1 -CD–phenolphthalein adduct: (A) isolation sweep attenuation and activation attenuation were 25 and 31 dB, respectively; (B) only isolation of the parent ion with isolation sweep attenuation as 25 dB; (C) isolation sweep attenuation and activation attenuation were 25 and 30 dB, respectively.
well as in the free solution was both changed with methanol addition. As shown in Fig. 8 and Scheme 1, only the quinonoid form could produce fragmentation at m/z 197. Expectedly the peak intensity was changed as the volume of methanol changed that was inconsistent with our hypothesis. Without methanol added the supramolecular species was colorless, and the phenolphthalein dianion existed as the benzenoid form. So no peak at m/z 197 was observed (as in Fig. 8A). The intensity of peak 197 was strongest when methanol was added to give a color (as in Fig. 8B), and the intensity of peak 197 was decreased when more methanol was added so that the color decreased (as in Fig. 8C). Finally the peak at m/z 197 disappeared
when sufficient methanol was added to remove all the color. It is well known that the host–guest complex would release the guest at a much high energy threshold so that the ion of phenolphthalein could be selected as the second step parent ion. Consequently the form of phenolphthalein dianion in the cavity of -CD could be identified. Unexpectedly, even at the highest possible collision energies, the parent ion of -CD–phenolphthalein did not yield any appreciable peaks corresponding to the components (as in Fig. 9). It was presumed that this complex was stable enough to survive in the gas phase under the selected conditions.
10
H.-J. Lu, Y.-L. Guo / Analytica Chimica Acta 495 (2003) 1–10 O
HO C
+
C
O
+
OH C
O
O
m/z 225
m/z 225
1
2
HO
OH
OH
O
OH
O
C +
O H+
OH
m/z 319
m/z 319
4 3 OH
O +
C
m/z 197 Scheme 1.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20175034), and by Natural Science Foundation of Shanghai (No. 00ZA14077). References [1] L. Gobbi, P. Seiler, F. Diederich, C. Angew, Chem. Int. Ed. 38 (5) (1999) 674–678. [2] O.A. Matthews, F.M. Raymo, J.F. Stoddart, A.J.P. White, D.J. Williams, New J. Chem. (1998) 1131–1134. [3] P.R. Ashton, R. Ballardini, V. Balzani, I. Baxter, A. Credi, M.C.T. Fyfe, J. Am. Chem. Soc. 120 (1998) 11932– 11942. [4] U. Pischel, W. Abraham, W. Schnabel, U. Müller, Chem. Commum. (1997) 1383–1384.
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