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Interaction of the univalent silver cation with [Gly6]-antamanide: Experimental and theoretical study
Journal of Molecular Structure 1155 (2018) 807e812
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Interaction of the univalent silver cation with [Gly6]-antamanide: Experimental and theoretical study € hm b, Jaroslav Kví ura b, *, Paolo Ruzza c Emanuel Makrlík a, Stanislav Bo cala b, Petr Van 129, 165 21 Prague 6 - Suchdol, Czech Republic Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamýcka 5, 166 28 Prague 6, Czech Republic University of Chemistry and Technology, Prague, Technicka c Institute of Biomolecular Chemistry of CNR, Padua Unit, Via Marzolo 1, 35131 Padua, Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2016 Received in revised form 13 October 2017 Accepted 14 November 2017 Available online 15 November 2017
On the basis of extraction experiments and g-activity measurements, the extraction constant corresponding to the equilibrium Agþ(aq) þ 1.Naþ(nb) % 1.Agþ (nb) þ Naþ(aq) occurring in the twoephase water e nitrobenzene system (1 ¼ [Gly6]-antamanide; aq ¼ aqueous phase, nb ¼ nitrobenzene phase) was determined as log Kex (Agþ,1·Naþ) ¼ 1.5 ± 0.1. Further, the stability constant of the 1·Agþ complex in nitrobenzene saturated with water was calculated for a temperature of 25 C: log bnb (1·Agþ) ¼ 4.5 ± 0.2. Finally, by using quantum chemical DFT calculations, the most probable structure of the cationic complex species 1·Agþ was derived. In the resulting complex, the “central” cation Agþ is coordinated by four noncovalent interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1·Agþ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1·Agþ complex was found to be 465.5 kJ/mol, confirming also the formation of this cationic species. © 2017 Elsevier B.V. All rights reserved.
Keywords: Univalent silver cation [Gly6]-antamanide Complexation Extraction and stability constants DFT calculations Structures
1. Introduction The cyclic decapeptide antamanide, cycl[-Val(1)-Pro(2) -Pro(3)Ala(4)-Phe(5)-Phe(6)-Pro(7)-Pro(8)-Phe(9)-Phe(10)-], consisting entirely of L-amino acids (see Scheme 1), forms 1:1 complexes with a variety of metal cations [1,2]. Antamanide was isolated from the poisonous mushroom Amanita phalloides [3] and it has the unique property of counteracting the toxin phalloidin, produced by the mentioned mushroom. Furthermore, it should be noted that antamanide prevents the inhibition of depolymerization of F-actin and G-actin in the liver cell membranes of mammals [4]. It was also found that antamanide inhibits tumor cell growth in vitro [2], displays an antitumor action in an animal model [5], and attenuates IL2-induced multisystem organ edema [6]. In general, nearly all cyclic peptides tend to be extremely resistant to the process of digestion, enabling them to survive in the human digestive tract [7]. This trait makes cyclic peptides attractive to designers of protein-based drugs that may be used as scaffolds which, in theory, could be engineered to incorporate any arbitrary protein domain of medicinal value to allow those components to be
* Corresponding author. ura). E-mail address: [email protected] (P. Van https://doi.org/10.1016/j.molstruc.2017.11.059 0022-2860/© 2017 Elsevier B.V. All rights reserved.
delivered orally. This is especially important for delivery of other proteins that would be destroyed without such implementation. Cyclic peptides are also somewhat more “rigid” compared to the corresponding linear peptides, and this attribute promotes binding by removing the “entropic penalty” [7]. The dicarbollylcobaltate anion (DCC) [8] and some of its halogen derivatives have been applied very often for the extraction of various metal cations (especially Csþ, Sr2þ, Ba2þ, Eu3þ, and Am3þ) from aqueous solutions into a polar organic phase, both under laboratory conditions for theoretical or experimental purposes [9e16], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste [17,18]. Ruzza et al. [19] synthesized and tested a series of linear and cyclic antamanide analogues modified in position 6 and 9 with glycine or tyrosine residue. The corresponding experiments demonstrated a cytotoxic and/or cytostatic action for antamanide and some of its synthetic analogues. Due to the different ability of antamanide and its analogues to exhibit antitoxic and/or anticancerogenic activity, these pharmacological effects may be ascribed to different mechanisms [19]. In acetonitrile medium, interactions of the cations Naþ, Kþ, Ca2þ, and Tb3þ with glycine6 antamanide analogue have been also
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Scheme 1. Structural formula of antamanide.
studied [2]. However, up to now, interaction between the Agþ ion and antamanide (or its analogue, respectively) has not been reported. Therefore, in the current work, the solvent extraction of Agþ into nitrobenzene by means of a mixture of sodium dicarbollylcobaltate (NaDCC) [7] and the mentioned glycine6 antamanide analogue, cycl[-Val(1)-Pro(2)-Pro(3)-Ala(4)-Phe(5)-Gly(6)-Pro(7)Pro(8)-Phe(9)-Phe(10)-], denoted by [Gly6]-antamanide (abbrev. 1; see Scheme 2), was investigated. In this context it is necessary to note that Agþ is a typical representative of the “soft” univalent cations; thus, this cation was chosen for the present study. Besides, we must add that the applied experimental method is very simple and quite unambiguous. Moreover, the stability constant of the proven 1·Agþ complex species in the organic phase of the waterenitrobenzene extraction system was evaluated. Finally, applying quantum chemical DFT calculations, the most probable structure of this cationic complex species was predicted.
Scheme 2. Structural formula of [Gly6]-antamanide (abbrev. 1).
the initial concentration of 1 in nitrobenzene, C1in;nb , was equal to in;nb the initial concentration of NaDCC in this medium, CNaDCC . The testtubes filled with the solutions were shaken for 3 h at 25 ± 1 C, using a laboratory shaker. Then the phases were separated by centrifugation. Finally, 1 mL samples were taken from each phase and their g-activities were measured by means of a well-type NaI(Tl) scintillation detector connected to a g-analyzer Triathler (Hidex, Turku, Finland). The equilibrium distribution ratios of sodium, DNa, were determined as the ratios of the corresponding measured radioactivities of 22Naþ in the nitrobenzene and aqueous samples (the uncertainties of these distribution ratios were always lower than 3%).
2. Experimental 3. Computational details Synthesis of [Gly6]-antamanide (1; see Scheme 2) was described elsewhere [19]. Cesium dicarbollylcobaltate (CsDCC) [8] was synthesized by means of the method published by Hawthorne et al. [20]. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A nitrobenzene solution of hydrogen dicarbollylcobaltate (HDCC) [8] was prepared from CsDCC by the procedure described in Ref. [21]. The equilibration of the nitrobenzene solution of HDCC with stoichiometric NaOH, which was dissolved in an aqueous solution of NaCl (0.20 M), yielded the corresponding NaDCC solution in nitrobenzene. The carrier-free radionuclide 22Naþ was obtained from DuPont, Belgium; its radionuclidic purity was 99.9%. The extraction experiments were carried out in 10 mL polypropylene test-tubes with polypropylene stoppers: 2 mL of an aqueous solution of AgNO3 of a concentration in the range from 1 103 to 3 103 M and 10 kBq of 22Naþ were added to 2 mL of a nitrobenzene solution of 1 and NaDCC, whose initial concentrations varied also from 1 103 to 3 103 M. In all these experiments,
The theoretical calculations were carried out at the density functional level of theory (DFT, B3LYP functional) [22,23], employing the Gaussian 09 suite of programs [24]. The LanL2DZ basis set was used, and the optimizations were unconstrained. In order to increase the numerical accuracy and to reduce oscillations during the molecular geometry optimization, two-electron integrals and their derivatives were calculated by using the pruned (99,590) integration grid, having 99 radial shells and 590 angular points per shell. This was ensured by means of the Gaussian 09 keyword “integral(ultrafinegrid)”. The most probable structure of the cationic complex 1·Agþ was predicted on the basis of the thorough conformational analysis (i. e., eight very different initial mutual positions of the ligand 1 and the Agþ cation were considered during the geometry optimization) and the respective vibrational frequency analysis. Localized molecular orbitals were computed using Firefly QC package [25], which is partially based on the GAMESS (US) [26]
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source code, using the Pipek-Mezey localization method [27]. Visualization of the localized molecular orbitals was performed with wxMacMolplt program [28]. 4. Results and discussion 4.1. Extraction experiments Regarding the results of previous papers [8,29,30], the twoephase watere AgNO3enitrobenzeneesodium dicarbollylcobaltate (NaDCC) extraction system can be described by the following equilibrium Agþ(aq) þ Naþ(nb) % Agþ(nb) þ Naþ(aq); Kex(Agþ,Naþ)
(1)
with the corresponding exchange extraction constant Kex (Agþ, Naþ); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (Agþ, Naþ), one can write [8,29,30].
i i log Kex Agþ ; Naþ ¼ log KAg þ log KNaþ
(2)
þ i i where KAg þ and KNaþ are the individual extraction constants for Ag þ
and Na , respectively, in the waterenitrobenzene system [29,30]. It i i should be noted that the mentioned constants KAg þ and KNaþ were
determined on the basis of a non-thermodynamic assumption of equal extractability of the tetraphenylarsonium cation, Ph4Asþ, and the tetraphenylborate anion, BPh-4, of the “reference” electrolyte Ph4AsBPh4 [31] from the aqueous into nitrobenzene phase, i i ¼ 6.3 [29]. Knowing expressed by identity log KPh þ ¼ log KBPh 4 As 4 i i the values log KAg þ ¼ 4.5 [30] and log KNaþ ¼ e 6.0 [29], the ex-
change extraction constant Kex (Agþ, Naþ) was simply calculated from Eq. (2) as log Kex (Agþ, Naþ) ¼ 1.5. Similarly as in our previous articles [32e34], the twoephase watereAgNO3enitrobenzenee1 ([Gly6]-antamanide) eNaDCC extraction system (see Section ‘Experimental’), chosen for determination of the stability constant of the 1·Agþ complex in nitrobenzene saturated with water, can be characterized by the main chemical equilibrium Agþ(aq) þ 1$Naþ(nb) %1$Agþ(nb) þ Naþ(aq); Kex(Agþ,1$Naþ) (3) with the respective Kex(Agþ,1$Naþ):
equilibrium
extraction
½1$Agþ nb ½Naþ aq Kex Agþ ; 1$Naþ Þ ¼ ½Agþ aq ½1$Naþ nb
constant
(4)
It is necessary to emphasize that 1 is a considerably lipophilic ligand, practically present in the nitrobenzene phase only, where this ligand forms e with Agþ and Naþ e the relatively stable complexes 1$Agþ and 1$Naþ, as given below. Employing the conditions of electroneutrality in the organic and aqueous phases of the system under study, the mass balances of the Agþ and Naþ cations at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratio of sodium, DNa ¼ [1$Naþ]nb/[Naþ]aq, combined with Eq. (4), we gain the final expression for Kex(Agþ,1$Naþ) in the form in;nb CNaDCC 1 Kex Agþ ; 1$Naþ ¼ DNa ð1 þ DNa Þ C in;aq C in;nb NaDCC AgNO 3
(5)
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in;aq
where CAgNO3 is the initial concentration of AgNO3 in the aqueous in;nb phase and CNaDCC denotes the initial concentration of NaDCC in the organic phase of the system under consideration. In this study, from the extraction experiments and g-activity measurements (see Section ‘Experimental’) by means of Eq. (5), the following value of the constant Kex (Agþ,1$Naþ) was determined as log Kex (Agþ, 1$Naþ) ¼ 1.5 ± 0.1 (see Table 1). At this point it should be stated that this constant experimentally proves the justifying of the extraction mechanism and the presentation of the corresponding species, expressed by the two-phase chemical equilibrium (3). Furthermore, with respect to previous results [32e34], for the extraction constants Kex (Agþ, Naþ) and log Kex (Agþ, 1$Naþ) defined above, as well as for the stability constants of the complexes 1$Agþ and 1$Naþ in nitrobenzene saturated with water, denoted by bnb(1$Agþ) and bnb (1$Naþ), corresponding to the following equilibria 1(nb) þ Agþ(nb) % 1·Agþ (nb) and 1(nb) þ Naþ(nb) % 1·Naþ (nb), respectively, one gets
log bnb (1$Agþ) ¼ log bnb (1$Naþ) þ log Kex(Agþ,1$Naþ) e log Kex(Agþ,Naþ)
(6)
Using the constants log Kex (Agþ, Naþ) and log Kex (Agþ,1$Naþ) given above, the value log bnb (1$Naþ) ¼ 4.5 ± 0.1 [35], determined from the distribution of sodium picrate in the water e nitrobenzene system containing also the ligand 1, and applying Eq. (6), we obtain the stability constant of the 1$Agþ complex in water-saturated nitrobenzene at 25 C as log bnb (1$Agþ) ¼ 4.5 ± 0.2. This means that in the mentioned nitrobenzene medium, the stability constants of the complexes 1$Agþ and 1$Naþ are practically the same. In this context we can add that the stability constant of the complex 2·Agþ, where 2 denotes nonactin (see Scheme 3), in nitrobenzene saturated with water is log bnb (2$Agþ) ¼ 6.6 ± 0.2 [36]. Thus, in this medium, the stability of the 1·Agþ complex under study is substantially lower than that of the cationic complex 2·Agþ (2 ¼ nonactin). 4.2. Quantum chemical DFT calculations In the model calculations, we optimized the molecular geometries of the parent [Gly6]-antamanide ligand 1 and its complex with Agþ, similarly as in our previous papers [37e40]. The optimized structure of the free ligand 1 with the carbonyl oxygen binding sites O(1), O(2), …,O(10) is illustrated in Fig. 1. It is necessary to state that this free ligand 1 is stabilized by two strong intramolecular hydrogen bonds O(3) … H(6)N (2.12 Å) and O(8) … H(1)N (2.23 Å). Moreover, we must emphasize that the only one structure was obtained by the full DFT-optimization of the 1·Agþ complex, which is depicted in Fig. 2. It should be noted that the respective vibrational calculations found no imaginary frequencies. In the resulting 1·Agþ cationic complex species, the “central” cation Agþ is coordinated by four noncovalent bonds to the respective four carbonyl
Table 1 Experimental data concerning determination of log Kex (Agþ, 1·Naþ) on the basis of Eq. (5). in;aq
CAgNO3 (M) 1.0 1.5 2.0 2.5 3.0
103 103 103 103 103
in;nb CNaDCC (M)
1.0 1.5 2.0 2.5 3.0
103 103 103 103 103
DNa
log Kex (Agþ, 1·Naþ)
0.16 0.17 0.16 0.18 0.19
1.6 1.5 1.6 1.5 1.4
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Scheme 3. Structural formula of nonactin (abbrev. 2).
Fig. 2. DFT optimized structure of the 1·Agþ complex (B3LYP/LanL2DZ); hydrogen atoms are omitted for clarity except two hydrogens taking place in two internal hydrogen bonds O(4) … H(6)N (1.99 Å) and O(8) … H(1)N (2.06 Å); the lengths of the bonding interactions Agþ … O(1), Agþ … O(3), Agþ … O(6), and Agþ … O(8) are 2.29, 2.44, 2.31, and 2.53 Å, respectively.
Fig. 1. DFT optimized structure of free ligand 1 (B3LYP/LanL2DZ); hydrogen atoms are omitted for clarity except two hydrogens taking place in two internal hydrogen bonds O(3) … H(6)N (2.12 Å) and O(8) … H(1)N (2.23 Å).
oxygen atoms O(1), O(3), O(6), and O(8) (2.29, 2.44, 2.31, and 2.53 Å) of the parent ligand 1 (Fig. 2). It is apparent that electrostatic interactions are the dominant forces, characterizing these bonds. Besides, as follows from Fig. 2, the whole 1$Agþ complex structure is also stabilized by two intramolecular H-bonds O(4) … H(6)N (1.99 Å) and O(8) … H(1)N (2.06 Å). We must add that analysis of localized orbitals of the 1·Agþ complex, obtained by the Pipek-Mezey localization method [27], revealed that the “central” Agþ cation is coordinated to four sp hybridized lone pairs of the corresponding oxygen atoms O(1), O(3), O(6), and O(8) (Fig. 3). No other conformation of the 1·Agþ complex was found using the above-mentioned theoretical procedure. Cartesian coordinates (in Å) for the free ligand 1 and the 1·Agþ complex are presented in Supplementary material. Finally, the interaction energy, E(int), of the 1·Agþ complex, involving the 7-point correction for the basis set superposition error (BSSE) [41,42], was calculated as e 465.5 kJ/mol, which confirms the formation of this cationic complex as well. In summary, we must state that by employing an extraction method in the water e nitrobenzene system, the stability constant of the 1.Agþ complex in nitrobenzene saturated with water was determined, while using quantum chemical DFT calculations, the most probable structure of this cationic complex was derived.
Fig. 3. Coordination of the “central” cation Agþ to four sp hybridized lone pairs of carbonyl oxygens O(1), O(3), O(6), and O(8) of the parent ligand 1 in the 1·Agþ complex.
5. Conclusions In this work, we have demonstrated that the combination of theoretical DFT calculations with an experimental extraction method in the twoephase waterenitrobenzene system can provide relevant data on the noncovalent interactions of the univalent silver cation (Agþ) with the [Gly6]-antamanide ligand (1). By using this extraction method, the stability constant of the cationic complex 1·Agþ in nitrobenzene saturated with water was determined as log bnb(1·Agþ) ¼ 4.5 ± 0.2 (t ¼ 25 C). On the other hand, applying quantum chemical DFT calculations, the most probable structure of this 1·Agþ cationic species was predicted. In the resulting complex,
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Scheme 4. Structural formula of valinomycin (abbrev. 3).
the “central” cation Agþ is coordinated by four interactions to the corresponding four carbonyl oxygens of the parent ligand 1. It is obvious that the present work may be an important contribution especially to both theoretical and experimental study of [Gly6]antamanide, as well as to structural chemistry in general. Finally, as follows from our previous results [36,43], the stability constants of the cationic complex species 2·Agþ (2 ¼ nonactin) and 3·Agþ, where 3 denotes valinomycin (see Scheme 4), in water-saturated nitrobenzene are log bnb(2·Agþ) ¼ 6.6 ± 0.2 [36] and log bnb(3·Agþ) ¼ 4.6 ± 0.1 [43]. It means that the complexation ability of the three considered macrocyclic ligands towards the Agþ cation in this nitrobenzene medium increases in the series of [Gly6]antamanide < valinomycin < nonactin. Acknowledgements This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No.: 42900/1312/3114 entitled “Environmental Aspects of Sustainable Development of Society,” as well as by the Czech Ministry of Education, Youth, and Sports (Project MSMT No.: 20/ 2015). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.molstruc.2017.11.059. References [1] T. Wieland, H. Faulstich, W. Burgemeister, Antamanide and analogs. Studies on selectivity and stability of complexes, Biochem. Biophys. Res. Commun. 47 (1972) 984e992. [2] P. Ruzza, A. Calderan, B. Biondi, M. Carrara, T. Tancredi, G. Borin, Ion-binding and pharmacological properties of Tyr6 and Tyr 9 antamanide analogs, J. Pept. Res. 53 (1999) 442e452. [3] T. Wieland, G. Lüben, H. Ottenheym, J. Faesel, J.X. de Vries, A. Prox, J. Schmid, The discovery, isolation, elucidation of structure, and synthesis of antamanide, Angew. Chem. Int. Ed. Engl. 7 (1968) 204e208. [4] T. Wieland, H. Faulstich, Amatoxins, phallotoxins, phallolysin, and antamanide: the biologically active components of poisonous amanita mushrooms, Crit. Rev. Biochem. 5 (1978) 185e260.
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Mol. Graph. Model. 16 (1998) 133e138, 164. [29] J. Rais, Individual extraction constants of univalent ions in the system waternitrobenzene, Collect. Czech. Chem. Commun. 36 (1971) 3253e3262. [30] E. Makrlík, F. Bo zek, Individual extraction constants of some univalent cations in the water-nitrobenzene system, Polish J. Chem. 72 (1998) 949e952. [31] O. Popovych, A.J. Dill, Single scale for ion activities and electrode potentials in ethanol-water solvents based on the triisoamylbutylammonium tetraphenylborate assumption, Anal. Chem. 41 (1969) 456e462. [32] E. Makrlík, J. H alov a, M. Kyrs, Contribution to the thermodynamics of complexes of alkali metal cations with dibenzo-18-crown-6 in the waternitrobenzene extraction system, Collect. Czech. Chem. Commun. 49 (1984) 39e44. kov ura, J. Radioanal. Nucl. Chem. 221 (1997) [33] M. Dan a, E. Makrlík, P. Van 251e253. ura, Experimental evidence for a calix[4]arene-proton [34] E. Makrlík, P. Van complex, Monatsh. Chem. 137 (2006) 1185e1188. ura, E. Makrlík, Unpublished Results, 2016. [35] P. Van ura, Experimental and theoretical study on interaction of the [36] E. Makrlík, P. Van silver cation with nonactin, Mol. Phys. 113 (2015) 3712e3716. [37] J. Krí z, P. Toman, E. Makrlík, J. Budka, R. Shukla, R. Rathore, Cooperative
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Interaction of the univalent silver cation with [Gly6]-antamanide: Experimental and theoretical study € hm b, Jaroslav Kví ura b, *, Paolo Ruzza c Emanuel Makrlík a, Stanislav Bo cala b, Petr Van 129, 165 21 Prague 6 - Suchdol, Czech Republic Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamýcka 5, 166 28 Prague 6, Czech Republic University of Chemistry and Technology, Prague, Technicka c Institute of Biomolecular Chemistry of CNR, Padua Unit, Via Marzolo 1, 35131 Padua, Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2016 Received in revised form 13 October 2017 Accepted 14 November 2017 Available online 15 November 2017
On the basis of extraction experiments and g-activity measurements, the extraction constant corresponding to the equilibrium Agþ(aq) þ 1.Naþ(nb) % 1.Agþ (nb) þ Naþ(aq) occurring in the twoephase water e nitrobenzene system (1 ¼ [Gly6]-antamanide; aq ¼ aqueous phase, nb ¼ nitrobenzene phase) was determined as log Kex (Agþ,1·Naþ) ¼ 1.5 ± 0.1. Further, the stability constant of the 1·Agþ complex in nitrobenzene saturated with water was calculated for a temperature of 25 C: log bnb (1·Agþ) ¼ 4.5 ± 0.2. Finally, by using quantum chemical DFT calculations, the most probable structure of the cationic complex species 1·Agþ was derived. In the resulting complex, the “central” cation Agþ is coordinated by four noncovalent interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1·Agþ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1·Agþ complex was found to be 465.5 kJ/mol, confirming also the formation of this cationic species. © 2017 Elsevier B.V. All rights reserved.
Keywords: Univalent silver cation [Gly6]-antamanide Complexation Extraction and stability constants DFT calculations Structures
1. Introduction The cyclic decapeptide antamanide, cycl[-Val(1)-Pro(2) -Pro(3)Ala(4)-Phe(5)-Phe(6)-Pro(7)-Pro(8)-Phe(9)-Phe(10)-], consisting entirely of L-amino acids (see Scheme 1), forms 1:1 complexes with a variety of metal cations [1,2]. Antamanide was isolated from the poisonous mushroom Amanita phalloides [3] and it has the unique property of counteracting the toxin phalloidin, produced by the mentioned mushroom. Furthermore, it should be noted that antamanide prevents the inhibition of depolymerization of F-actin and G-actin in the liver cell membranes of mammals [4]. It was also found that antamanide inhibits tumor cell growth in vitro [2], displays an antitumor action in an animal model [5], and attenuates IL2-induced multisystem organ edema [6]. In general, nearly all cyclic peptides tend to be extremely resistant to the process of digestion, enabling them to survive in the human digestive tract [7]. This trait makes cyclic peptides attractive to designers of protein-based drugs that may be used as scaffolds which, in theory, could be engineered to incorporate any arbitrary protein domain of medicinal value to allow those components to be
* Corresponding author. ura). E-mail address: [email protected] (P. Van https://doi.org/10.1016/j.molstruc.2017.11.059 0022-2860/© 2017 Elsevier B.V. All rights reserved.
delivered orally. This is especially important for delivery of other proteins that would be destroyed without such implementation. Cyclic peptides are also somewhat more “rigid” compared to the corresponding linear peptides, and this attribute promotes binding by removing the “entropic penalty” [7]. The dicarbollylcobaltate anion (DCC) [8] and some of its halogen derivatives have been applied very often for the extraction of various metal cations (especially Csþ, Sr2þ, Ba2þ, Eu3þ, and Am3þ) from aqueous solutions into a polar organic phase, both under laboratory conditions for theoretical or experimental purposes [9e16], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste [17,18]. Ruzza et al. [19] synthesized and tested a series of linear and cyclic antamanide analogues modified in position 6 and 9 with glycine or tyrosine residue. The corresponding experiments demonstrated a cytotoxic and/or cytostatic action for antamanide and some of its synthetic analogues. Due to the different ability of antamanide and its analogues to exhibit antitoxic and/or anticancerogenic activity, these pharmacological effects may be ascribed to different mechanisms [19]. In acetonitrile medium, interactions of the cations Naþ, Kþ, Ca2þ, and Tb3þ with glycine6 antamanide analogue have been also
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Scheme 1. Structural formula of antamanide.
studied [2]. However, up to now, interaction between the Agþ ion and antamanide (or its analogue, respectively) has not been reported. Therefore, in the current work, the solvent extraction of Agþ into nitrobenzene by means of a mixture of sodium dicarbollylcobaltate (NaDCC) [7] and the mentioned glycine6 antamanide analogue, cycl[-Val(1)-Pro(2)-Pro(3)-Ala(4)-Phe(5)-Gly(6)-Pro(7)Pro(8)-Phe(9)-Phe(10)-], denoted by [Gly6]-antamanide (abbrev. 1; see Scheme 2), was investigated. In this context it is necessary to note that Agþ is a typical representative of the “soft” univalent cations; thus, this cation was chosen for the present study. Besides, we must add that the applied experimental method is very simple and quite unambiguous. Moreover, the stability constant of the proven 1·Agþ complex species in the organic phase of the waterenitrobenzene extraction system was evaluated. Finally, applying quantum chemical DFT calculations, the most probable structure of this cationic complex species was predicted.
Scheme 2. Structural formula of [Gly6]-antamanide (abbrev. 1).
the initial concentration of 1 in nitrobenzene, C1in;nb , was equal to in;nb the initial concentration of NaDCC in this medium, CNaDCC . The testtubes filled with the solutions were shaken for 3 h at 25 ± 1 C, using a laboratory shaker. Then the phases were separated by centrifugation. Finally, 1 mL samples were taken from each phase and their g-activities were measured by means of a well-type NaI(Tl) scintillation detector connected to a g-analyzer Triathler (Hidex, Turku, Finland). The equilibrium distribution ratios of sodium, DNa, were determined as the ratios of the corresponding measured radioactivities of 22Naþ in the nitrobenzene and aqueous samples (the uncertainties of these distribution ratios were always lower than 3%).
2. Experimental 3. Computational details Synthesis of [Gly6]-antamanide (1; see Scheme 2) was described elsewhere [19]. Cesium dicarbollylcobaltate (CsDCC) [8] was synthesized by means of the method published by Hawthorne et al. [20]. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A nitrobenzene solution of hydrogen dicarbollylcobaltate (HDCC) [8] was prepared from CsDCC by the procedure described in Ref. [21]. The equilibration of the nitrobenzene solution of HDCC with stoichiometric NaOH, which was dissolved in an aqueous solution of NaCl (0.20 M), yielded the corresponding NaDCC solution in nitrobenzene. The carrier-free radionuclide 22Naþ was obtained from DuPont, Belgium; its radionuclidic purity was 99.9%. The extraction experiments were carried out in 10 mL polypropylene test-tubes with polypropylene stoppers: 2 mL of an aqueous solution of AgNO3 of a concentration in the range from 1 103 to 3 103 M and 10 kBq of 22Naþ were added to 2 mL of a nitrobenzene solution of 1 and NaDCC, whose initial concentrations varied also from 1 103 to 3 103 M. In all these experiments,
The theoretical calculations were carried out at the density functional level of theory (DFT, B3LYP functional) [22,23], employing the Gaussian 09 suite of programs [24]. The LanL2DZ basis set was used, and the optimizations were unconstrained. In order to increase the numerical accuracy and to reduce oscillations during the molecular geometry optimization, two-electron integrals and their derivatives were calculated by using the pruned (99,590) integration grid, having 99 radial shells and 590 angular points per shell. This was ensured by means of the Gaussian 09 keyword “integral(ultrafinegrid)”. The most probable structure of the cationic complex 1·Agþ was predicted on the basis of the thorough conformational analysis (i. e., eight very different initial mutual positions of the ligand 1 and the Agþ cation were considered during the geometry optimization) and the respective vibrational frequency analysis. Localized molecular orbitals were computed using Firefly QC package [25], which is partially based on the GAMESS (US) [26]
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source code, using the Pipek-Mezey localization method [27]. Visualization of the localized molecular orbitals was performed with wxMacMolplt program [28]. 4. Results and discussion 4.1. Extraction experiments Regarding the results of previous papers [8,29,30], the twoephase watere AgNO3enitrobenzeneesodium dicarbollylcobaltate (NaDCC) extraction system can be described by the following equilibrium Agþ(aq) þ Naþ(nb) % Agþ(nb) þ Naþ(aq); Kex(Agþ,Naþ)
(1)
with the corresponding exchange extraction constant Kex (Agþ, Naþ); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (Agþ, Naþ), one can write [8,29,30].
i i log Kex Agþ ; Naþ ¼ log KAg þ log KNaþ
(2)
þ i i where KAg þ and KNaþ are the individual extraction constants for Ag þ
and Na , respectively, in the waterenitrobenzene system [29,30]. It i i should be noted that the mentioned constants KAg þ and KNaþ were
determined on the basis of a non-thermodynamic assumption of equal extractability of the tetraphenylarsonium cation, Ph4Asþ, and the tetraphenylborate anion, BPh-4, of the “reference” electrolyte Ph4AsBPh4 [31] from the aqueous into nitrobenzene phase, i i ¼ 6.3 [29]. Knowing expressed by identity log KPh þ ¼ log KBPh 4 As 4 i i the values log KAg þ ¼ 4.5 [30] and log KNaþ ¼ e 6.0 [29], the ex-
change extraction constant Kex (Agþ, Naþ) was simply calculated from Eq. (2) as log Kex (Agþ, Naþ) ¼ 1.5. Similarly as in our previous articles [32e34], the twoephase watereAgNO3enitrobenzenee1 ([Gly6]-antamanide) eNaDCC extraction system (see Section ‘Experimental’), chosen for determination of the stability constant of the 1·Agþ complex in nitrobenzene saturated with water, can be characterized by the main chemical equilibrium Agþ(aq) þ 1$Naþ(nb) %1$Agþ(nb) þ Naþ(aq); Kex(Agþ,1$Naþ) (3) with the respective Kex(Agþ,1$Naþ):
equilibrium
extraction
½1$Agþ nb ½Naþ aq Kex Agþ ; 1$Naþ Þ ¼ ½Agþ aq ½1$Naþ nb
constant
(4)
It is necessary to emphasize that 1 is a considerably lipophilic ligand, practically present in the nitrobenzene phase only, where this ligand forms e with Agþ and Naþ e the relatively stable complexes 1$Agþ and 1$Naþ, as given below. Employing the conditions of electroneutrality in the organic and aqueous phases of the system under study, the mass balances of the Agþ and Naþ cations at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratio of sodium, DNa ¼ [1$Naþ]nb/[Naþ]aq, combined with Eq. (4), we gain the final expression for Kex(Agþ,1$Naþ) in the form in;nb CNaDCC 1 Kex Agþ ; 1$Naþ ¼ DNa ð1 þ DNa Þ C in;aq C in;nb NaDCC AgNO 3
(5)
809
in;aq
where CAgNO3 is the initial concentration of AgNO3 in the aqueous in;nb phase and CNaDCC denotes the initial concentration of NaDCC in the organic phase of the system under consideration. In this study, from the extraction experiments and g-activity measurements (see Section ‘Experimental’) by means of Eq. (5), the following value of the constant Kex (Agþ,1$Naþ) was determined as log Kex (Agþ, 1$Naþ) ¼ 1.5 ± 0.1 (see Table 1). At this point it should be stated that this constant experimentally proves the justifying of the extraction mechanism and the presentation of the corresponding species, expressed by the two-phase chemical equilibrium (3). Furthermore, with respect to previous results [32e34], for the extraction constants Kex (Agþ, Naþ) and log Kex (Agþ, 1$Naþ) defined above, as well as for the stability constants of the complexes 1$Agþ and 1$Naþ in nitrobenzene saturated with water, denoted by bnb(1$Agþ) and bnb (1$Naþ), corresponding to the following equilibria 1(nb) þ Agþ(nb) % 1·Agþ (nb) and 1(nb) þ Naþ(nb) % 1·Naþ (nb), respectively, one gets
log bnb (1$Agþ) ¼ log bnb (1$Naþ) þ log Kex(Agþ,1$Naþ) e log Kex(Agþ,Naþ)
(6)
Using the constants log Kex (Agþ, Naþ) and log Kex (Agþ,1$Naþ) given above, the value log bnb (1$Naþ) ¼ 4.5 ± 0.1 [35], determined from the distribution of sodium picrate in the water e nitrobenzene system containing also the ligand 1, and applying Eq. (6), we obtain the stability constant of the 1$Agþ complex in water-saturated nitrobenzene at 25 C as log bnb (1$Agþ) ¼ 4.5 ± 0.2. This means that in the mentioned nitrobenzene medium, the stability constants of the complexes 1$Agþ and 1$Naþ are practically the same. In this context we can add that the stability constant of the complex 2·Agþ, where 2 denotes nonactin (see Scheme 3), in nitrobenzene saturated with water is log bnb (2$Agþ) ¼ 6.6 ± 0.2 [36]. Thus, in this medium, the stability of the 1·Agþ complex under study is substantially lower than that of the cationic complex 2·Agþ (2 ¼ nonactin). 4.2. Quantum chemical DFT calculations In the model calculations, we optimized the molecular geometries of the parent [Gly6]-antamanide ligand 1 and its complex with Agþ, similarly as in our previous papers [37e40]. The optimized structure of the free ligand 1 with the carbonyl oxygen binding sites O(1), O(2), …,O(10) is illustrated in Fig. 1. It is necessary to state that this free ligand 1 is stabilized by two strong intramolecular hydrogen bonds O(3) … H(6)N (2.12 Å) and O(8) … H(1)N (2.23 Å). Moreover, we must emphasize that the only one structure was obtained by the full DFT-optimization of the 1·Agþ complex, which is depicted in Fig. 2. It should be noted that the respective vibrational calculations found no imaginary frequencies. In the resulting 1·Agþ cationic complex species, the “central” cation Agþ is coordinated by four noncovalent bonds to the respective four carbonyl
Table 1 Experimental data concerning determination of log Kex (Agþ, 1·Naþ) on the basis of Eq. (5). in;aq
CAgNO3 (M) 1.0 1.5 2.0 2.5 3.0
103 103 103 103 103
in;nb CNaDCC (M)
1.0 1.5 2.0 2.5 3.0
103 103 103 103 103
DNa
log Kex (Agþ, 1·Naþ)
0.16 0.17 0.16 0.18 0.19
1.6 1.5 1.6 1.5 1.4
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Scheme 3. Structural formula of nonactin (abbrev. 2).
Fig. 2. DFT optimized structure of the 1·Agþ complex (B3LYP/LanL2DZ); hydrogen atoms are omitted for clarity except two hydrogens taking place in two internal hydrogen bonds O(4) … H(6)N (1.99 Å) and O(8) … H(1)N (2.06 Å); the lengths of the bonding interactions Agþ … O(1), Agþ … O(3), Agþ … O(6), and Agþ … O(8) are 2.29, 2.44, 2.31, and 2.53 Å, respectively.
Fig. 1. DFT optimized structure of free ligand 1 (B3LYP/LanL2DZ); hydrogen atoms are omitted for clarity except two hydrogens taking place in two internal hydrogen bonds O(3) … H(6)N (2.12 Å) and O(8) … H(1)N (2.23 Å).
oxygen atoms O(1), O(3), O(6), and O(8) (2.29, 2.44, 2.31, and 2.53 Å) of the parent ligand 1 (Fig. 2). It is apparent that electrostatic interactions are the dominant forces, characterizing these bonds. Besides, as follows from Fig. 2, the whole 1$Agþ complex structure is also stabilized by two intramolecular H-bonds O(4) … H(6)N (1.99 Å) and O(8) … H(1)N (2.06 Å). We must add that analysis of localized orbitals of the 1·Agþ complex, obtained by the Pipek-Mezey localization method [27], revealed that the “central” Agþ cation is coordinated to four sp hybridized lone pairs of the corresponding oxygen atoms O(1), O(3), O(6), and O(8) (Fig. 3). No other conformation of the 1·Agþ complex was found using the above-mentioned theoretical procedure. Cartesian coordinates (in Å) for the free ligand 1 and the 1·Agþ complex are presented in Supplementary material. Finally, the interaction energy, E(int), of the 1·Agþ complex, involving the 7-point correction for the basis set superposition error (BSSE) [41,42], was calculated as e 465.5 kJ/mol, which confirms the formation of this cationic complex as well. In summary, we must state that by employing an extraction method in the water e nitrobenzene system, the stability constant of the 1.Agþ complex in nitrobenzene saturated with water was determined, while using quantum chemical DFT calculations, the most probable structure of this cationic complex was derived.
Fig. 3. Coordination of the “central” cation Agþ to four sp hybridized lone pairs of carbonyl oxygens O(1), O(3), O(6), and O(8) of the parent ligand 1 in the 1·Agþ complex.
5. Conclusions In this work, we have demonstrated that the combination of theoretical DFT calculations with an experimental extraction method in the twoephase waterenitrobenzene system can provide relevant data on the noncovalent interactions of the univalent silver cation (Agþ) with the [Gly6]-antamanide ligand (1). By using this extraction method, the stability constant of the cationic complex 1·Agþ in nitrobenzene saturated with water was determined as log bnb(1·Agþ) ¼ 4.5 ± 0.2 (t ¼ 25 C). On the other hand, applying quantum chemical DFT calculations, the most probable structure of this 1·Agþ cationic species was predicted. In the resulting complex,
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Scheme 4. Structural formula of valinomycin (abbrev. 3).
the “central” cation Agþ is coordinated by four interactions to the corresponding four carbonyl oxygens of the parent ligand 1. It is obvious that the present work may be an important contribution especially to both theoretical and experimental study of [Gly6]antamanide, as well as to structural chemistry in general. Finally, as follows from our previous results [36,43], the stability constants of the cationic complex species 2·Agþ (2 ¼ nonactin) and 3·Agþ, where 3 denotes valinomycin (see Scheme 4), in water-saturated nitrobenzene are log bnb(2·Agþ) ¼ 6.6 ± 0.2 [36] and log bnb(3·Agþ) ¼ 4.6 ± 0.1 [43]. It means that the complexation ability of the three considered macrocyclic ligands towards the Agþ cation in this nitrobenzene medium increases in the series of [Gly6]antamanide < valinomycin < nonactin. Acknowledgements This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No.: 42900/1312/3114 entitled “Environmental Aspects of Sustainable Development of Society,” as well as by the Czech Ministry of Education, Youth, and Sports (Project MSMT No.: 20/ 2015). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.molstruc.2017.11.059. References [1] T. Wieland, H. Faulstich, W. Burgemeister, Antamanide and analogs. Studies on selectivity and stability of complexes, Biochem. Biophys. Res. Commun. 47 (1972) 984e992. [2] P. Ruzza, A. Calderan, B. Biondi, M. Carrara, T. Tancredi, G. Borin, Ion-binding and pharmacological properties of Tyr6 and Tyr 9 antamanide analogs, J. Pept. Res. 53 (1999) 442e452. [3] T. Wieland, G. Lüben, H. Ottenheym, J. Faesel, J.X. de Vries, A. Prox, J. Schmid, The discovery, isolation, elucidation of structure, and synthesis of antamanide, Angew. Chem. Int. Ed. Engl. 7 (1968) 204e208. [4] T. Wieland, H. Faulstich, Amatoxins, phallotoxins, phallolysin, and antamanide: the biologically active components of poisonous amanita mushrooms, Crit. Rev. Biochem. 5 (1978) 185e260.
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