Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion

Subscriber access provided by Vanderbilt Libraries

Article

Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion Transfer Voltammetry and DFT Calculations Jan Langmaier, Martin Pizl, Zdenek Samec, and Stanislav Zalis J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04786 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion Transfer Voltammetry and DFT Calculations Jan Langmaier,a Martin Pižl,a,b Zdeněk Samec*a and Stanislav Záliš*a a

J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, 182 23 Prague 8, Czech Republic

b

Department of Inorganic Chemistry, University of Chemistry and Technology, Prague 6, Technická 5, 166 28, Czech Republic

e-mail: [email protected], [email protected]

Abstract

Ion transfer voltammetry is used to estimate the acid dissociation constants Ka1 and Ka2 of the mono- and di-protonated forms of the biguanide drugs metformin (MF), phenformin (PF) and 1-phenylbiguanide (PB) in an aqueous solution. Measurements gave the pKa1 values for MFH+, PFH+ and PBH+ characterizing the basicity of MF, PF and PB, which are significantly higher than those reported in literature. As a result, the mono-protonated forms of these biguanides should prevail in a considerably broader range of pH 1-15 (MFH+, PFH+) and 2-13 (PBH+). DFT calculations with solvent correction were performed for possible tautomeric forms of neutral, mono and di-protonated species. Extreme basicity of all drugs is confirmed by DFT calculations of pKa1 for the most stable tautomers of the neutral and protonated forms with explicit water molecules in the first solvation sphere included.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction

Biguanides metformin (MF, N,N-dimethylimidodicarbonimidic diamide) and phenformin (PF, N-(2-phenylethyl)imidodicarbonimidic diamide) have been widely used for the treatment of type 2 diabetes mellitus.1 MF has become one of the most commonly prescribed drugs worldwide.2 Recently, its role in scavenging of methylglyoxal in humans has been recognized.3 On the other hand, PF has been withdrawn from the clinical practice in many countries due to the high incidence of lactic acidosis.4 At physiological pH, MF and PF studied exist as the cationic species MFH+ and PFH+, (Chart 1), respectively, which are distributed into body tissues (intestine, liver, kidney) by organic cation transporters, while their passive diffusion through the cell membranes is very limited.2,

5

1-phenylbiguanide (PB, 1-

(diaminomethylidene)-2-phenylguanidine) is a 5-HT3 (serotonin) receptor agonist.6

The acid-base properties of MF, PF and PB are characterized by the acid dissociation constants Ka1 and Ka2 for the mono- and di-protonated biguanide, respectively. In the case of MF, the most frequently cited values pKa1 = 11.5 and pKa2 = 2.85,

7-11

are rarely7 referred to the original work by Sarma,12 but often to the

compilations and reviews.5, 13 The pKa values above were obtained by the classical pH measurements of the aqueous solutions containing MF and various amounts of NaOH and HCl at 32 °C, with the mixture being eventually incubated for 48 hours.12 More recently, the acid dissociation constants of mono- and di-protonated MF and PF have been determined by 1H NMR-pH titration method.14 The pH dependence of the chemical shift provided a remarkably higher value of pKa1 = 13.85, and a comparable value of pKa2 = 3.14 for MFH+ and MFH22+, respectively.14 Similar values pKa1 = 13.27 and pKa2 = 3.26 were obtained for PFH+ and PFH22+, respectively.14 Strong basicity of


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MF was confirmed by the combined DFT and Raman scattering methods showing that Raman markers of the neutral MF species do not dominate even at very high pH values (>13), while the di-protonated form appears as the major population at very low pH (< 1.5).8 Basicity of PB and PBH+ is slightly lower, as indicated by the values pKa1 = 10.76 and pKa2 = 2.13, respectively, which were obtained by an acid-base titration, as in the case of MF.12

The X-ray crystal analysis of MF and PF suggests that the structures without hydrogen on the bridging nitrogen N4 cf. Chart 1, should be the most preferred one for both the neutral biguanides15 and their HCl salts.16 This conclusion was corroborated by the ab initio MO and density functional calculations revealing the most stable tautomers,17 and by the most recent crystal structure study of mono-protonated metformin salt.18 In the latter study, the four potential tautomers of MFH+ were optimized, and their single point energies were calculated by the DFT/B3LYP method based on the Polarized Continuum Model (PCM) in water. These results led to the identification of the most stable tautomer with the non-protonated bridging nitrogen N4 also in the crystal structure.18 In contrast, the calculations probing the conformational properties of MFH+, and apparently ignoring previous electronic and structural analysis,17 suggested that the N4 protonated tautomer is the lowest energy one.10 This questionable conclusion was anticipated in a study of the protonation-deprotonation features and structural dynamics of MF by using jointly Raman scattering and DFT methodology.8

Present investigation was motivated by the lack of agreement in the values of the acid dissociation constants of the protonated biguanides, and by the contradictory identification of their most stable tautomers in the aqueous solutions. It is notable that



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

the existence of the protonation-deprotonation equilibria was omitted in studies of the chemical reactivity of MF, e.g., in the experimental19 and DFT20 studies of the reaction between MF and methylglyoxal. Here, we used a novel experimental approach that is based on the ion transfer voltammetry at a polarized ionic liquid membrane21-22 to inspect the changes in concentrations of the protonated MF, PF and PB as a function of pH (pH titration curves), and to evaluate the pKa1 and pKa2 values.

This method is

similar to the electron transfer voltammetry at solid electrodes and involves the application of an electrical potential difference from an external voltage source across an ion conducting membrane, which induces the transfer of the selected ion from the adjacent aqueous phase into the membrane. The corresponding electrical current is measured, which is proportional to the ion concentration in the aqueous phase.21 Besides, we used the DFT methodology to extend the search for the lowest energy tautomers of the neutral systems as well as their mono- and di-protonated forms in aqueous solutions. DFT calculations with solvent correction were successfully used for estimation of pKa values in aqueous solution.23-32 It was shown that including explicit water molecules in the vicinity of the protonated site is important for reliable estimation of pKa.33-35 Schlegel et.al. in the benchmark study of thiols in aqueous solution shown that the variation of functional strongly influence the mean errors of calculated pKa and the proper choice of explicit water molecules is necessary for correct description of pKa values.36

In the present work the Gibbs free energies

calculated with the solvent PCM correction for the most stable forms have yielded the pKa1 and pKa2 values for the most stable tautomers including up to twenty explicit water molecules in the first solvation sphere. We shall show that the range of pH where the protonated forms prevail is significantly broader than that indicated by the literature values of pKa1 and pKa2.


Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental Section

Measurements

Metformin hydrochloride (N,N-dimethylimidodicarbonimidic diamide monohydrochloride),

phenformin

hydrochloride

(N-(2-phenylethyl)imidodicarbonimidic

diamide

monohydrochloride) and 1-phenylbiguanide hydrochloride (1-(diaminomethylidene)-2phenylguanidine monohydrochloride)

were obtained from AlfaAesar, Fluka and Sigma-

Aldrich, respectively. All other salts, acids and buffers were of analytical grade quality and were

used

as

received.

Tridodecylmethylammonium

tetrakis[3,5-

bis(trifluoromethyl)phenyl]borate (TDMATFPB) was prepared by the metathesis of the corresponding salts in acetone.37

The membrane cell, which was used for cyclic voltammetric measurements, can be represented by the Scheme I: Ag’|AgCl | 1 mM LiCl (w’) |m| XY, y mM RHCl (w) | AgCl|Ag

(Scheme I)

where m represents a room-temperature ionic-liquid (RTIL) membrane, RHCl is the hydrochloride form of the drug studied (y = 0–15), XY is 1 mM KCl or 1 mM LiCl (pH 7), 10 mM HCl (pH 2.05), 1 mM HCl (pH 3.02), 1 mM LiOH (pH 10.98), 10 mM LiOH (pH 11.96), 0.1 M LiOH (pH 12.89), 1 M LiOH (pH 13.8), 1 mM borate (pH 9.18), 3.9 mM phosphate buffer (pH 7.41), or 5.5 mM Clark-Lubs buffer (pH 6.0). The two Ag|AgCl reference



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

electrodes were connected to the aqueous phase by means of the Luggin capillary, the tip of which was filled with the agar gel containing 0.1 M LiCl. The membrane was made from a polyvinylidenfluoride microporous filter (Millipore, type GVHP 1300, pore size of 0.22 µm, thickness of ~112 µm), which was impregnated by soaking with liquid TDMATFPB (viscosity 3.55 Pa·s at 23.4o C).37 The membrane disk was cut off the impregnated filter (diameter of 0.9 cm) and mounted in a four-electrode cell.22 The area of the membrane exposed to the aqueous electrolyte solution was 0.071 cm2. Aqueous electrolyte solutions were prepared from highly purified water with resistivity of 18.2 MΩ (Millipore).

The voltammetric measurements were carried out with the help of the four-electrode potentiostat (Model CHI920C, CH Instruments, Inc., USA) equipped with the automatic compensation of the ohmic potential drop. This instrument was also used to measure the complex impedance of the cell enabling to estimate the solution resistance for the adjustment of the ohmic potential drop compensation (~80-150 kΩ). The resistance of 100 kΩ for the exposed membrane area and the thickness of the membrane of 112 µm correspond to the RTIL specific conductivity of 1.56 µS cm-1. All measurements were performed at the ambient temperature of 25 ± 2 oC. Effect of pH on the shape and position of the cyclic voltammograms of the TEA+ ion and MFH+ ion is demonstrated in Fig. S1 (SI). Cyclic voltammogram of a univalent ion can be characterized by the mid-point potential, which is defined as Em = (Ep+ + Ep-)/2, where Ep+ and Ep- is the positive and negative voltammetric peak potential, respectively, and by the positive peak current Ip+. The transfer of the TEA+ ion or a mono-protonated biguanide is likely to be controlled by the linear diffusion. The peak current Ip+ (in A) is then described by the equation37:

I p+ = (2.31x105 ) A Diw v ci0,w

(1) 6

ACS Paragon Plus Environment

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where Diw is the ion diffusion coefficient in the aqueous phase in cm2 s-1 , A is the interfacial area in cm2, ν is the sweep rate in V s-1 and ci0,w is the bulk ion concentration in mol cm-3. The values of the peak currents and the mid-point potentials Em = 0.33 V (pH 13.8), 0.29 V (pH 7.41) and 0.34 V (pH 2.05) as inferred from the voltammograms of MFH+ are comparable. The peak current measured in the cell containing the aqueous solution of 1 M LiOH (pH 13.8) was corrected for the increased viscosity ηw of the aqueous phase on assuming the validity of the Walden rule stating that Diw is inversely proportional to ηw.

Quantum Chemical Calculations

DFT calculations on metformin, phenformin, 1-phenylbiguanide and their protonized forms were performed using Gaussian 09. Revision D.01 (G09) program package.38 Density functional M06-2X39 designed for the correct description of the non-covalent interactions together with the D3 version of Grimme’s dispersion40 correction parametrized for M06-2X functional were chosen. Triple ζ 6-311+G* polarized basis sets including diffuse function were used for geometry optimizations and calculations of Gibbs free energies. Geometry optimizations were confirmed by means of the vibrational analysis in order to characterize the stationary points (for minima no imaginary frequencies were found). G09 uses vibrational frequencies for calculation of the zero-point energies, and the vibrational contributions to the partition functions needed for enumeration of Gibbs free energies. Electrostatic part of the solvent effect was modeled by the polarizable continuum model (PCM),41 specific interactions with water molecules were modeled by putting up to 20 of water molecules into the first solvation sphere, comparable with previously published models.42

The acid dissociation constants were calculated from the differences between the Gibbs free energies G of the protonated and deprotonated forms in the aqueous solution: 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

pKa1 = [Gaq(H+)+Gaq(MF)-Gaq(MFH+)]/2.203RT

(2)

pKa2 = [(Gaq(H+)+Gaq(MFH+)-Gaq(MFH2+)]/2.203RT

(3)

using the value of the Gibbs free energy of -264 kcal/mol for the solvated proton.25, 43 Prior the calculations of the pKa values of the series of guanidines the performance of the above mentioned approach was tested on guanidine where the experimental pKa1 value of 13.7 was detemined.44 Test DFT calculations using the equations above gave the pKa1 value of 15.3 for the guanidine (Chart 1) with 20 water molecules in the first solvation sphere, which indicate that model used reasonably describe experimenal value of pKa1. The simultaneous calculation of pKa1 and pKa2 requires the comparison of G for three different oxidations states. In the real experimental arrangement the charge is compensated by a presence of an excess of counterions and the neglecting this effect can lead to substantial errors.

Results and discussions

In order to characterize the pKa values of mono- and di-protonated biguanides, cyclic voltammetry and quantum chemical calculations were performed on mono- and di-protonated biguanides whose mono-protonated forms are listed in Chart 1.


Page 9 of 23

Chart 1. Structures of the most stable mono-protonated forms of metformin, 1phenylbiguanide, phenformin and guanidine.

1.0 a

b

+

Ip / µA

1 I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-1 -0.5

0.5

0.0 0.0

0.0 0.5 + E / V (vs. TEA )

0.1 c / mM

0.2

Figure 1. (a) Cyclic voltammograms (10 mV s-1) of the TDMATFPB membrane in the absence (dashed black line) and presence of 0.2 mM TEA+ (solid red line) or 0.2 mM MFH+ (solid black line) in the aqueous phase (pH 7.41). (b) Plot the backgroundcorrected positive peak current Ip+ vs. the bulk concentration c of MFH+ (○), PFH+ (●) or PBH+(+).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1a shows the cyclic voltammograms (CVs) of the tetraethylammonium ion (TEA+) and MFH+. The positive or negative current peak corresponds to the transfer of a cation from the aqueous to the membrane phase or the reverse, respectively. Apart from a difference in the position on the potential scale, the shape of both CVs is very similar, which suggests that MF (and analogously PF or PB) is present in the form of a univalent ion. It is notable, that irrespective of the composition of the aqueous phase (w), cf. Scheme I, the transfer of a cation from the aqueous phase (w) to the membrane phase (m) is coupled to the transfer of the chloride anion from the aqueous phase (w’) to the membrane phase (m).35 The mid-point potential Em23 on the ion transfer CV is determined by the standard Gibbs energy of ion transfer from water to the organic solvent phase (here TDMATFPB), ∆Gi0.21 Since the value of ∆Gi0 for the TEA+ ion transfer from water to TDMATFPB is close to zero (-1.7 kJ mol-1),22 the difference of the voltammetric mid-point potentials between MFH+ and TEA+, ∆Em = 0.29 V (pH 7.41), can be considered as a measure of ∆Gi0 for the MFH+ transfer (∆Gi0 = 28 kJ mol1

). The value of ∆Gi0 can be used to characterize the hydrophilicity of the transferred

ion.21 Increasing hydrophilicity of a cation is accompanied by an increasing value of ∆Gi0 (or Em). For MFH+, this quantity is practically independent of pH and composition of the aqueous solution, cf. Fig. S1 (SI), and indicates that MFH+ is considerably more hydrophilic than TEA+. The presence of the phenyl group in the structure of the PFH+ ion (Chart I) suggests that this ion should be less hydrophilic than MFH+. This is confirmed by the voltammetric measurements showing the CV peak that is shifted negative by 0.05 V with respect to the peak of MFH+. On the other hand, the PBH+ ion gives rise to a CV that is shifted positive by 0.03 V with respect to the peak of MFH+, and appears to be slightly more hydrophilic than MFH+ without an obvious relation to the chemical structure of the ion. The positive peak current Ip+ is proportional to the 10 ACS Paragon Plus Environment

Page 10 of 23

Page 11 of 23

concentration of MFH+, PFH+, or PBH+, cf. Fig. 1b. A small difference in the slope can be ascribed to a smaller diffusion coefficient of PFH+ (SI).

2.0

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

a

1.5

1.5

1.0

1.0

0.5

0.5

0.0 0

5

pH

10

b

0.0 0

15

5

pH

10

15

Figure 2. Plot of y = Ip / Ip0 vs. pH, where Ip and Ip0 represent the peak currents measured at the given pH and at pH 7.41, respectively, for MF (panel a) and PF (panel b). Filled points represent the data obtained after aging the aqueous phase for 24 hours. Solid line represented a model dependence calculated assuming pKa1 = 15.3 and pKa2 = 0.7 (panel a), or pKa1 = 14.9 and pKa2 = 1.1 (panel b). The effect of the ion concentration on Ip+, cf. eq 1, was used to follow the changes in the concentrations of MFH+, PFH+ and PBH+ with pH of the aqueous phase. Fig. 2 shows the plot of y = Ip / Ip0 vs. pH, where Ip and Ip0 represent the peak currents measured at the given pH and at pH 7.41, respectively, for MF (panel a) and PF (panel b). The data displayed in Figure 2 suggest that in the range of pH 2-14 the change in the concentrations of MFH+ or PFH+ is less than 10%, and that the effect of aging of the aqueous phase for 24 hours is negligible, cf. the empty and filled points in Fig. 2a. An increase in the hydroxide concentration above 1 mol L-1 led to a decomposition of MFH+ indicated by the amine or ammonia smell. Provided that the pH titration curve, of which the accessible part is displayed in Figure 2a, has the theoretical shape, less 11 ACS Paragon Plus Environment


than 10% increase or decrease in the titrated ion concentration at pH < 2 or pH > 14 yields an estimate of pKa2 < 0.7 and pKa1 > 15.3 for MFH22+ and MFH+, respectively, cf. the solid line in Figure 2a. Analogous consideration based on the data shown in Fig. 2b leads to the slightly different estimates of pKa2 < 1.1 and pKa1 > 14.9 for PFH22+ and PFH+, respectively, cf. the solid line in Figure 2b. The basicity of PB and PBH+ is somewhat less pronounced, as it can be seen from the dependence of Ip/Ip0 vs. pH in Fig. 3. In this case, the model calculations suggest that pKa2 = 2 and pKa1 = 13 for PBH22+ and PBH+, respectively.

2.0 1.5 y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0 0.5 0.0

0

5

10

15

pH Figure 3 Plot of y = Ip / Ip0 vs. pH, where Ip and Ip0 represent the peak currents measured at the given pH and at pH 7.41, respectively, for PB. Solid line represented a model dependence calculated assuming pKa1 = 13 and pKa2 = 2. The experimentally determined pKa1 and pKa2 are supported by the DFT calculations. Prior to calculating the pKa values, the structures of all possible tautomers of the neutral and protonated forms of MF, PF and PB were optimized including the PCM correction. The structures of lowest energy tautomers of MF, MFH+ and MFH22+ are depicted in Figure 4, analogous tautomers of PF, PFH+, PFH22+, PB, PBH+ and PBH22+ are depicted in Figures S2-S3 (SI). 12 ACS Paragon Plus Environment

Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 DFT optimized structures of the most stable tautomers of MF, MFH+ and MFH22+.

Calculations on the neutral forms of all biguaninides suggest that the tautomer (a) with the non-protonated bridging nitrogen N4 has the highest stability in the case of systems studied. Calculated Gibbs free energy differences between the most stable MF tautomer (a) with the non-protonated bridging nitrogen N4 and the tautomer (b), (c) or (d) are given in Table S1, together with energy differences of individual MFH+ tautomers. These differences are quite appreciable and make 2.9-3.0 kcal/mol for the neutral forms, and 10.0-14.1 kcal/mol for the protonated form. Analogous results were found for PF and PB (Tables S2 and S3). Geometry optimization of the MFH+ tautomer (a) and the complex formed between MF and HCl (MFH+Cl-) well reproduces the experimental structure16,

18, 45

, cf. Figure 4, and Tables S4 and S5 (SI). The good

description of the experimental structure is found also for mono-protonated forms of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PF and PB (Tables S4 – S8). The energetic order of MFH+ tautomers is in agreement with that based on the theoretical study of protomeric tautomerism of biguanide and its mono- and di-protonated forms.16,

18, 46-47

The acid dissociation constants were

calculated for the lowest energy tautomers involved in the superstructures with up to 20 explicit water molecules. In the case of MF three and in the case of PF and PB six different starting conformations including the fully optimized isolated conformers were tested. Also the influence of twisting the NH groups which can influence H-bonding was checked. The fully optimized lowest energy superstructures of the neutral, monoprotonated and di-protonated MF with 20 water molecules are displayed in Figure 5, optimized structures of neutral and protonated forms PF with explicit water molecules are depicted in Figure 6 and analogous forms of PB in Figure S4. Calculated pKa values strongly depend on the number of explicit water molecules, analogously to recently published pKa variations calculated for series of thiols in,36 which apparently reflects the contribution of hydrogen bonds between MF/PF/PB and water sites to the relative energies of the superstructures. The strongest hydrogen bonds were found in mono- and di- protonated forms with H-O and N-H bond lengths between 1.732 and 1.867 Å. In this way, the pKa1 value of 15.1 was calculated for {(MFH)·(H2O)20}+.


Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

DFT/M06-2X/G3/PCM

optimized structure of (a) {(MF)·(H2O)20}, (b)

{(MFH)·(H2O)20}+ and (c) {(MFH2)·(H2O)20}2+ systems. The shortest hydrogen bond distance drawn as dashed line was calculated to be of 1.838 Å and 1.754 Å for system (b) and (c), respectively. The circles indicate the groups including protons in the monoand di-protonated MF forms.

Figure 6

DFT/M06-2X/G3/PCM

optimized structure of (a) {(PF)·(H2O)20}, (b)

{(PFH)·(H2O)20}+ and (c) {(PFH)·(H2O)20}2+. The circles indicate the groups including protons in the mono- and di-protonated PF forms. The shortest hydrogen bond distances were calculated to be of 1.884 Å for system (b) and 1.732 Å and 1.857 Å for system (c).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

Analogously, the value of pKa2 = -0.6 was obtained for {(MFH2)·(H2O)20}2+. Similar effect of the number of water molecules is demonstrated in case of PF by the DFT calculations yielding pKa1 value of 12.6 for {(PFH)·(H2O)20}+, and pKa2 value of 2.7 for {(PFH2)·(H2O)20}2+. Calculations for {(PBH)·(H2O)20}+ gave pKa1 value of 11.1 and pKa2 values of -0.2 for {(PBH2)·(H2O)20}2+. In the case of pKa2 values the calculations show the necessity of inclusion of larger number of explicit water molecules within the model in order to get qualitatively correct results.

Table 1. A comparison of the experimental pKa1 and pKa2 values with those calculated for MF, PF and PB surrounded by 20 water molecules.

MF

pKa1

PF

PB

Exp.

Calc.

Exp.

Calc.

Exp.

Calc.

> 15.3a

15.1a

> 14.9a

12.6a

13a

11.1a

11.5b

13.27c

10.76b

13.85c > 13d pKa2

< 0.7a 2.8b

-0.6a

< 1.1a

2.7a

3.26c

3.14c < 1.5d a

present study, b ref. 12, c ref. 14, d ref. 8


2a 2.13b

-0.2a

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 compares the experimental and calculated values of pKa1 and pKa2. It can be seen that the present experimental values of pKa1 are significantly higher, and those of pKa2 significantly lower (except for PB), than the experimental values reported in the literature.8,12,14. Consequently, the mono-protonated forms of MF, PF and BP should exist in an aqueous solution over a pH range as broad as 1-15 (MFH+, PFH+) and 2-13 (PBH+). It is noteworthy that the basicity of PF and PB appears to be evidently lower than that of MF, which can be also seen from Figures 2 and 3 indicating that on increasing pH the PFH+ and PBH+ concentrations starts to decay at lower pH values. Theoretical estimates of pKa1 are consistent with the experimental data for MFH+, PFH+ and PBH+. Calculations slightly underestimate the pKa1 values, but their decrease following the order MFH+ > PFH+ > PBH+ is reasonably reproduced. Differences between pKa1 and pKa2 are also well reproduced by calculations.

Conclusions

In summary, we have demonstrated that ion transfer voltammetry can be used to estimate the acid dissociation constants Ka1 and Ka2 of mono- and di-protonated biguanidines metformin, phenformin and 1-phenylbiguanide in an aqueous solution. The pKa1 values for MFH+, PFH+ and PBH+, which characterize the basicity of MF, PF and PB, are significantly higher than those reported in literature. On the other hand the pKa2 values for MFH22+ and PFH22+ are lower than literature date. As a result, the mono-protonated forms of the studied biguanides should prevail in a considerably broader range of pH 1-15 (MFH+, PFH+) and 2-13 (PBH+). Strong basicity of the neutral biguanides should be emphasized by DFT calculations of pKa1 for the most stable tautomers of the neutral and protonated biguanide forms with explicit water molecules in the first solvation sphere. Calculations also well describe the second dissociation constant Ka2. Both



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experimental and computational results emphasize the extraordinary stability of monoprotonated forms of all drugs, which should play key role in their chemical transformations in aqueous solutions.

Author information Corresponding Authors Z. Samec, Email: [email protected], Tel. 00420266052017 S. Záliš, , Email: [email protected], Tel. 00420266053268

Acknowledgements

This work was supported by the Czech Science Foundation (project number 15-03139S).

Supporting Information Available

Optimized structures of all complexes under study, the comparison of experimental and optimized structures of all mono- and di-protonated biguanidines under study, graphical representation of structures not listed in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1. Madiraju, A. K., et al., Metformin Suppresses Gluconeogenesis by Inhibiting Mitochondrial Glycerophosphate Dehydrogenase. Nature 2014, 510, 542-. 2. Gong, L.; Goswami, S.; Giacomini, K. M.; Altman, R. B.; Klein, T. E., Metformin Pathways: Pharmacokinetics and Pharmacodynamics. Pharmacogenet. Genom. 2012, 22, 820827. 3. Kinsky, O. R.; Hargraves, T. L.; Anumol, T.; Jacobsen, N. E.; Dai, J.; Snyder, S. A.; Monks, T. J.; Lau, S. S., Metformin Scavenges Methylglyoxal to Form a Novel Imidazolinone Metabolite in Humans. Chem. Res. Toxicol. 2016, 29, 227-234. 4. Kwong, S. C.; Brubacher, J., Phenformin and Lactic Acidosis: A Case Report and Review. J. Emerg. Med. 1998, 16. 5. Graham, G. G., et al., Clinical Pharmacokinetics of Metformin. Clin. Pharmacokinet. 2011, 50, 81-98. 6. Higgins, G. A.; Joharchi, N.; Sellers, E. M., Behavioral Effects of the 5Hydroxytryptamine3 Receptor Agonists 1-Phenylbiguanide and M-Chlorophenylbiguanide in Rats. J. Pharmacol. Exp. Ther. 1993, 264, 1440-1449. 7. Dutta, R. L.; Sengupta, N. R., J. Ind. Chem. Soc. 1961, 38, 741. 8. Hernadez, B.; Pfluger, F.; Kruglik, S. G.; Cohen, R.; Ghomi, M., ProtonationDeprotonation and Structural Dynamics of Antidiabetic Drug Metformin. J. Pharm. Biomed. Anal. 2015, 114, 42-48. 9. Tao, C. A.; Wang, J. F.; Qin, S. Q.; Lv, Y. A.; Long, Y.; Zhu, H.; Jiang, Z. H., Fabrication of Ph-Sensitive Graphene Oxide-Drug Supramolecular Hydrogels as Controlled Release Systems. J. Mater. Chem. 2012, 22, 24856-24861. 10. Trouillas, P.; Marchetti, C.; Bonnefont-Rousselot, D.; Lazzaroni, R.; Jore, D.; GardesAlbertd, M.; Collin, F., Mechanism of One-Electron Oxidation of Metformin in Aqueous Solution. Phys. Chem. Chem. Phys. 2013, 15, 9871-9878. 11. Clarke, G. S.; Bretnall, A. E., Metformin Hydrochloride. In Analytical Profiles of Drug Substances and Excipients, Brittain, H. G., Ed. Academic Press: San Diego, 1998; pp 243-293. 12. Sarma, B. D., Acid Dissotiation Constants and Basicity of Biguanides and Dibiguanides. J. Ind. Chem. Soc. 1952, 29, 217. 13. Craig, P. N., In Comprehensive Medicinal Chemistry, Hansch, C.; Sammes, P. G.; Taylor, J. B., Eds. Pergamon Press: Oxford, 1990; p 658. 14. Orgovan, G.; Noszal, B., Electrodeless, Accurate pH Determination in Highly Basic Media Using a New Set of H-1 Nmr Ph Indicators. J. Pharm. Biomed. Anal. 2011, 54, 958964. 15. Ernst, S. R.; Cagle, F. W., Jnr, Biguanide. Acta Cryst. B 1977, 33, 235-237. 16. Ernst, S., Biguanide Hydrochloride. Acta Cryst. B 1977, 33, 237-240. 17. Bharatam, P. V.; Patel, D. S.; Iqbal, P., Pharmacophoric Features of Biguanide Derivatives: An Electronic and Structural Analysis. J. Med. Chem. 2005, 48, 7615-7622. 18. Wei, X. D.; Fan, Y. H.; Bi, C. F.; Yan, X. C.; Zhang, X.; Li, X., Crystal Structure and Tautomerism Study of the Mono-Protonated Metformin Salt. Bull. Korean Chem. Soc. 2014, 35, 3495-3501. 19. Ruggiero-Lopez, D.; Lecomte, M.; Moinet, G.; Patereau, G.; Lagarde, M.; Wiernsperger, N., Reaction of Metformin with Dicarbonyl Compounds. Possible Implication in the Inhibition of Advanced Glycation End Product Formation. Biochem. Pharmacol. 1999, 58, 1765-1773. 20. Solis-Calero, C.; Ortega-Castro, J.; Frau, J.; Munoz, F., Scavenger Mechanism of Methylglyoxal by Metformin. A Dft Study. Theor. Chem. Acc. 2015, 134. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

21. Samec, Z.; Langmaier, J.; Kakiuchi, T., Charge-Transfer Processes at the Interface between Hydrophobic Ionic Liquid and Water. Pure Appl. Chem. 2009, 81, 1473-1488. 22. Samec, Z.; Trojánek, A.; Samcová, E., Ion and Pore Fluid Transport-Properties of a Nafion(R) Membrane Separating 2 Electrolyte-Solutions .1. Kinetics of the Proton and AlkaliMetal Cation-Transport. J. Electroanal. Chem. 1995, 389, 1-11. 23. Cramer, C. J.; Truhlar, D. G., A Universal Approach to Solvation Modeling. Acc. Chem. Res. 2008, 41, 760-768. 24. Eckert, F.; Diedenhofen, M.; Klamt, A., Towards a First Principles Prediction of pK(a): COSMO-RS and the Cluster-Continuum Approach. Mol. Phys. 2010, 108, 229-241. 25. Houari, Y.; Jacquemin, D.; Laurent, A. D., Methodological Keys for Accurate pK(a)* Simulations. Phys. Chem. Chem. Phys. 2013, 15, 11875-11882. 26. Houari, Y.; Jacquemin, D.; Laurent, A. D., Td-Dft Study of the pK(a)* for Coumarins. Chem. Phys. Letters 2013, 583, 218-221. 27. Saracino, G. A. A.; Improta, R.; Barone, V., Absolute pK(a) Determination for Carboxylic Acids Using Density Functional Theory and the Polarizable Continuum Model. Chemical Physics Lett. 2003, 373, 411-415. 28. Brown, T. N.; Mora-Diez, N., Computational Determination of Aqueous pKa Values of Protonated Benzimidazoles (Part 2). J. Phys. Chem. B 2006, 110, 20546-20554. 29. Casasnovas, R.; Fernandez, D.; Ortega-Castro, J.; Frau, J.; Donoso, J.; Munoz, F., Avoiding Gas-Phase Calculations in Theoretical pK(a) Predictions. Theoret. Chem. Acc. 2011, 130, 1-13. 30. Schuurmann, G.; Cossi, M.; Barone, V.; Tomasi, J., Prediction of the Pk(a) of Carboxylic Acids Using the Ab Initio Continuum-Solvation Model PCM-UAHF. J. Phys. Chem. A 1998, 102, 6706-6712. 31. Sutton, C. C. R.; Franks, G. V.; da Silva, G., First Principles pK(a) Calculations on Carboxylic Acids Using the Smd Solvation Model: Effect of Thermodynamic Cycle, Model Chemistry, and Explicit Solvent Molecules. J. Phys. Chem. B 2012, 116, 11999-12006. 32. Rebollar-Zepeda, A. M.; Campos-Hernandez, T.; Ramirez-Silva, M. T.; RojasHernandez, A.; Galano, A., Searching for Computational Strategies to Accurately Predict pK(a)s of Large Phenolic Derivatives. J. Chem. Theory Computat. 2011, 7, 2528-2538. 33. Kelly, C. P.; Cramer, C. J.; Truhlar, D. G., Adding Explicit Solvent Molecules to Continuum Solvent Calculations for the Calculation of Aqueous Acid Dissociation Constants. J. Phys. Chem. A 2006, 110, 2493-2499. 34. Rebollar-Zepeda, A. M.; Galano, A., First Principles Calculations of pKa Values of Amines in Aqueous Solution: Application to Neurotransmitters. Int. J. Quant. Chem. 2012, 112, 3449-3460. 35. Thapa, B.; Schlegel, H. B., Calculations of pK(a)'s and Redox Potentials of Nucleobases with Explicit Waters and Polarizable Continuum Solvation. J. Phys. Chem. A 2015, 119, 5134-5144. 36. Thapa, B.; Schlegel, H. B., Density Functional Theory Calculation of pKa’s of Thiols in Aqueous Solution Using Explicit Water Molecules and the Polarizable Continuum Model. J. Phys. Chem. A 2016, 120, 5726–5735. 37. Langmaier, J.; Samec, Z., Voltammetry of Ion Transfer across a Polarized RoomTemperature Ionic Liquid Membrane Facilitated by Valinomycin: Theoretical Aspects and Application. Anal. Chem. 2009, 81, 6382-6389. 38. Frisch, M. J., et al. Gaussian 09, Revision D.01, Gaussian, Inc.: Wallingford CT, 2009. 39. Zhao, Y.; Truhlar, D. G., The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215-241. 20 ACS Paragon Plus Environment

Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements HPu. J. Chem. Phys. 2010, 132. 41. Cossi, M.; Rega, N.; Scalmani, G.; Barone, V., Energies, Structures, and Electronic Properties of Molecules in Solution with the C-PCM Solvation Model. J. Computat. Chem. 2003, 24, 669-681. 42. Munoz Frean, S.; Alcolea Palafox, M.; Rastogi, V. K., Effect of the Microhydration on the Tautomerism in the Anticarcinogenic Drug 5-Fluorouracil and Relationships with Other 5Haloderivatives. J. Mol. Struct. 2013, 1054, 32-45. 43. Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R., The Proton's Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster-Ion Solvation Data. J. Phys. Chem. A 1998, 102, 77877794. 44. Albert, A.; Goldacre, R.; Phillips, J., The Strength of Heterocyclic Bases. J. Chem. Soc. 1948, 2240-2249. 45. Hariharan, M.; Rajan, S. S.; Srinivasan, R., Structure of Metformin Hydrochloride. Acta Cryst. Section C 1989, 45, 911-913. 46. Hatanaka, M., Tautomerism in Polyguanide. Comput. Theor. Chem. 2011, 971, 58-64. 47. Hatanaka, M., Role of Topological Charge Stabilization in Protomeric Tautomerism. J. Phys. Chem. A 2015, 119, 1074-1086.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic


Page 22 of 23

Page 23 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Paragon Plus Environment

Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion

Recommend Documents