Acid–base and hydrogen-bonding equilibria in aliphatic amine and carboxylic acid systems in non-aqueous solutions

Zielińska, J.; Makowski, Mariusz; Maj, K.; Liwo, Adam; Chmurzyński, Lech

doi:10.1016/s0003-2670(99)00509-7

Cited by 35 publications

(41 citation statements)

References 17 publications

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“…Their pK a values in water, methanol, 95% methanol/5% water mixture and DMSO are listed in Table 2. 31,45,48,49 In the MD/GB/pH approach, canonical sampling of both conformation and protonation state was carried out. Therefore, the average number of protons attached to the peptide molecule at a given pH is computed as a direct average over conformations.…”

Section: 22theoretical Calculations-mentioning

confidence: 99%

Assessment of Two Theoretical Methods to Estimate Potentiometric Titration Curves of Peptides: Comparison with Experiment

et al. 2006

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We compared the ability of two theoretical methods of pH-dependent conformational calculations to reproduce experimental potentiometric-titration curves of two models of peptides: Ac-K 5 -NHMe in 95% methanol (MeOH)/5% water mixture and Ac-XX(A) 7 OO-NH 2 (XAO) (where X is diaminobutyric acid, A is alanine, and O is ornithine) in water, methanol (MeOH) and dimethylsulfoxide (DMSO), respectively. The titration curve of the former was taken from the literature, and the curve of the latter was determined in this work. The first theoretical method involves a conformational search using the Electrostatically Driven Monte Carlo (EDMC) method with a low-cost energy function (ECEPP/3 plus the SRFOPT surface-solvation model, assumming that all titratable groups are uncharged) and subsequent reevaluation of the free energy at a given pH with the Poisson-Boltzmann equation, considering variable protonation states. In the second procedure, MD simulations are run with the AMBER force field and the Generalized-Born model of electrostatic solvation, and the protonation states are sampled during constant-pH MD runs. In all three solvents, the first pK a of XAO is strongly downshifted compared to the value for the reference compounds (ethyl amine and propyl amine, respectively); the water and methanol curves have one, and the DMSO curve has two jumps characteristic of remarkable differences in the dissociation constants of acidic groups. The predicted titration curves of Ac-K 5 -NHMe are in good agreement with the experimental ones; better agreement is achieved with the MD-based method. The titration curves of XAO in methanol and DMSO, calculated using the MD-based approach, trace the shape of the experimental curves, reproducing the pH jump, while those calculated with the EDMC-based approach, and the titration curve in water calculated using the MD-based approach, have smooth shapes characteristic of the titration of weak multifunctional acids with small differences between the dissociation constants. Nevertheless, quantitative agreement between theoretically predicted and experimental titration curves is not achieved in all three solvents even with the MD-based approach which is manifested by a smaller pH range of the calculated titration curves with respect to the experimental curves. The poorer agreement obtained for water than for the non-aqueous solvents suggests a significant role of specific solvation in water, which cannot be accounted for by the meanfield solvation models.

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Section: 22theoretical Calculations-mentioning

confidence: 99%

Assessment of Two Theoretical Methods to Estimate Potentiometric Titration Curves of Peptides: Comparison with Experiment

et al. 2006

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“…For comparison, it was also found [22] that the pK a values of BH + and HA studied in this work change in different solvents according to the donor numbers of solvents. The pK a values of the protonated n-butylamine (cationic acid) are 18.26, 16.95, 11.92, 9.70, and 11.22 in acetonitrile (AN), propylene carbonate (PC), acetone (AC), dimethyl sulfoxide (DMSO), and methanol (MeOH), respectively.…”

Section: Resultsmentioning

confidence: 83%

“…The binary solvent mixtures of predetermined composition were prepared on a volume basis and stored in bottles with ground-in stoppers, additionally tightened with a high-viscosity silicone grease and parafilm. Acetic acid and n-butylamine were purified as usual [22], and auxiliary materials were obtained as described previously [23]. The detailed description of our laboratory equipment for potentiometric titrations, as well as the experimental procedure could be found in one of our previous papers [19,20,23,24].…”

Section: Measurementsmentioning

confidence: 99%

Molecular Heteroconjugation Equilibria in (n-Butylamine + Acetic Acid) Systems in Binary (Dimethyl Sulfoxide + 1,4-Dioxane) Solvent Mixtures

et al. 2007

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The acidity constants of molecular acid, K a (HA), cationic acid, K a (BH + ), as well as the equilibrium constants of anionic homoconjugation, K AHA − , cationic homoconjugation, K BHB + , and molecular heteroconjugation, K AHB , have been determined in (n-butylamine + acetic acid) systems without proton transfer in binary [dimethyl sulfoxide (DMSO) + 1,4-dioxane (D)] solvent mixtures. The constants were determined by using the potentiometric titration method at a fixed ionic strength. It is concluded that the molecular heteroconjugation constants in the mixed solvent systems studied are linearly related to the 1,4-dioxane content. Furthermore, in the (acid + base) systems without proton transfer, the direction of titration (direct B + HA or reverse HA + B) has been found to affect the precision of determination of reliable values of molecular heteroconjugation constants. Moreover, it has been found that the relative dielectric constants of the solvent mixtures studied change linearly as a function of solvent composition, as well as solvent components do not show interactions of solvent-solvent type.

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“…Another issue to be ensured is that there are free acetate ions present in the solution. Reported pK a values of acetic acid (conjugate acid of acetate) in ACN are 22.3 [38], 24.7 6 1.3 [39], 22.31 [40], 21.56 [41] and 23.51 [42]. Seemingly there is a huge scatter in published values.…”

Section: Selection Of Experimental Conditionsmentioning

confidence: 99%

“…homoconjugation). Some homoconjugation constants are reported for acetic acid in ACN (expressed as log K AHA À ): 3.67 [38], 2.34 [39], 3.87 6 0.03 [40] and 3.21 6 0.01 [41]. Even though the scatter in literature data is again very wide (K AHA À ,220-7400!…”

Section: Effect Of Homoconjugation Of Acetic Acidmentioning

confidence: 99%

Electromigration of a heteroconjugated imidazole–acetate complex in ACN

Porras

Jussila

2007

Electrophoresis

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ACN is an extremely poor hydrogen bond donor and therefore the anions dissolved in it are solvated mainly by other hydrogen bond donors (e.g. uncharged acids) possibly present in the solution. Under properly selected experimental conditions stabilization via hydrogen bonding can be used for separation in CE as has been demonstrated for uncharged acids by several authors. Electromigration based on heteroconjugation can be of importance, e.g. when aqueous separation medium cannot be used due to stability reasons. It also allows CE to be used as a tool for solution chemistry measurements, if the required physicochemical properties of the studied system are known or they can be predicted with sufficient accuracy by existing theories. In the present work we showed that also an uncharged base can stabilize an anion via hydrogen bonding in ACN. In the setup imidazole was chosen as a model base and acetate ion as complexing anion in equimolar acetic acid-acetate buffer. The resulted hydrogen-bonded imidazole-acetate complex (i.e. heteroconjugate) possesses a charge and can thus migrate in CE. It was shown that the studied complexation in ACN is sensitive to competition by other hydrogen bond donors such as water and methanol. On the other hand, acetone, which is a poor hydrogen bond donor, did not have much effect on the complexation. To take the effect of ionic strength on mobility into account, mobilities of the imidazole-acetate complex measured at various ionic strengths were corrected to zero ionic strength by the aid of conductivity equation. A fit of the 1:1 binding isotherm to the ionic strength corrected mobility versus acetate concentration data led to rather good correlation. However, x-reciprocal linear transformation of the binding isotherm showed nonlinearity, which could be partly explained by homoconjugation of acetic acid and acetate ion. Since the homoconjugation constant for acetic acid under present experimental conditions was not available, theoretical simulations were used to demonstrate the effects of homoconjugation. The possibility of multiple complexation of imidazole was discussed as well.

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Acid–base and hydrogen-bonding equilibria in aliphatic amine and carboxylic acid systems in non-aqueous solutions

Cited by 35 publications

References 17 publications

Assessment of Two Theoretical Methods to Estimate Potentiometric Titration Curves of Peptides: Comparison with Experiment

Assessment of Two Theoretical Methods to Estimate Potentiometric Titration Curves of Peptides: Comparison with Experiment

Molecular Heteroconjugation Equilibria in (n-Butylamine + Acetic Acid) Systems in Binary (Dimethyl Sulfoxide + 1,4-Dioxane) Solvent Mixtures

Electromigration of a heteroconjugated imidazole–acetate complex in ACN

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