Enthalpies of Dilution of Penicillamines in N,N-Dimethylformamide + Water Mixtures at 298.15 K

Abstract

Dilution enthalpies of two penicillamine enantiomers, namely, d-penicillamine, l-penicillamine, and their racemate d,l-penicillamine, in N,N-dimethylformamide (DMF) + water mixtures of various compositions have been determined at 298.15 K respectively, using an isothermal titration calorimeter (MicroCal ITC200). According to the McMillan–Mayer theory, homochiral enthalpic pairwise interaction coefficients (h XX) of the three penicillamines in DMF + water mixtures of different mass fractions (w DMF = 0 to 0.3) have been calculated regressively. It is found that h XX coefficients of each enantiomer and their racemate change increasingly with w DMF to the maxima at w DMF = 0.20 and then decrease relatively rapidly, which follows the order h LL > h DD ≈ h RR > 0 (R represents the racemate mixture of penicillamine). The results are discussed from the point of view of competitive equilibrium between hydrophobic and hydrophilic interactions in solutions.

Thermodynamic Difference between Protocatechualdehyde and p-Hydroxybenzaldehyde in Aqueous Sodium Chloride Solutions

“…The enthalpic interaction coefficients, derived from McMillan−Mayer's theory, 33 and modified by Franks et al, 34 characterize the total energetic effects of interactions between the investigated solute molecules with the competitive participation of solvent molecules. 35 Therefore, these coefficients can contribute to a better understanding of the effects of the molecular structures of solute molecules on their hydrophobic/hydrophilic properties.…”

Temperature Dependences and Enthalpic Discrimination of Homochiral Pairwise Interactions of Six α-Amino Acid Enantiomers in Pure Water

“…However, among various possible interaction configurations of two homochiral hydrated α-amino acid molecules, only the one formed between the opposite charges of zwitterions is hypothesized to be favorable and prevailing in aqueous solutions, resulting in the depression of the cooperativity of hydrophobic interaction. The possibility of detecting chiral recognition in solutions of small molecules by microcalorimetry has been our recent ongoing interest. − We have found that, when homochiral pairwise interaction occurs in aqueous solutions containing highly polar aprotic cosolvents like dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), a small but significant energetic difference between S–S and R–R (or L–L and D–D) enantiomeric pairs can be found, i.e., | h SS | > | h RR | or | h LL | > | h DD |. − In the present case, though the largest difference of h xx between L–L and D–D pairs of each α-amino acid is only up to 47 J·kg·mol –2 , and the existence of possible experimental errors cannot be ruled out completely, we still believe that the enthalpic discrimination (i.e., | h LL | > | h DD |) is a real phenomenon for the systems under study. The L–L pair is considered to be the more preferentially selected configuration than the D–D pair because of the best juxtaposition of hydrophobic or hydrophilic side-chains on L-α-carbons, in coordination with electrostatic interaction between the “heads” of α-amino acids.…”

“…As is well-known, the stability of biological macromolecules like proteins is controlled by various weak nonbonding interactions between α-amino acid residues and other biochemical components in aqueous solutions. − Homotactic or heterotactic enthalpic pairwise interaction coefficients ( h xx or h xy ) obtained from the framework of the McMillan–Mayer theory are particularly useful for understanding solute–solute and solute–solvent noncovalent interactions. , Over the past several decades, great efforts have been made by many research groups to gain insight into the thermodynamics of complex nonbonding interactions in biological model solution systems. − However, most of the studies have been carried out only at T = 298.15 K, and therefore the temperature dependences of h xx or h xy coefficients and the relevant mechanisms have remained unknown or not very clear, though some attempts have been made for this problem. − Considering that the first derivative of enthalpic pairwise interaction coefficient ( h 2 ) on temperature T ((∂ h 2 /∂ T ) p ) equals the second virial coefficient of isobaric heat capacity ( c p,2 ), and the latter seems to be more sensitive to the structural change of water in the hydration shell of solutes, it is of greater interest to investigate the temperature dependence of h 2 . Recently, we have paid much attention to the solvent and substituent effects on homotactic enthalpic pairwise interactions of small biochemical compounds in aqueous mixed solvents containing highly polar cosolvents like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) at 298.15 K, and the phenomena of enthalpic discrimination in these systems has been explored. − In the present work, we have measured the dilution enthalpies of six enantiomers of typical α-amino acids (Scheme ) in pure water at different temperatures by isothermal titration calorimetry (ITC), and the temperature dependences and enthalpic discrimination of homochiral pairwise interactions are discussed mainly. We hope to gain insight into how the studied α-amino acids play key roles on the stability of proteins like collagens …”