Energetics of pairwise interaction between glycidol enantiomers in (dimethylformamide + water) mixtures rich in water at T = 298.15 K

Highlights

• Dilution enthalpies of glycidols in (DMF + water) mixtures were determined by ITC.

• Enthalpic pairwise interaction coefficients (h_XX) of each compound were evaluated.

• The h_XX values are all positive and decrease gradually with the mass fraction of DMF.

• The pairwise interaction of S–S pair is slightly stronger than that of R–R pair.

• The addition of DMF favours endothermic pairwise interactions of these compounds.

Abstract

Successive dilution enthalpies ($\mathrm{\Delta }H(m_{N-1}\to m_N)$) of glycidol enantiomers (R-(+)-glycidol, S-(−)-glycidol) and the racemate (R/S-(±)-glycidol) in (dimethylformamide (DMF) + water) mixtures rich in water (mass fractions of DMF, w_DMF = 0 to 0.25) have been determined respectively by isothermal titration calorimetry (ITC) at T = 298.15 K. The corresponding homotactic enthalpic pairwise interaction coefficients (h_XX) of each compound have been evaluated from the framework of McMillan–Mayer theory. Across the studied composition range of (DMF + water) mixtures the h_XX values of these compounds are all positive, and decrease gradually with the mass fraction of DMF by the order h_RR > h_RS ≈ h_MM > h_SS > 0 (M represents the racemate R/S-(±)-glycidol, and h_RS is the estimated value for the heterochiral pair R–S). The positive values of h_XX indicate that pairwise interactions of these compounds are endothermic and unfavourable from the point of view of enthalpy. The interaction of S–S pair is considered to be slightly stronger than M–M, R–S and R–R pairs since the former absorbs less heat than the latter in pairwise interactions. The addition of cosolvent (DMF) is in favour of pairwise interactions of these compounds in the mixtures rich in water.

Introduction

It is well known that effective interactions between a pair of chiral objects or molecules depend greatly on their relative handedness [1], [2], [3], [4], [5]. As a basis for insights into complex biochemical processes, chiral recognition or discrimination at molecular level is an ongoing challenge to researchers in different fields [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Massive experimental and theoretical data for a series of central and simple molecules in biological systems have shown that α-amino acids, peptides, sugars, alcohols, etc. have chiral discrimination effects in favor of homochirality [19], [20], [21], [22], [23], [24], [25], which is considered to be associated closely with the origin of life.

Following the framework of McMillan–Mayer theory [26], Castronuovo and her coworkers [27], [28], [29], [30], [31], [32], [33], [34] have investigated chiral recognition of some small compounds of biological interest like α-amino acids and short peptides in pure water and aqueous solutions of HCl, urea and ethanol, etc. by microcalorimetry. We have also carried out microcalorimetric investigations on chiral self-recognition of some small enantiomeric compounds in aqueous solutions containing highly polar cosolvents such as dimethylformamide (DMF) and dimethylsulfoxide (DMSO), and have found that the pairwise interaction between two righty molecules are usually stronger than that between two lefty ones [35], [36], [37], [38], [39]. In the present work, we focus our interest on homotactic enthalpic pairwise interactions of glycidol enantiomers and their racemate in (DMF + water) mixtures rich in water. Glycidol, also known as oxiranemethanol, bears a rigid oxirane ring (scheme 1). We hope that the selection of these compounds help us gain fresh insight into the effects of geometry and surroundings of chiral center on energetics of chiral self-recognition.