Fourier-transform nuclear magnetic resonance studies of the effects of 2-chloroethanol on the association of N-acetyl-L-amino acid N′,N′-dimethylamides in aqueous solutions

Abstract

The effects of 2-chloroethanol (ClEtOH) on the self-association of three N-acetyl-L-amino acid N′,N′-dimethylamides (Val, Leu, Phe as amino acids) have been studied in water–deuterium oxide solutions by ¹H and ¹³C F.t.n.m.r. These amides were found to self-associate through hydrogen bonding to a small extent. Hydrogen-bonding association of the amides takes place when [ClEtOH]/[amide] > 1. The hydroxyl proton peak of water–ClEtOH mixtures shifts to high field regardless of the presence of the amide owing to the disruption of water structure by ClEtOH. The decrease in the hydrogen-bonding capability of the solvent leads to the hydrogen-bonding association of the amide and ClEtOH induces the hydrogen-bonding association of the amides by decreasing the hydrogen-bonding capability of the solvent around the amides.

An apparent dissociation of the amide complexes was found to occur at low concentrations of ClEtOH. This is attributed to the hydrophobic interaction between ClEtOH and the alkyl part of the amides to minimize the energy loss due to the disruption of water structure, and the interaction results in a further decrease in the hydrogen-bonding capability of the solvent around the amides and leads to the start of hydrogen-bonding association of the amides.