A Basic Study of the Amino Acid Residue in Protein. The Molecular Interaction Responsible for the Differentiation of Hydrocarbon Residues

Abstract

Abstract A pair of primary amines was competitively acylated with acid anhydride in water–dioxane mixed solvents. Based on the product distribution, the mode of differentiation of hydrocarbon residues in the competing substrates with a reagent was studied. In solvents with a low water content, where both reactants and products were completely soluble, no appreciable differentiation of the alkyl-chain length of alkylamines occurred. On the other hand, the differentiation of a phenyl group from an alkyl group occurred. From the dependency of the product distribution on the water content in reaction media, an attractive interaction induced by water was postulated between alkyl and phenyl groups, but no such interaction was observed between alkyl and alkyl groups. The differences in functions of the phenyl and alkyl groups in the differentiation process are discussed. In solvents with a very high water content, the acylation proceeded under heterogeneous conditions, since acid anhydrides became insoluble and provided an organic phase in the reaction mixture. In this case, the differentiation was controlled by the hydrophobicity of hydrocarbon residues of amines indexed by the partition coefficients of amines between the aqueous phase and the organic phase.