Interaction between carboxylic acids and the peptide group

Abstract

The interaction between the peptide group and carboxylic acids is one of either protonation or hydrogen bonding. Both interactions will affect the vibrational frequency of the peptide group, resulting in changes in the characteristic amide vibrations. Infra-red spectroscopy has been used to examine the change in the amide I and II absorption bands of the model compound N-methylacetamide (NMA) when it is dissolved in deuterochloroform and various deuterochloroform/carboxylic acids, such as acetic, formic, dichloroacetic and trifluoroacetic acid. The results indicate that the interaction is one of hydrogen bonding except when NMA is dissolved in the strongest acid used, trifluoroacetic acid (TFA). Studies on the interaction between TFA and the peptide group of poly( l-alanine) various poly( dl-alanines) and poly(γ-benzyl- l-glutamate) show that the hydrogen bond interaction occurs, with little evidence of protonation taking place. In poly(γ-benzyl- l-glutamate) it was found that the TFA interacted both with the peptide group and the side chain carboxyl group.