A “hidden” role of amino and imino groups is unveiled during the micro-solvation study of three biomolecule groups in water

Abstract

The 13C longitudinal relaxation times (T1) of three biomolecule groups of major significance to proteins and cells – protein amino acids (AAs), acetyl-amino acids (Ac-AAs) and betaines – and the 14N linewidths (Δν1/2) of Ac-AAs and betaines were measured in aqueous solutions at acidic and neutral pH, by NMR spectroscopy, to estimate the effect of the molecular weight (Mw) on 13C longitudinal relaxation times and 14N linewidths, respectively. 13C relaxation times indicate that AAs and Ac-AAs strongly interact with the same number of water molecules at acidic and neutral pH, respectively, whereas both 13C and 14N results indicate that their Mw values at acidic pH (protonated, positively charged AAs and zero-charged Ac-AAs) increase relatively to those at pH 6.0 (zwitterionic AAs and negatively charged Ac-AAs) that translates into their extra hydration with an excess of one water molecule. Both 13C and 14N relaxation times revealed that betaines retain their hydration grade in both their ionization states at two pH values, while exhibiting their hydration differences from AAs and Ac-AAs and pointing out the “controlling” role of the amino and imino groups in the extra hydration of protonated AAs and Ac-AAs, enlightening the so far unknown significant role of the N-terminus and the −NH near the C-terminus in peptide solvation. Moreover, DFT calculations of the interacting water molecules through hydrogen bonds with Ala, Ac-Ala and Ala betaine molecules in their protonated and neutral pH forms are in absolute agreement with NMR results. Finally, a fully promising method arises with a view on hydration–solvation studies of oligopeptides and other bio-organic molecules by 13C relaxation.