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 (Δν_1/2) 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 (M_w) 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 M_w 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.