Fig. 1. (Top) Amino acid residues in their zwitterionic form with three-letter codes along with the portion of sidechain highlighted in blue used for model system studies. (Bottom) The four HB conformations studied for both representative model systems, acetamide–methanol and acetamide–p-cresol. The anti N–H⋯O HB, syn N–H⋯O HB, ambifunctional HB, and O–H⋯O HB interactions are shown from left to right with methanol and p-cresol distinguished by the R group as indicated at bottom left. Hydrogen-bonding interactions are shown as green dotted lines, and participating electronegative atoms are colored red for oxygen and blue for nitrogen.