Tautomerization of HKPGG: Entropic Consequences of Strong Hydrogen-Bond Networks in Peptides.

Ion mobility spectrometry-mass spectrometry and quantum chemical calculations are used to determine the structures and stabilities of the singly protonated peptide HKPGG. The two peaks making up the IMS distribution are shown to be tautomers differing by the location of the extra proton on either the lysine side chain or the N-terminus. The lysine-protonated tautomer is strongly preferred entropically while being disfavored in terms of the electronic energy and enthalpy. This relationship is shown, through comparison of all low-lying conformers of both tautomers, to be related to the strong hydrogen-bond network of the N-terminally protonated tautomer. A general relationship is demonstrated wherein stronger cross-peptide hydrogen-bond networks result in entropically disfavored conformers. Further effects of the HKPGG hydrogen-bond network are probed by computationally examining singly and doubly methylated analogues. These results demonstrate the importance of the entropic consequences of hydrogen bonds to peptide stability as well as techniques for perturbing the hydrogen-bond network and folding preferences of peptides via minimal chemical modification.