Proton Migration and Tautomerism in Protonated Triglycine

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Abstract
Proton migration in protonated glycylglycylglycine (GGG) has been investigated by using density functional theory at the B3LYP/6-31++G(d,p) level of theory. On the protonated GGG energy hypersurface 19 critical points have been characterized, 11 as minima and 8 as first-order saddle points. Transition state structures for interconversion between eight of these minima are reported, starting from a structure in which there is protonation at the amino nitrogen of the N-terminal glycyl residue following the migration of the proton until there is fragmentation into protonated 2-aminomethyl-5-oxazolone (the b2 ion) and glycine. Individual free energy barriers are small, ranging from 4.3 to 18.1 kcal mol-1. The most favorable site of protonation on GGG is the carbonyl oxygen of the N-terminal residue. This isomer is stabilized by a hydrogen bond of the type O−H···N with the N-terminal nitrogen atom, resulting in a compact five-membered ring. Another oxygen-protonated isomer with hydrogen bonding of the type O−H···O, resulting in a seven-membered ring, is only 0.1 kcal mol-1 higher in free energy. Protonation on the N-terminal nitrogen atom produces an isomer that is about 1 kcal mol-1 higher in free energy than isomers resulting from protonation on the carbonyl oxygen of the N-terminal residue. The calculated energy barrier to generate the b2 ion from protonated GGG is 32.5 kcal mol-1 via TS(6→7). The calculated basicity and proton affinity of GGG from our results are 216.3 and 223.8 kcal mol-1, respectively. These values are 3−4 kcal mol-1 lower than those from previous calculations and are in excellent agreement with recently revised experimental values.

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