title: Proton migration and tautomerism in protonated triglycine.
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 b(2) 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 b(2) 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.
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3.
authors.forename: C F
authors.initials: CF
authors.lastname: Rodriquez
authors.forename: A
authors.initials: A
authors.lastname: Cunje
authors.forename: T
authors.initials: T
authors.lastname: Shoeib
authors.forename: I K
authors.initials: IK
authors.lastname: Chu
authors.forename: A C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: K W
authors.initials: KW
authors.lastname: Siu
mesh_terms: D004735:Energy Transfer; D007536:Isomerism; D009842:Oligopeptides / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D011522:Protons; D011789:Quantum Theory
chemical_list: D009842:Oligopeptides; D011522:Protons; C035647:glycyl-glycyl-glycine