title: Quantum chemical studies of a model for peptide bond formation.  3.  Role of magnesium cation in formation of amide and water from ammonia and glycine.
abstract: The SN2 reaction between glycine and ammonia molecules with magnesium cation Mg2+ as a catalyst has been studied as a model reaction for Mg(2+)-catalyzed peptide bond formation using the ab initio Hartree-Fock molecular orbital method.  As in previous studies of the uncatalyzed and amine-catalyzed reactions between glycine and ammonia, two reaction mechanisms have been examined, i.e., a two-step and a concerted reaction.  The stationary points of each reaction including intermediate and transition states have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics of each reaction.  Substantial decreases in free energies of activation were found for both reaction mechanisms in the Mg(2+)-catalyzed amide bond formation compared with those in the uncatalyzed and amine-catalyzed amide bond formation.  The catalytic effect of the Mg2+ cation is to stabilize both the transition states and intermediate, and it is attributed to the neutralization of the developing negative charge on the electrophile and formation of a conformationally flexible nonplanar five-membered chelate ring structure.
authors.affiliation: The Rockefeller University, Molecular Theory Laboratory, Palo Alto, California 94304, USA.
authors.forename: T
authors.initials: T
authors.lastname: Oie
authors.forename: G H
authors.initials: GH
authors.lastname: Loew
authors.forename: S K
authors.initials: SK
authors.lastname: Burt
authors.forename: R D
authors.initials: RD
authors.lastname: MacElroy
mesh_terms: D000577:Amides / Q000138:chemical synthesis* / Q000737:chemistry; D000641:Ammonia / Q000737:chemistry*; D002384:Catalysis; D002412:Cations / Q000737:chemistry; D019418:Evolution, Chemical; D019143:Evolution, Molecular; D005998:Glycine / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D000069497:Origin of Life; D010455:Peptides / Q000138:chemical synthesis* / Q000737:chemistry; D011789:Quantum Theory
chemical_list: D000577:Amides; D002412:Cations; D010455:Peptides; D000641:Ammonia; D008274:Magnesium; D005998:Glycine