Gas-phase ionic syntheses of amino acids: beta versus alpha.
Both theoretical and experimental studies are reported for the gas-phase reactions of protonated hydroxylamine with acetic and propanoic acids which yield protonated glycine and alanine, GlyH+ and AlaH+, respectively. The key step for these reactions is an insertion of the amino group into a C-H bond. For the formation of AlaH+, the reaction barrier for insertion into a Cbeta-H bond is ca. 5 kcal.mol-1 lower than that for the insertion into a Calpha-H bond; the product beta-AlaH+ is ca. 6 kcal mol-1 lower in energy than alpha-AlaH+. Thus, both kinetics and thermodynamics favor formation of the beta-form. The energetic preference for the beta-form is due to more efficient hydrogen bonding between the amino group and the carbonyl oxygen in the limiting transition structure and in the beta-AlaH+ product. These theoretical results are in excellent accord with selected ion flow tube measurements of the gas-phase synthesis which show striking specificity for the beta-isomer according to multi-collision-induced dissociation of the AlaH+ product ion. The results suggest that Gly and beta-Ala found in carbonaceous chondrite meteorites are products of interstellar chemistry.
Snow
Jamie L
JL
Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia, Canada.
Orlova
Galina
G
Blagojevic
Voislav
V
Bohme
Diethard K
DK
D019342:Acetic Acid / Q000737:chemistry; D000596:Amino Acids / Q000138:chemical synthesis* / Q000737:chemistry; D005740:Gases; D005998:Glycine / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D006898:Hydroxylamines / Q000138:chemical synthesis* / Q000737:chemistry; D008958:Models, Molecular; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry; D011422:Propionates / Q000737:chemistry; D011522:Protons; D013816:Thermodynamics
D000596:Amino Acids; D005740:Gases; D006898:Hydroxylamines; D010455:Peptides; D011422:Propionates; D011522:Protons; D019342:Acetic Acid; D005998:Glycine; C006368:aceturic acid