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.