The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues.
The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
R M
RM
Moision
P B
PB
Armentrout
D002412:Cations / Q000737:chemistry; D006860:Hydrogen Bonding; D008024:Ligands; D008672:Metals, Alkali / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D011392:Proline / Q000737:chemistry*
D002412:Cations; D008024:Ligands; D008672:Metals, Alkali; D009942:Organometallic Compounds; D011392:Proline