title: First principle computational study on the full conformational space of L-proline diamides.
abstract: Ab initio molecular orbital computations were carried out at three levels of theory: RHF/3-21G, RHF/6-31G(d), and B3LYP/6-31G(d), on four model systems of the amino acid proline, HCO-Pro-NH2 [I], HCO-Pro-NH-Me [II], MeCO-Pro-NH2 [III], and MeCO-Pro-NH-Me [IV], representing a systematic variation in the protecting N- and C-terminal groups. Three previously located backbone conformations, gammaL, epsilonL, and alphaL, were characterized together with two ring-puckered forms syn (gauche+ = g+) or "DOWN" and anti (gauche- = g-) or "UP", as well as trans-trans, trans-cis, cis-trans, and cis-cis peptide bond isomers. The topologies of the conformational potential energy cross-sections (PECS) of the potential energy hypersurfaces (PEHS) for compounds [I]-[IV] were explored and analyzed in terms of potential energy curves (PEC), and HCO-Pro-NH2 [I] was also analyzed in terms of potential energy surfaces (PESs). Thermodynamic functions were also calculated for HCO-Pro-NH2 [I] at the CBS-4M and G3MP2 levels of theory. The study confirms that the use of the simplest model, compound [I] with P(N) = P(C) = H, along with the RHF/3-21G level of theory, is an acceptable practice for the analysis of peptide models because only minor differences in geometry and stability are observed.
authors.affiliation: Department of Medical Biophysics, University of Toronto, Ontario Cancer Institute, Princess Margaret Hospital, 610 University Avenue, Toronto, Ontario, Canada M5G 2M9. michelle.sahai@utoronto.ca
authors.forename: Michelle A
authors.initials: MA
authors.lastname: Sahai
authors.forename: Tara A K
authors.initials: TA
authors.lastname: Kehoe
authors.forename: Joseph C P
authors.initials: JC
authors.lastname: Koo
authors.forename: David H
authors.initials: DH
authors.lastname: Setiadi
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Chass
authors.forename: Bela
authors.initials: B
authors.lastname: Viskolcz
authors.forename: Botond
authors.initials: B
authors.lastname: Penke
authors.forename: Emil F
authors.initials: EF
authors.lastname: Pai
authors.forename: Imre G
authors.initials: IG
authors.lastname: Csizmadia
mesh_terms: D003198:Computer Simulation; D003958:Diamide / Q000031:analogs & derivatives* / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D003958:Diamide; D011392:Proline