Conformers of dehydrogenated glycine isomers

Abstract

We report a comprehensive ab initio investigation of the conformers of dehydrogenated glycine radicals using the STO-3G, 3-21G, and aug-cc-pVDZ (aVDZ) basis sets and the UHF and UMP2 (H₂ N-CH-COOH and HN-CH₂ -COOH) as well as MCSCF and MRCI (H₂ N-CH₂ -COO) methods via two different conformational search strategies generating initial structures for optimizations by (a) removing H atoms from glycine conformers and (b) scanning torsional angles describing internal rotation along the CC, CN, and CO (except for H₂ N-CH₂ -COO) bonds of the radicals. We find four H₂ N-CH-COOH {In_CH , IIn_CH , IIIn_CH , IVn_CH } and seven HN-CH₂ -COOH {Ip_NH , IIp_NH , IIIn_NH , IVp_NH Vn_NH , VIp_NH , VIIp_NH } conformers with classical(adiabatic) relative energies of {0.00(0.00), 1.57(1.55), 5.25(5.03), 9.85(9.72)} and {0.00(0.00), 0.78(1.06), 1.93(2.08), 3.34(3.16), 3.39(3.29), 5.00(4.86), 9.27(8.87)} kcal/mol, respectively, obtained with UCCSD(T)-F12b/aug-cc-pVTZ(+UCCSD(T)-F12b/aVDZ ZPE correction) and four H₂ N-CH₂ -COO {Ip_COO , IIn_COO , IIIp_COO , IVn_COO } conformers with MRCI-F12+Q/aVDZ(+MRCI/aVDZ ZPE correction) energies of {0.00(0.00), 1.65(1.64), 1.78(1.75), 2.21(2.21)} kcal/mol, where n and p denote C₁ and C_s symmetry. The MRCI-F12+Q[UCCSD(T)-F12b] In_CH → Ip_NH and In_CH → Ip_COO classical(adiabatic) isomerization energies are 18.51(17.32)[21.20(20.01)] and 31.88(31.66) kcal/mol, respectively.

Keywords: ab initio; amino acids; conformational search; explicitly correlated; glycine radical conformers.