Acid–base thermochemistry of gaseous oxygen and sulfur substituted amino acids (Ser, Thr, Cys, Met)

摘要: Acid–base thermochemistry of isolated amino acids containing oxygen or sulfur in their side chain (serine, threonine, cysteine and methionine) have been examined by quantum chemical computations. Density functional theory (DFT) was used, with B3LYP, B97-D M06-2X functionals using the 6-31+G(d,p) basis set for geometry optimizations larger 6-311++G(3df,2p) energy Composite methods CBS-QB3, G3B3, G4MP2 G4 were applied to large sets neutral, protonated deprotonated conformers. Conformational analysis these species, based on approach AMOEBA force field calculations, has used identify lowest conformers estimate population expected be present at thermal equilibrium 298 K. It is observed that G4, G4MP2, CBS-QB3 composite DFT lead similar conformer energies. Thermochemical parameters computed either most stable populations Comparison experimental theoretical proton affinities ΔacidH shows method provides better agreement deviations less than 1.5 kJ mol−1. From this point view, a evaluated thermochemical quantities serine, methionine may proposed: PA = 912, 919, 903, 938; GB 878, 886, 870, 899; 1393, 1391, 1396, 1411; ΔacidG 1363, 1362, 1367, 1382 This study also confirms non-negligible ΔpS° associated protonation acidic hydrogen gas phase SH group. In several instances new identified thus suggesting re-examination IRMPD spectra.