Benchmark Ab Initio Determination of the Conformers, Proton Affinities, and Gas-Phase Basicities of Cysteine

A systematic conformational mapping combined with literature
data
leads to 85 stable neutral cysteine conformers. The implementation
of the same mapping process for the protonated counterparts reveals
21 N-(amino-), 64 O-(carbonyl-), and 37 S-(thiol-)protonated cysteine
conformers. Their relative energies and harmonic vibrational frequencies
are given at the MP2/aug-cc-pVDZ level of theory. Further benchmark
ab initio computations are performed for the 10 lowest-lying neutral
and protonated amino acid conformers (for each type) such as CCSD(T)-F12a/cc-pVDZ-F12
geometry optimizations (and frequency computations for cysteine) as
well as auxiliary correction computations of the basis set effects
up to CCSD(T)-F12b/cc-pVQZ-F12, electron correlation effects up to
CCSDT(Q), core correlation effects, second-order Douglass–Kroll
relativistic effects, and zero-point energy contributions. Boltzmann-averaged
0 (298.15) K proton affinity and [298.15 K gas-phase basicity] values
of cysteine are predicted to be 214.96 (216.39) [208.21], 201.83 (203.55)
[194.16], and 193.31 (194.74) [186.40] kcal/mol for N-, O-, and S-protonation,
respectively, also considering the previously described auxiliary
corrections.