Gaseous Arginine Conformers and Their Unique Intramolecular Interactions

Abstract

Extensive ab initio calculations were employed to characterize stable conformers of gaseous arginine, both the canonical and zwitterionic tautomers. Step-by-step geometry optimizations of possible single-bond rotamers at the B3LYP/6-31G(d), B3LYP/6-31++G(d,p), and MP2/6-31++G(d,p) levels yield numerous structures that are more stable than any known ones. The final electronic energies of the conformers were determined at the CCSD/6-31++G(d,p) level. The lowest energies of the canonical and zwitterionic structures are lower than the existing values by 2.0 and 2.3 kcal/mol, respectively. The relative energies, rotational constants, dipole moments, and harmonic frequencies of the stable conformers remain for future experimental verification. The conformational distributions at various temperatures, estimated according to thermodynamic principles, consist almost exclusively of the newly found structures. One striking feature is the occurrence of blue-shifting hydrogen bonds in all six of the most stable conformers. A unique feature of important conformations is the coexistence of dihydrogen and blue- and red-shifting hydrogen bonds. In addition to the hydrogen bonds, the stereoelectronic effects were also found to be important stabilization factors. The calculated and measured proton affinities agree within the theoretical and experimental uncertainties, affirming the high quality of our conformational search. The theoretical gas-phase basicity of 245.9 kcal/mol is also in good agreement with the experimental value of 240.6 kcal/mol. The extensive searches establish firmly that gaseous arginine exists primarily in the canonical and not the zwitterionic form.

Formation of Peptide Radical Cations (M+·) in Electron Capture Dissociation of Peptides Adducted with Group IIB Metal Ions

“…By replacing the tryptophan residue with valine residue, the ionization energy of the peptide should be increased to a value close to the ionization energy of arginine residue (i.e., I.E=9.20 eV) [62]. Figure 3a-c shows typical ECD spectra of divalent Group IIB metal ion adducted RGGGVGGGR.…”

Comparison of some representative density functional theory and wave function theory methods for the studies of amino acids

“…For example, the relative stabilities of arginine conformations by the B3LYP calculations may be in error of more than 3 kcal/mol. 21 The difficulty in choosing a proper DFT forms for the problem at hand is mainly due to (a) the testing set of systems for the construction or evaluation of a specific DFT form is not representative to the interested system; (b) current DFT forms are generally not designed to accurately treat the nonbonded interactions.…”

Proton Transfer in Biomolecules Facilitated by Water: Quantum Chemical Investigations

“…We find that the low energy conformers of Arg -H 2 O with canonical Arg core are produced mainly from the lowest energy canonical Arg, (C4) and (C5) (see Ref. [36]). The three lowest energy canonical conformers (C4-1), (C4-7), and (C5-1) are very similar in energy, to within 0.3 kcal/mol.…”