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Low-Energy Tautomers and Conformers of Neutral and Protonated Arginine
journal contribution
posted on 2001-11-03, 00:00 authored by Janusz Rak, Piotr Skurski, Jack Simons, Maciej GutowskiThe relative stabilities of zwitterionic and canonical forms of neutral arginine and of its protonated
derivative were studied by using ab initio electronic structure methods. Trial structures were first identified at
the PM3 level of theory with use of a genetic algorithm to systematically vary geometrical parameters. Further
geometry optimizations of these structures were performed at the MP2 and B3LYP levels of theory with basis
sets of the 6-31++G** quality. The final energies were determined at the CCSD/6-31++G** level and corrected
for thermal effects determined at the B3LYP level. Two new nonzwitterionic structures of the neutral were
identified, and one of them is the lowest energy structure found so far. The five lowest energy structures of
neutral arginine are all nonzwitterionic in nature and are clustered within a narrow energy range of 2.3 kcal/mol. The lowest energy zwitterion structure is less stable than the lowest nonzwitterion structure by 4.0 kcal/mol. For no level of theory is a zwitterion structure suggested to be the global minimum. The calculated
proton affinity of 256.3 kcal/mol and gas-phase basicity of 247.8 kcal/mol of arginine are in reasonable agreement
with the measured values of 251.2 and 240.6 kcal/mol, respectively. The calculated vibrational characteristics
of the low-energy structures of neutral arginine provide an alternative interpretation of the IR-CRLAS spectrum
(Chapo et al. J. Am. Chem. Soc. 1998, 120, 12956−12957).
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canonical formsPM 3 levelenergy zwitterion structurealternative interpretationenergy structuresB 3LYP levelsenergy structureMP 2kcalvibrational characteristicsProtonated ArginineCCSDab initiononzwitterionic structuresargininestructure methodsgeometry optimizationsTrial structuresnonzwitterion structurebasis setsB 3LYP levelzwitterion structureenergy rangeproton affinity
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