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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 6
BKCSDE 29(6)
June 20, 2008 

Population and Interconversion of Neutral and Zwitterionic Forms of L-Alanine in Solution
Young Kee Kang*, Byung Jin Byun, Yong Hyun Kim, Yun Ho Kim, Dong Hwa Lee, Joo Yun Lee
L-Alanine, Population, Interconversion, Solvent effects
The conformational study on neutral and zwitterionic L-alanines (N-Ala and Z-Ala, respectively) and the transition state (TS) for their interconversion is carried out using ab initio HF and density functional B3LYP methods with the self-consistent reaction field method in the gas phase and in solution. At both the HF and B3LYP levels of theory, the local minimum N1 for N-Ala is found to be most preferred in the gas phase and a weak asymmetric bifurcated hydrogen bond between the amino hydrogens and the carbonyl oxygen appears to play a role in stabilizing this conformation. The local minima N2a and N2b are found to be the second preferred conformations, which seem to be stabilized by a hydrogen bond between the amino nitrogen and the carboxylic hydrogen. The relative stability of the local minimum N2b is remarkably increased in solution than that in the gas phase. The local minimum N2b becomes more stable than the local minimum N2a in most of the solution. On the whole the relative free energies of Z-Ala and TS become more lowered, as the solvent polarity increases. N-Ala prevails over Z-Ala in aprotic solutions but Z-Ala is dominantly populated in ethanol and water. In aprotic solutions, the population of Z-Ala increases somewhat with the increase of solvent polarity. The barrier to Z-Ala-to-N-Ala interconversion increases on the whole with the increase of solvent polarity, which is caused by the increase of stability for Z-Ala.
1149 - 1156
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