title: Including thermal disorder of hydrogen bonding to describe the vibrational circular dichroism spectrum of zwitterionic L-alanine in water.
abstract: The vibrational circular dichroism (VCD) spectrum of l-alanine amino acid in aqueous solution in ambient conditions has been studied. The emphasis has been placed on the inclusion of the thermal disorder of the solute-solvent hydrogen bonds that characterize the aqueous solution condition. A combined and sequential use of molecular mechanics and quantum mechanics was adopted. To calculate the average VCD spectrum, the DFT B3LYP/6-311++G(d,p) level of calculation was employed, over one-hundred configurations composed of the solute plus all water molecules making hydrogen bonds with the solute. Simplified considerations including only four explicit solvent molecules and the polarizable continuum model were also made for comparison. Considering the large number of vibration frequencies with only limited experimental results a direct comparison is presented, when possible, and in addition a statistical analysis of the calculated values was performed. The results are found to be in line with the experiment, leading to the conclusion that including thermal disorder may improve the agreement of the vibrational frequencies with experimental results, but the thermal effects may be of greater value in the calculations of the rotational strengths.
authors.affiliation: †Instituto de Química, Universidade Federal Fluminense Campus do Valonguinho, CEP 24020-141, Niterói, RJ, Brazil.
authors.forename: Ednilsom
authors.initials: E
authors.lastname: Orestes
authors.affiliation: ‡Instituto de Física, Universidade de São Paulo, CP 66318, 05914-370 São Paulo, SP, Brazil.
authors.forename: Carlos
authors.initials: C
authors.lastname: Bistafa
authors.affiliation: ‡Instituto de Física, Universidade de São Paulo, CP 66318, 05914-370 São Paulo, SP, Brazil.|§Instituto de Física, Universidade Federal da Bahia, 40210-340 Salvador, BA, Brazil.
authors.forename: Roberto
authors.initials: R
authors.lastname: Rivelino
authors.affiliation: ‡Instituto de Física, Universidade de São Paulo, CP 66318, 05914-370 São Paulo, SP, Brazil.
authors.forename: Sylvio
authors.initials: S
authors.lastname: Canuto
mesh_terms: D000409:Alanine / Q000737:chemistry*; D002942:Circular Dichroism / Q000379:methods; D006860:Hydrogen Bonding; D008958:Models, Molecular; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D012997:Solvents; D014867:Water; D000409:Alanine