Extensive Computational Study on Conformations of Microsolvated Leucine Complexes

The conformations for leucine (Leu) hydrated with one to three water molecules, Leu-(H₂O)_n (n=1-3), were carefully searched by considering the trial structures generated by all possible combinations of rotamers of Leu combined with all likely hydration modes. The structures were optimized at the BHandHLYP/6-31+G* level and the single point energies were calcu-lated at the BHandHLYP/6-311++G** level. Good correspondence between the conforma-tions of Leu-(H₂O)_n and bare Leu is found, showing that the conformations of Leu-(H₂O)_nmay be effciently and reliably determined by the hydration of Leu conformers. The simu-lated IR spectra of canonical and zwitterionic conformers of Leu-(H₂O)_n are compared with the experimental result of Leu in aqueous solution. The IR spectrum of zwitterionic Leu-(H₂O)₃ provides the best description of the experiment. The result demonstrates that the IR spectrum of solute in solution may be simulated by the solute hydrated with an adequate number of water molecules in the gas phase.