Infrared spectral, structural, and conformational studies of zwitterionic L-tryptophan

The infrared spectrum, molecular structure, and conformations of L-tryptophan in isolated zwitterionic form have been studied by means of a recently developed infrared sampling technique and ab initio molecular orbital calculations using a nonaqueous self-consistent reaction field (SCRF). A complete mid-IR spectrum of monomeric tryptophan in zwitterionic form was obtained in a solid solvent of KBr. SCRF calculations using KBr as the continuum at the HF/6-31G(d,p) level were performed on zwitterionic tryptophan to predict its structure and vibrational frequencies. Three possible conformers were found, and one of them was identified to be predominant in the prepared sample on the basis of spectral and energy comparisons. Good agreement in terms of both frequencies and intensities was established between the calculated and observed infrared spectra. The detailed molecular structure of the predominant conformer of zwitterionic tryptophan was predicted and discussed.