Vibrational Circular Dichroism of Matrix-Assisted Amino Acid Films in the Mid-Infrared Region

Abstract

Vibrational circular dichroism (VCD) spectra in the mid-infrared region of amino acid films are reported here for the first time. Amino acid films are formed from aqueous solutions with α-cyclodextrin (CD) serving as the matrix to facilitate the film formation. This film method eliminates the strong interfering water absorption seen in the solution study and makes it easier to measure the VCD in the 1800–1200 cm⁻¹ region. VCD spectra for films of six amino acids, L-alanine, L-proline, L-methionine, L-histidine, L-phenylalanine, and L-tryptophan, are obtained. For amino acids with low solubility (L-phenylalanine and L-tryptophan), VCD could not be measured in solution, so the film method is the only means of obtaining the VCD spectra for such amino acids. For amino acids with moderate solubility (L-alanine, L-proline, L-methionine, and L-histidine), VCD spectra are also obtained in the solution state and compared with their corresponding spectra in the film state. A good correlation is found between the film and solution spectra for both absorption and VCD. The VCD spectra of L-methionine, L-histidine, L-phenylalanine, and L-tryptophan are reported here in the mid-infrared region for the first time. The present study broadens the application range of the VCD technique and enhances its role in the detection and analysis of biologically important compounds.