Two-dimensional vibrational circular dichroism correlation spectroscopy: pH-induced spectral changes in l-alanine

Two-dimensional correlation spectroscopy (2D-COS) has been applied to Fourier transform (FT) vibrational circular dichroism (VCD) in multiple spectral regions for the first time. FT-IR and FT-VCD measurements of l-alanine in D2O have been carried out in the mid-IR (800–2000 cm−1) region, the hydrogen-stretching region (2000–4000 cm−1), and the near-IR region (4000–6000 cm−1) as a function of pD. A variety of different 2D-COS plots are possible from the IR and VCD spectral data in these three spectral regions, and examples are presented that show particularly interesting features of these spectra. Homo-spectral-region and hetero-spectral-region VCD–VCD and IR–IR 2D-COS plots are presented for l-alanine. Relative sensitivities to spectral changes in the methine and methyl vibrational regions are elucidated by analysis of both IR and VCD 2D-COS spectra. Near-IR band assignments are facilitated by hetero-spectral-region 2D-COS spectra that correlate the combination band region to that of the underling fundamental modes. These studies demonstrate that the method of 2D-COS IR-VCD is a useful tool that provides enhanced chemical information over traditional 1D spectra.