Structure of Condensed Phase Peptides: Insights from Vibrational Circular Dichroism and Raman Optical Activity Techniques.

Peptides and proteins are naturally chiral molecular systems so that sensing their structure and conformation with chirality-based spectral methods is an obvious and long-used diagnostic application. Extending chiroptical techniques to measurement of vibrational transitions, in the form of vibrational circular dichroism (VCD) and Raman optical activity (ROA), expands the number and types of excitations available that might provide structural insight and can provide an alternate and, in some cases, a more distinctive conformational probe. Since the dominant repeating structural element in peptides is the locally achiral amide group, VCD senses the polymeric structure through amide coupling, which is directly dependent on secondary structure. Determination of the type and relative contribution of these structural components through empirical correlation with spectral character has been the main application of VCD for peptides and proteins, although this is now reinforced by extensive theoretical modeling. Monitoring structural and conformational change induced by environmental perturbations provides another important application. More recently, VCD has been used to detect morphological variations in fibril states of aggregated peptides and proteins. ROA has parallel secondary structural sensitivities, with more applications for proteins than peptides, and has more sensitivity to local configuration and side chains. This review covers the range of peptide studies done with VCD and extends them to compare with example protein and ROA applications.