ABSTRACT
Determination of protein secondary structure has long been a major application of optical spectroscopic studies of biopolymers (1-3). In most cases these efforts have been aimed at evaluating the average fractional amount of helix and sheet contributions to the overall secondary structure in a protein or peptide. In some cases further interpretations in terms of turns and specific helix and sheet segment types have developed. This focus on average secondary structure is a consequence of the interactions of primary importance to these techniques and their relatively low resolution, which ordinarily does not provide site-specific information with out technologically challenging selective isotopic substitution. Such a limit is in contrast to x-ray crystallography and nuclear magnetic resonance (NMR) spec troscopy, which naturally yield site-specific structural information, due to their very high resolution (at least for NMR), requiring at most uniform labeling. How ever, these invaluable structural biological techniques are very slow in terms of both data acquisition and completion of the complex interpretive process. Further more, the intrinsic time scales of the measurements are slow, thereby not per mitting reliable analysis of dynamic structures or of conformations undergoing fast-changing events. Only more limited applications of optical spectra to deter mination of the tertiary structure or fold of the secondary structural elements have appeared, and these have typically used fluorescence or near-UV electronic
circular dichroism (ECD) of aromatic residues to sense a change in the fold rather than to determine its nature (4,5).
