Spectral Properties of “Disordered” and Polyproline II Structures Defined by Circular Dichroism Spectroscopy

Circular dichroism spectroscopy is a well-used method for studying the secondary structure of proteins in solution. Synchrotron radiation circular dichroism (SRCD) offers the advantage of improving accuracy, due to the additional low wavelength data that can be measured.Intrinsically disordered proteins (IDPs), which lack significant amounts of their structure in canonical secondary structures such as helixes, sheets, or turns, are an important class of protein that have not, to date, been accurately analysed by circular dichroism spectroscopy because of the lack of an appropriate reference data base of proteins available for empirical analyses. Furthermore, the “other” category, often considered to represent disordered structures generally also includes polyproline-II (PPII) structures, because until now their spectral properties have not been well-distinguished from unordered structures.In this study, SRCD spectroscopy was used to characterize the spectral properties of PPII structures, using collagen and polyproline as representatives of this type of structure. The spectra of the polypeptides were obtained under “native” and a variety of denaturing conditions, such as thermal unfolding and chemical unfolding (by the presence of chaotropic or reducing agents, pH or detergents, and dehydration), with the aim of defining the differences between polyproline and disordered structures.The single most characteristic spectroscopic feature distinguishing the spectra of polyproline and unordered structures was found to be a positive peak around 220 nm in the former but not in the latter. As a consequence this data is being used, in addition to new spectra on IDPs to create a improved reference data set for the empirical analyses of IDPs, and PPII structures.Supported by: BBSRC, and CNPq grants and beamtime grants from ANKA, ISA, BSRF, and Soleil synchrotrons (to BAW and/or JLSL).