Semistiff polymer model of unfolded proteins and its application to NMR residual dipolar couplings

We present a new statistical model of unfolded proteins in which the stiffness of polypeptide backbone is taken into account. We construct and solve a mean field equation which has the form of a diffusion equation and derive the distribution function for conformations of unfolded polypeptides. Accounting for the stiffness of the protein backbone results in a more accurate description of general properties of a polypeptide chain, such as its gyration radius. We then use the distribution function of a semistiff protein within a previously developed theoretical framework [J. Biomol. NMR 39, 1 (2007)] to determine the nuclear magnetic resonance (NMR) residual dipolar couplings (RDCs) in unfolded proteins. The calculated RDC profiles (dependence of the RDC value on the residue number) exhibit a more prominent bell-like shape and a better agreement with experimental data as compared to the previous results obtained with the random flights chain model. PACS. 87.10.-e General theory and mathematical aspects – 82.56.Pp NMR of biomolecules – 82.56.Dj