Electronic Circular Dichroism of Proteins from First-Principles Calculations

The circular dichroism (CD) spectra of 47 proteins in the far-ultraviolet have been calculated from first principles, using a parameter set derived from ab initio calculations on N -methylacetamide. These spectra agree well with experimental data, as shown by the Spearman rank correlation coefficients of 0.86, 0.80, and 0.94 between the computed and experimental intensities at 190, 208, and 220 nm, respectively. The computed spectra are most accurate for proteins that have a high α-helical content and are least accurate for a class of β-sheet-rich proteins, which have some irregular structure and are known as β-II proteins. To address the lack of resolution between the two negative peaks around 208 and 220 nm in the calculated spectra of α-helical proteins, narrower bandwidths have been explored. Other factors were investigated, including the dielectric constant of the protein, higher energy transitions of the amide chromophore, and the orientation of the ππ* electric transition dipole moment vector. Combining some of these aspects made it possible to obtain accurate spectra with the desired resolution between the negative peaks. Although not fully quantitative, the first-principles calculations of protein CD presented in this study are the most accurate reported to date.