Dr. Applequist and his group have gained a theoretical understanding of certain optical properties that are useful in studying structures of biological molecules.
The diagram of the chiral molecule CHFClBr illustrates the theory as it applies to the phenomena of optical rotation and circular dichroism. The arrows show the dipole moments induced in each atom by the electric field E of the circularly polarized light wave. These induced moments are calculated from the atom polarizabilities and the interactions between neighboring induced moments. The model predicts that the molecule is more polarizable for left-circular light than right-circular light, which would lead to positive optical rotation and positive circular dichroism.
Polypeptides and proteins show ultraviolet absorption and circular dichroic spectra that are sensitive to conformation. Some examples are shown below for the globular proteins myoglobin, porin, and lactate dehydrogenase. The structures found by X-ray crystallography are shown at the left. The solid curves are the ultraviolet circular dichroic spectra calculated by the model described above. The circles show the experimental spectra measured in solution. The comparison shows that the model is a reasonable description of the origin of the spectra in the band system centered at about 200 nm. These studies provide information on the sensitivity of circular dichroism to distortions of backbone structures and to interactions between regions of the folded molecule. They also provide new information on the contributions of various conformational components of the protein to the observed spectra.
Click here to obtain the program package for calculating peptide and protein spectra.