Analysis of circular dichroism spectra.

Publisher Summary The chapter discusses mathematical methods for the analysis of circular dichroism (CD) spectra. Circular dichroism (CD) is the difference in absorption between left and right circularly polarized light so that CD spectral bands occur wherever there are normal electronic absorption bands in an asymmetric molecule. The chapter focuses on electronic absorption—that is, the spectra that can be measured for a molecule undergoing a transition from its ground electronic state to some higher energy electronic state. Most biological molecules contain a number of electronic units that absorb light nearly independently, called “chromophores,” which are asymmetrically disposed in space. Such asymmetric molecules will absorb left circularly polarized light differently from right circularly polarized light. Thus, CD spectra are usually interpreted empirically, and the chapter discusses two mathematical methods that will help in empirical analysis. Taylor series fitting separates the two distinct types of interactions between chromophores that give rise to CD spectra. For oligomers and polymers, this separation gives insight into the type of secondary structure, facilitates comparison among different molecules, or provides an accurate measure of the changes in a specific molecule when some parameter is altered. Taylor series fitting has been applied primarily to oligo- and polynucleotides as a function of solution conditions. Singular value decomposition (SVD) is an extremely powerful method that can be applied to any series of any type of spectra.