Circular Dichroism of Polymers: Theory and Practice

Abstract

The goal of understanding the circular dichroism (CD) of a polymer is to be able to interpret the measured spectrum in terms of the conformation of the polymer. We want to know if the polymer is rigid or flexible. Is it a helix? What are the helical parameters? To deduce a polymer conformation from a measured CD we must calculate the CD for different assumed conformations. When we get agreement between calculation and experiment, we say this is a possible conformation. There may be other conformations which are also consistent.

Applications to polypeptides and nucleic acids will be discussed.

A new method which is more specific than circular dichroism is fluorescence detected circular dichroism (FDCD). In this method the intensity of light emitted by a fluorescent chromophore excited by left and right circularly polarized light is used to measure the circular dichroism. The FDCD method thus gives information about the conformation of a polymer in the immediate neighborhood of fluorescent groups.

Phenomena related to circular dichroism which may be useful for studying conformations in polymers are the circular intensity differential (CID) for scattered light. The difference in scattered intensity is measured for left and light circularly polarized incident light. At this time these effects have only been reported for small molecules. In principle, however, they may be very useful for the study of polymers.