The linear dichroism of oriented helical and superhelical polymers.

Abstract

AbstractExpressions are derived to relate the linear dichroism of oriented helical and super‐helical polymers to the vector equation describing a transition moment in a reference frame based on the position of a single monomer unit in a simple helix and the super‐helix tilt angle β. The reduced dichroism of a perfectly oriented superhelical polymer, (Δε/ε)0, is related to the reduced dichroism of a perfectly oriented helical polymer, (Δε/ε)1, by the expression (Δε/ε)0 = ½ (Δε/ε)1(3 cos2 β − 1). Hence the linear dichroism of a helical polymer can be significantly altered by supercoiling. The application of the general expressions is illustrated by the derivation of specific equations which relate the dichroism of helical and superhelical DNA to the tilt and twist of the DNA bases, the directions of the transition moments within the base planes, and the superhelix tilt angle. Calculations of the reduced dichroism of perfectly oriented, non‐super‐helical DNA in the A and B configurations suggest that it will be difficult to distinguish between these two forms on the basis of dichroism measurements.