Classical Theory and Calculations of Circular Dichroism in Infinite Helical Polymers.

摘要

A new theory of optical activity in helical polymers is derived. Helical symmetry and periodic boundary conditions are applied to classical polarizability theory in order to express the infinite polymer result in closed form. Result is all order in inter-monomer interactions and gives the polymer circular dichroism bandshape in terms of monomer transition bandshapes. A complex unit of symmetry containing many optical transitions is also explicitly treated. This theory is applied to the following polynucleotide sequences:poly(A),poly(T),poly(G),poly(C),poly(A . T),poly(G . C),poly((A-T) . (A-T)),poly((G-C) . (G-C)),poly(A-T),poly(G-C),poly((A-G) . (C-T)),poly((A-C) . (G-T)),poly(A-C),poly(G-T),poly(A-G),poly(C-T) in RNA,and B and C form DNA geometries. Calculations are carried out for poly((A-A-T) . (A-T-T)) and poly((A-G-C) . (G-C-T)) in RNA and B-DNA geometries,and for polyadenylic acid and polyionsinic acid in nonstandard geometries. The CD difference spectra obtained upon melting of the two double-stranded polymers are very similar. Comparison of these difference spectra with calculated ones suggests that optical transitions near 272nm (on A) and 288nm (on T) are present. Premelting changes of the CD spectrum of poly(d(A-T) . d(A-T)) are due to a change in conformation in which the secondary structure goes from C-to B-type spectrum by increasing the A-type nature of the polymer. Such a change is not observed for poly(d(A) . d(T));instead, a transition between two different B-type geometries occurs. 13figures, 34references. (ERA citation 02:014259)