Electric dichroism and birefringence of poly(ribocytidylic acid). I. Optical properties and stability

Measurements of the uv absorption and melting profiles of poly(rC) at low‐salt conditions confirm the existence of a double‐stranded complex at low pH and imply, but do not prove, the structure proposed by Akinrimisi et al. (1963) Biochemistry 2, 340–343. Electric dichroism and birefringence provide additional supporting evidence and indicate that the complex is one in which the bases are tilted considerably from the major molecular axis. The base‐stacking tendency of single‐stranded poly(rC), unlike poly(rA), is found to be insufficient to maintain a rodlike structure; the persistence length of stacked bases in poly(rC) must be very small. The existence of an out‐of‐plane transition in poly(rC) in the vicinity of 300 nm is conclusively demonstrated by both dichroism and birefringence measurements.