Circular dichroism study of the conformation of ultraviolet-irradiated ribonuclease A

RNAase A irradiated by ultraviolet light at 254 nm shows a linear dependence between loss of activity and destruction of cystine. At least one of the cystine modified forms in irradiated RNAase is catalytically active. Circular dichroism spectra of irradiated RNAase show a marked decrease in ellipticity between 210 nm and 230 nm, an increased ellipticity between 230 nm and 240 nm, and a blue shift of the 210-nm minimum toward 205 nm. These circular dichroism changes indicate a pariial disorganization of the native secondary and tertiary changes with irradiation. The temperature dependency of the circular dichroism shows the irradiated enzyme to be conformationally less stable to thermal perturbation than native RNAase. Differences in the polypeptide conformations of unirradiated RNAase denatured by heat and sodium dodecylsulfate, and irradiated RNAase treated with heat and sodium dodecylsulfate are discussed.