POLY-5-METHYLDEOXYCYTIDYLIC ACID AND SOME ALKYLAMINO ANALOGS

5-Methyldeoxycytidine triphosphate and several alkylamino analog triphosphates have been polymerized with the aid of terminal deoxynucleotidyl transferase to give poly-5-methyldeoxycytidylic acid and the N4-methyl and N4-ethyl polymer analogs of 5-methyldeoxycytidylic acid. All three of these were polymerized by the enzyme at the same rate as polydeoxycytidylic acid, and the final products have about the same sedimentation constants, s20,w = 4.5 S. Poly-5-methyldeoxycytidylic acid forms an acid twin-stranded helix similar to that formed by polydeoxycytidylic acid, the 5-methyl substituent being without any appreciable effect on the stability. The alkylamino polymer analogs do not form such structures. All three new polymers are considerably more resistant than polydeoxycytidylic acid to phosphodiesterase I. Poly-5-methyldeoxycytidylic readily complexes with polydeoxyinosinic acid, and the Tm for the twin-stranded helix is 17° higher than that for the corresponding polydeoxyinosinic acid :polydeoxycytidylic acid, thus confirming the significant stabilizing effect of a pyrimidine 5-methyl substituent, even in the absence of a 2′- hydroxyl. An extensive analysis of the complexes between poly-5-methyldeoxycytidylic acid and polydeoxyinosinic acid, as a function of the ionic strength of the medium, demonstrated the existence of four equilibrium reactions between the homopolymers and the twin- and triple-stranded helices formed by them. The alkylamino polymer analogs did not complex with polydeoxyinosinic acid, probably as a result of hindered rotation of the alkylamino groups by the 5-methyl substituent. The nature of the various complexes is discussed with emphasis on the so-called acid form of poly-5-methyldeoxycytidylic acid (and polydeoxycytidylic acid) in relation to the acid form of poly-5-methylribocytidylic acid (and polyribocytidylic acid). © 1969, American Chemical Society. All rights reserved.