To study the mech a nisms of cytidine and its bi o log i cally ac tive an a logues bind ing to DNA we an a lyzed the bind ing of these lig ands to DNA in the pres ence of well-known intercalator ethidium bro mide (EtBr). Thereto, we have car ried out the de tailed spec tro pho to met ric re search of EtBr-DNA mix tures ab sorp tion in the pres ence of cytidine and its an a logues in the wide range of wave lengths and DNA con cen tra tions. Cytidine de riv a tives con tain ing aza group in the cy to sine ring (6AZC, AZAfur and AZAxyl) com pete with EtBr for the DNA bind ing sites. The bind ing con stants and bind ing site sizes of ligand-DNA com plexes were cal cu lated via ab sorp tion spec tra op ti mi za tion pro grams COMP and DALSMOD. Un mod i fied in the cy tosine ring lig ands (cytidine and Ara-C) do not com pete with EtBr for the DNA bind ing sites, how ever, they con trib ute to the change in con cen tra tion de pend en cies of ti tra tion curves in the re gion of low DNA con centra tions. This phe nom e non can be ex plained by the cytidine and Ara-C in flu ence on the DNA con for ma tion in the pres ence of EtBr at low P/D EtBr val ues, where P/D EtBr is the phos phate/EtBr ra tio.

The investigation of complexation features of ethidium bromide (EB) with calf thymus DNA at the high and low ratios of biopolymer/ligand molar concentrations (P/D) was carried out using Raman spectroscopy and VIS-spectrophotometry. It was shown that EB binds to DNA with formation of two types of complexes: intercalation and exterior binding. The analysis of Raman spectra revealed that the amino groups of EB form the hydrogen bonds with acceptor atom groups of DNA in both types of complexes. A low extent of filling DNA structure by the ligand (P/D = 20) does not change the DNA B-form, while a high extent (P/D = 3) results in the conformation B-A transitio