Comparative CD and X-ray diffraction studies of DNA compact particules which were obtained in PEG-containing water-salt solutions, have been carried out. Compact particles, formed from native DNA, produce a psi CD spectrum (characterized by a negative band at lambda-270 nm) and a small-angle X-ray diffraction pattern, which shows two reflections: I at 34-40 A and II at 80-90 A (together with its second-order reflection). Compact particules, formed from DNA molecules with partially disordered secondary structure, do not produce the psi CD spectrum and the reflection I, while the reflection II remains unchanged. It is suggested that the spacing of 34-40 A is associated with a side-by-side packing of DNA fragments in "microcrystallization' regions in compact particules and that such "microcrystallization' accounts for the generation of the psi CD spectrum.

SynopsisBased on equilibrium binding studies, as well as on kinetic investigations, two types of interactions of Cu2+ ions with native DNA a t low ionic strength could be characterized, namely, a nondenaturing and a denaturing complex formation. During a fast nondenaturing complex formation a t low relative ligand concentrations and a t low temperatures, different binding sites at the DNA bases become occupied by the metal ions. This type of interaction includes chelate formation of Cu2+ ions with atoms N(7) of purine bases and the oxygens of the corresponding phosphate groups, chelation between atoms N(7) and 0 of C(6) of the guanine bases, as well as the formation of specific interstrand crosslink complexes a t adjacent G.C pairs of the sequence dGpC. CD spectra of the resulting nondenatured complex (DNA-CU~+),,~ may be interpreted in terms of a conformational change of DNA from the B-form to a C-like form on ligand binding. A slow cooperative denaturing complex formation occurs a t increased copper concentrations and/or a t increased temperatures. The uv absorption and CD spectra of the resulting complex, (DNA-Cu2+)denat, indicate DNA denaturation during this type of interaction. Such a conclusion is confirmed by microcalorimetric measurements, which show that the reaction consumes nearly the same amount of heat as acid denaturation of DNA.From these and the kinetic results, the following mechanism for the denaturing action of the ligands is suggested: binding of Cu2+ ions to atoms N(3) of the cytosine bases takes place when the cytosines come to the outside of the double helix as a result of statistical fluctuations. After the completion of the binding process, the bases cannot return to their initial positions, and thus local denaturation at the G-C pairs is brought about. The probability of the necessary fluctuations occurring is increased by chelation of Cu2+ ions between atoms N(7) and 0 of C(6) of the guanine bases during nondenaturing complex formation, which loosens one of the hydrogen bonds within the G-C pairs, as well as by raising the temperature. The implications of the new binding model, which comprises both the sequence-specific interstand crosslinks and the described mechanism of denaturing complex formation, are discussed and some predictions are made. The model is also used to explain the different renaturation properties of the denatured complexes of Cu2+, Cd2+, and Zn2+ ions with DNA.In temperature-jump experiments with the nondenatured complex (DNA-CU~+),,~, a specific kinetic effect is observed, namely, the appearance of a lag in the response to the per-

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The formation of liquid crystalline microphases (0.3 M NaC104, and 120 and 170 mg PEG/ml) from lowMr DNA (salmon sperm) complexed with cis and trans dichlorodiamine-platinum(I1) was investigated. It was shown that the amplitude of the negative band in the CD spectrum, characteristic of a liquid crystalline microphase of DNA, decreased upon complexing with platinum compounds. It was estimated that the influence of cis Pt(I1) on the optical properties of liquid crystalline microphase of DNA molecules strongly differed from the effect of trans Pt(I1); the phenomenon did not depend on [PEG]. The reasons of the decrease of the negative band in the CD spectra of the DNA liquid crystalline microphases are discussed.

Some peculiarities of compactization of double-stranded DNA molecules containing methylated nitrogen bases have been studied in water-salt solutions of PEG. It is shown that the methylation of N7-atoms of guanyl residues in original DNA molecules does not prevent the formation of DNA compact particles, but results in a decrease of the amplitude of the negative band in the CD spectrum of compact particles. The influence of N7-guanine methylation on the shape of the CD spectrum being the greater, the lower is the concentration of PEG. The dependence of the negative band amplitude in the CD spectrum on the content of methylated guanyl residues is practically the same for low-molecular weight DNA's from different sources. The observed decrease in the negative band amplitude is interpreted as a result of alterration of guanyl residue orientation relative to the helix axis which leads to diminished optical activity of the "microcrystalline" domains of compact particles. The evidence obtained suggests that changes in the secondary structure of DNA lead to considerable difference between CD spectra of compact particles of methlated DNA and psi-form of DNA. (The changes in the CD spectrum of the DNA compact particles occur also as a result of methylation of C5-atoms of cytosine residues). It is suggested that the negative band in the CD spectrum can be used a criterion for detection of negligible alterations in the DNA secondary structure.