Phase diagrams for DNA crystallization systems.

Phase diagrams for several oligonucleotide duplex-spermine systems have been constructed. These diagrams characterize the duplex and spermine concentrations ranges in which crystalline precipitates are formed. All of them are wedge-like form. The slope of the upper branch of the diagram is determined by the oligonucleotide length. The position of the lower branch depends on both the nucleotide sequence and its length. The position of the lower branch depends on both the nucleotide sequence and its length. It has been shown that the addition to the system of MgCl2 and NaCl salts and MPD results in specific changes in the diagrams. A model for oligonucleotide duplex-spermine system has been suggested which explains the main characteristic features of the obtained phase diagrams. The experimental phase diagrams for the (pGpT)n (pApC)n-spermine system (n = 2,3,4) have been analyzed ion terms of this model and the values of the binding constants of spermine and Mg2+ ions binding to duplexes have been determined. It permitted to identify the complexes that precipitated in different regions of the phase diagrams under various conditions. The diagram obtained in the presence of a cobalt hexammine counterion is also considered. It has been shown that this phase diagram, in general, is similar to those obtained for the oligonucleotide duplex-spermine system.

[The use of phase diagrams in the crystallization of oligonucleotide duplexes. I. A model of crystallization of the (pGpT)n.(pApC)n + spermine system]. / Ispol'zovanie fazovykh diagramm pri kristallizatsii oligonukleotidnykh dupleksov. I. Modelirovanie povedeniia kristallizatsionnoi sistemy "(pGpT)n.(pApC)n + spermin".

A set of experimental phase diagrams revealing the region of existence of microcrystals in mixture "(pGpT)n.(pApC)n+spermine", n = 2,3,4, was obtained. All diagrams are wedge-like with the slope of the upper branch and the level of the lower one depending on the oligonucleotidd length. The presence of MPD, MgCl2 and NaCl changes the form of the diagrams in a different manner. A model explaining the peculiar features of the diagrams for mixture "oligonucleotide duplex+spermine" is proposed. The analysis of the diagrams was carried out on the basis of this model and the values of the binding constants for binding of spermine and Mg2+ to duplexes were estimated. Some conclusions about the types of complexes, which may form microcrystals in different regions of diagrams were made.