Conformational state and molecular ionic transformations of polycytidyl acid immobilized in multilayer Langmuir- and polyelectrolyte films

Multilayer films of complexes of polycytidylic acid with dioctadecyldimetyl ammonium were obtained by the Langmuir-Blodgett method (LB films), and comp lexes of poly(C) with polycations (poly-L-lysisne, polyethyleneimine, polya llylamine) were obtained by the method of alternate adsorption (polyionic a ssembly) from solutions of oppositely charged polyelectrolytes on the solid carrier (SA films). It was shown that poly(C) exists in SA films in a sing le-stranded state irrespective of whether in the starting solution it occur red in the single-stranded nonprotonated or double-stranded protonated conf ormation. Conversely, in the LB film poly(C) preferred to be in a double pr otonated conformation. UV-spectra of water-insoluble LB and SA films at dif ferent pH values of surrounding water medium were investigated. Proton titr ation curves of poly(C) immobilized in LB films were obtained. The analysis of the shape of titration curves showed that the molecular-ionic transform ation of poly(C) in LB films is accompanied by both the conformational tran sition of the polynucleotide and the molecular rearrangement in the whole f ilm. Poly(C) was found to transform from the double- to single-stranded sta te and vice versa in the <<deprotonation-protonation>> cycle of LB film due to cooperative release/binding of hydrogen ions by cytosine bases. In cont rast, poly(C) <<protonation-deprotonation>> in SA films occurred without co nformational transitions of the polynucleotide. As opposed to poly(C) in so lution a rather big hysteresis of forward and back titration curves was fou nd for both types of multilayer films, indicating molecular rearrangements in films. The reason for the structural transformations of poly(C) upon fab rication of LB or SA films and the mechanism of molecular ionic transformat ions of poly(C) in films art: discussed ill terms of a simple model of ion exchange. An assumption about the nature of structural transformations of L B and SA films during their protonation-deprotonation is put forward.