[Reaction capabilities and structure of poly(rG) and poly(rG)-poly(rC) in solution by the method of the kinetics of...

Data on the kinetics of 1H greater than 3H exchange between water and C(8)H groups of guanylic residues in the poly(rG) and poly poly(rG)-poly(rC) are presented. Furthermore, optical properties (CD spectra and hyperchromism) of neutral solutions of these polymers from 20 to 100 degrees C are described. It is shown that the exchange in poly(rG) within the temperature range from 20 to 80 degrees C proceeds faster than in rGMP. Within the temperature range from 20 to 40 degrees C such an acceleration of the exchange is observed also in poly(rG)-poly(rC). According to the ylide mechanism of the exchange reaction the observed accleration of the exchanged in in C(8)H groups of guanylic residues is considered as a consequence of an increase of the positive charge at N(7) atoms. This effect is due to formation of additional hydrogen bonds in which N(7) atoms take part. The exchange in poly(rG)-poly(rG) at temperatures hihger than 75 degrees C, when these additional hydrogen bonds are absent, proceeds more slowly than in rGMP. Such picture is usual in other previously studied polynucleotides whose structure in solution is stabilized only by Watson - Crick hydrogen bonds and stacking interactions. The data obtained support a Guschelbauer's model of the four-stranded stranded poly(rG). They also indicate the posibility of associates formation in poly(rG)-poly(rC) solutions at temperature lower than 40 degrees C being stabilized by hydrogen bonds in which N(7) atoms of guanylic residues take part.