The solution structure of the circular trinucleotide cr(GpGpGp) determined by NMR and molecular mechanics calculation

The 3'-5' circular trinucleotide cr(GpGpGp) was studied by means of 1D and 2D high resolution NMR techniques and molecular mechanics calculations. Analysis of the J-couplings, obtained from the 1H and 13C-NMR spectra, allowed the determination of the conformation of the sugar rings and of the 'circular' phosphate backbone. In the course of the investigations it was found that the Karplus-equation most recently parametrized for the CCOP J-coupling constants could not account for the measured J(C4'P) of 11.1 Hz and a new parametrization for both HCOP and CCOP coupling constants is therefore presented. Subsequent analysis of the coupling constants yielded 'fixed' values for the torsion angles beta and delta (with beta = 178 degrees and delta = 139 degrees). The value of the latter angle corresponds to an S-type sugar conformation. The torsion angles gamma and epsilon are involved in a rapid equilibrium in which they are converted between the gauche(+) and trans and between the trans and gauche(-) domain respectively. We show that the occurrence of epsilon in the gauche(-) domain necessitates S-type sugar conformations. Given the aforementioned values for beta, gamma, delta and epsilon the ring closure constraints for the ring, formed by the phosphate backbone can only be fulfilled if alpha and zeta adopt some special values. After energy minimization with the CHARMm force field only two combinations of alpha and zeta result in energetically favourable structures, i.e. the combination alpha (t)/zeta(g-) in case gamma is in a gauche(+) and epsilon is in a trans conformation, and the combination alpha (t)/zeta (g+) for the combination gamma (t)/epsilon (g-). The results are discussed in relation to earlier findings obtained for cd(ApAp) and cr(GpGp), the latter molecule being a regulator of the synthesis of cellulose in Acetobacter xylinum.