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 calcula tions. Analysis of the J-couplings, obtained from the H-1 and C-13-NMR spectra, allowed the determination of the conformation of the sugar r ings and of the 'circular' phosphate backbone. In the course of the in vestigations it was found that the Karplus-equation most recently para metrized for the COOP J-coupling constants could not account for the m easured J(C4'P) of 11.1 Hz and a new parametrization for both HCOP and CCOP coupling constants is therefore presented. Subsequent analysis o f 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 equilib rium in which they are converted between the gauche(+) and trans and b etween the trans and gauche(-) domain respectively. We show that the o ccurrence 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 favour able 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 comb ination alpha(t)/zeta(g+) for the combination gamma(t)/epsilon(g-). Th e results are discussed in relation to earlier findings obtained for c d(ApAp) and cr(GpGp), the latter molecule being a regulator of the syn thesis of cellulose in Acetobacter xylinum.