The measurement of proton relaxation time & in a series of purine and pyrimidine 5'-nucleotides has been carried out to investigate their conformation in dilute neutral aqueous solutions.The interpretation of relaxation data has been performed with the help of computer calculations taking into account the different conformers of the ribose ring and of the exocyclic group. It has been found that all nucleotides under study show nearly the same preferential orientations of the base defined by Yo = 70" 10" in the syn range. A more elaborate treatment, using an angular distribution derived from calculated potentials on 5'-GMP gives theoretical relaxation times in close agreement with experimental ones.
SynopsisT h e preferential orientations of the purine hases in dinucleoside monophosphates snch as ApA, ApG. and GpA in 10-2M neutral aqueous solutions have heen investigated hy proton relaxation a t 250 MHz. These orientations are deduced from computer simulations of the magnetization recovery curves following a 180° nonselective pulse. T h e distances between the H(8) proton of a base and the ribose ring protons which are used in these calculations are obtained by minimization as a function of the glycosyl torsion angle T of the standard deviation between the isotropic reorientation correlation times rn derived from the relaxation rates of these protons. T h e average H(1') -H(8) distance obtained hy this procedure may he readily verified from the reduction of the H(1') relaxation rate when H(8) is substituted by a deuteron. T h e limits of validity of the assumption o f a single correlation time rH governing t h e proton relaxation have been estimated, taking into account several possible internal motions, e.g., the rotation of the base, of the methylene exocyclic group and the N .: S interconversion of the ribose ring. For lo-'" < T R < 2 X 10-"'sec, it appears that the influence of these motions on the proton relaxation becomes perceptible when the jump rates among equilibrium positions exceed ca. loy sec-l T h e whole of the experimental results show that for the ribose ring N conformer, the orientation of the bases is found in the ranges GOo < T < 80' ( s y n ) and 1x0' < T < 210' ( a n t i ) . For ribose S conformer, it is observed t h a t this orient.ation is mainly s?.n with 5' < T < 90'.
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