Dinucleosides, monophosphates, proton

Many extensive studies110-126 at 60, 100, or 220 MHz of dinucleotides and dinucleoside monophosphates have confirmed quite conclusively that the magnetic anisotropy of one base moiety influences the chemical shifts of the protons of the other base and, therefore, that these compounds exist in one or more folded conformations, in which the rings of the bases are stacked in parallel planes. Intra- and inter-molecular base-stacking both appear to be extremely common,103,122-126 and the chemical shifts experienced by H-l, H-2, and H-8 of adenylyl-(3 - 5 )-adenosine (44) on inter-... 

Early NMR studies on dinucleoside monophosphates and conformational conclusions were reported in the years 1968-1970 (5-9). The only data that could be extracted from the spectra at that time were the chemical shifts of the base and HI protons (as well as the coupling constant Jl 2 ... 

The experimental value of u in solution cannot be obtained from NMR data but Cheng and Sarma [1][17] have interpreted some of their measurements on the variation of the shift of H3 upon temperature or polymerization by a variation of the value of o>. They assume that in 3 -nucleotides the PO group rotates freely about the O31- P bond and that the restriction of this rotation brought about by polymerization produces an upfield shift on H3 x since an increase of temperature which can increase the free rotation about the 031 P bond in dinucleoside monophosphates does produce a downfield shift for this proton. We cannot say that the calculated A6 for H3 fully support this interpretation since, if our results are significant, they predict for this proton an upfield A6, between a conformation with a freely rotating PO and one with a definite value of u, which has little chance to be of some importance. Ue shall come back to this point later in the discussion. 

Although conformations and of 3 -ANP and d and of 5 -ANP differ only by the value of X(>m (difference of 20 T for the four conformations of ApA that we consider in TabLe II, our resuLts show that for most protons the calculated values of the dimerization shifts are quite different according to the conformation chosen as reference state of the monomer. Some experimental results on dinucleoside monophosphates require for their interpretation the hypothesis that some "rearrangement effect" contributes to the measured dimerization shifts [ 1][ 7][ 8][ 13], but the feature illustrated by the resuLts of Table II is that the exact conformation of the monomer might be as important as the conformation of the dimer in the determination of the dimerization shift. As an exampLe, for HI of the Ap unit we see that for conformation 1 we calculate a dimerization shift differing by0.17tpbp.m. according to the choice or b for the conformation of 3 -AM. For this... 

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