SynopsisWe report the temperature dependence of the H2 and H8 purine ring proton resonances of oligoriboadenylates up to chain length 11, with or without a single guanosine residue a t the 5'-end, second position, or 3'-end. The results suggest the following generalizations: (1) Stacking of the bases in a right-handed single-stranded helix is more extensive in the interior of the chain than at the chain ends. (2) The tendency of the terminal base to unstack is greater at the 5'-end than a t the 3'-terminus. (3) G stacks more weakly than A, as evidenced by weak stacking of 3'-terminal G. Anomalies were also observed in the unstacking profile of G a t the second position in the chain, indicating a conformational anomaly such as looping out of G, thereby allowing adjacent A's to stack together, or adoption by G of some other alternative structure. (4) The results imply that the environment a t a given base is influenced by effects of longer range than nearest-or next-nearest-neighbor. Increasing ion condensation as chain length increases may be responsible for the slow approach of oligomer behavior to the properties of the high polymer.
We report simplified methods for large scale enzymatic synthesis of oligoribonucleotides using polynucleotide phosphorylase. The main features of the method are use of RPC-5 chromatography, including chromatography at two pH values to deal with the problem of primer phosphorolysis, rapid dialysis for large scale desalting, simplified methods for enzyme removal, and high resolution 1H and 31P NMR for product identification and demonstration of purity. The capacity of the method is adequate to allow beginning with grams of material in the first polymerization step, so that product yields of several milligrams, sufficient for many physical studies, are possible after as many as three separate polymerization reactions.
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