Oligonucleotide studies. Optical rotatory dispersion of five homodinucleotides

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Abstract
1. The optical rotatory dispersion (ORD) of five homodinucleotides, ApAp(3′), CpCp(3′), GpGp(3′), IpIp(3′) and UpUp(3′) (where A, C, G, I and U represent adenosine, cytidine, guanosine, inosine and uridine respectively, and p to the left of a nucleoside symbol indicates a 5′-phosphate and to the right it indicates a 3′-phosphate), were measured as a function of pH, ionic strength and Mg2+ concentration. 2. The ORD titrations of ApAp(3′) and CpCp(3′), which were made by measuring the ORD curves at closely spaced pH intervals, exhibit a maximum at approx. pH5·0 and 5·7 for ApAp(3′) and CpCp(3′) respectively in the profile of the magnitude of the first Cotton effect versus pH. The results indicate that the conformational rigidity of these dinucleotides depends on the ionization state of a 3′-terminal phosphate group. 3. ApAp(3′) was shown to exist as an approximately 1:1 equilibrium mixture of the two major ionic species represented by Ap(−1)Ap(−1) and Ap(−1)Ap(−2) at pH6·16, whereas at pH7·5 it exists exclusively as a form of Ap(−1)Ap(−2). 4. To ascertain the effects of the presence of a terminal phosphate group and of the ionization of the secondary phosphate on the conformation of adenylate dimer, we measured the ORD of ApA, ApAp(3′)CH3 and ApAp(2′). The rotatory power of the first Cotton effect in the above series of dinucleotides decreased at 20° in the order ApA> ApAp(3′)CH3≈ApAp(3′)(−1)> ApAp(2′) at pH7> ApAp(3′) at pH7. 5. The pH–rotation profiles were also obtained for ApAp(2′), CpCp(2′) and UpUp(3′), but no corresponding maximum was observed. Although simple nearest-neighbour calculations based on the ORD data of IpIp(3′) and 5′-IMP account for the observed ORD spectrum of polyinosinic acid at low salt concentration, there were large discrepancies between calculated and experimental results of the polyguanylic acid ORD even at low ionic strength. 6. The extent to which the amplitude of the Cotton effects of IpIp(3′) increases with salt concentration, especially by the addition of Mg2+, was much greater than that observed for ApAp(3′). The implication of such salt effects on the ORD is considered.