Circular dichroism and ordered structure of bisnucleoside oligophosphates and their Zn2+ and Mg2+ complexes

Circular dichroism, absorbance, hypochromicity, and the formation of Mg2+ and Zn2+ complexes have been measured for a series of bisnucleoside oligophosphates that contain adenosine, guanosine, and mixed guanosine/adenosine, guanosine/cytidine, and guanosine/uridine, as well as 7-methylguanosine and ribose-methylated purine nucleosides. All of the metal complex ions have stacking interactions at 2 degrees C, 10 mM tris(hydroxymethyl)aminomethane hydrochloride, pH 8.0. There is a measurable degree of base stacking for all unsubstituted purine nucleotides that differs, however, from that of bases in nucleic acids. The degree of base stacking varies with the length of oligophosphate chains and the state of methylation. The effect of 7-methylation of guanosine is interpreted as causing a switch of nucleic acid base stacking from an atypical to a typical mode, which could be important for cap function in mRNA. The Mg2+ and Zn2+ complexes give rise to characteristic circular dichroism. In all instances excepting 7-methylated bisguanosine oligophosphates, the active secondary structures are disrupted, and in this regard, Zn2+ is more effective than Mg2+. At least two sets of binding sites are involved. A single metal ion is bound tightly. Stability, in terms of equilibrium constants, increases by more than 1000-fold as a function of chain length varying from two to six phosphates. The consequences of methylation are only minor. Electrostatic attraction between metal ions and phosphates is the most likely mechanism of these phenomena as judged by the effect of high ionic strength.