The complexing of guanine nucleosides by poly(inosinic acid)

Under conditions where it is single-stranded (pH 7.0, 0.05 M Na+), poly(inosinic acid) complexes guanosine and 2'-deoxyguanosine in a strongly cooperative process that ultimately results in gel formation. The binding isotherm for formation of the poly(I).deoxyguanosine complex at 3.5 degrees C shows that it contains approximately one molecule of deoxyguanosine bound per IMP residue of the poly(I). This is compatible with the complex having a structure similar to that of the four-stranded secondary structure of poly(I) in which two of the poly(I) strands are replaced by helical arrays of deoxyguanosine molecules. The complex is laevorotatory at wavelengths longer than 300 nm. Optical rotation and ultraviolet absorbance versus temperature profiles show that on thermal dissociation the complex undergoes a co-operative helix-coil transition with Tm in the range 10--20 degrees C.