The existence of unique B structures in polynucleotides with alternating purine-pyrimidine sequences

The infrared spectra of the sodium salts of calf-thymus DNA, poly(dA-dC).poly(dG-dT), poly(dA-dT) and poly(dG-dC) were measured for the samples as highly hydrated, nonoriented gels. The bands from the sugar-phosphate vibrational modes show that poly(dG-dC) assumes a B-family structure which is different from the B structures of the other samples. Poly(dG-dC) most likely assumes a wrinkled B structure. The other samples retain a smooth B structure. An alternating purine-pyrimidine sequence is not a sufficient condition for the formation of wrinkled B structure in a polynucleotide.