Proton nuclear magnetic resonance investigations and ring current calculations of guanine N-1 and thymine N-3 hydrogen-bonded protons in double-helical deoxyribonucleotides in aqueous solution.

Methods are outlined for assigning the guanine-N(1)H and thymine-N(3)H protons to particular base pairs in the proton nuclear magnetic resonance spectra of double-stranded oligodeoxyribonucleotides of known sequence in aqueous solution. Ring current calculations have been used to evaluate the upfield shifts of the guanine-N(1)H and thymine-N(3)H protons from the pyrimidine and purine rings of nearest-neighbor base pairs in DNA B-type double-helical structures. Chemical shifts of 13.6 +/- 0.1 ppm and 14.6 +/- 0.2 ppm are assigned to the guanine-N(1)H proton of an isolated G.C base pair and the thymine-N(3)H proton of an isolated A.T base pair, respectively.