NMR Structural Studies on a Nonnatural Deoxyribonucleoside Which Mediates Recognition of GC Base Pairs in Pyrimidine·Purine·Pyrimidine DNA Triplexes

As a part of our ongoing efforts to define the structural aspects of unusual pairing alignments in DNA triplexes by nuclear magnetic resonance spectroscopy, we have examined the structural role of a nonnatural deoxyribonucleoside, PI, that has been shown to mediate the recognition of GC base pairs in pyrimidine·purine·pyrimidine DNA triplexes [Koh, J. S., & Dervan, P. B. (1992) J. Am. Chem Soc. 114, 1470]. A qualitative interpretation of the NMR data indicates that this analog of protonated cytosine is readily accommodated in the third strand segment of an intramolecular triplex system. Furthermore, the observed NOE patterns position the imino and amino protons of PI opposite the N⁷ and O⁶ atoms of guanine, respectively, consistent with the previously proposed pairing scheme.