Nucleic acid triple helices constitute a chemically and biologically important class of structures, but no triplex structure at atomic resolution has previously been determined. We had previously obtained well-formed crystals of DNA triple helices which gave only fiber-type X-ray diffraction patterns. We have now synthesized a novel design of DNA sequences which permits the first crystallographic study, in collaboration with Drs. Davies and Rhee, of a DNA molecule containing triple helices. In the 1.8 A resolution structure the bases of the third strand associate with the Watson-Crick duplex via Hoogsteen-type hydrogen bonds, resulting in three consecutive C+.GC, T.AT, and C+.GC triplets.We have previously studied effects of chemical modificationon the chemistry and structure of oligonucleotide triple helices. We have now studied the effects 2-O-methylation on these complexes. Complete O- methylation of the purine strand prevents triplex formation, but partial methylation does not. 2O-methylation of the pyrimidine strands increases the stability but otherwise has has relatively little effect. - chemistry,structure,nucleic acids,spectroscopy,infrared,triple helix,nmr,X-ray
