The simulation of the B-Z-DNA transition by using space-filling models of the dimer d(C-G) shows the possibility of hydrogen-bond formation between the N-2 amino group of the partially rotated guanine and one of the 5'-phosphate oxygens of deoxyguanylic acid. To probe the importance of this postulated interaction, analogs of the hexamer d(C-G)3 were synthesized. These analogs contained a methylphosphonate linkage, of distinct stereochemistry, which replaced the first 5'-phosphate linkage of deoxyguanosine. The CD spectra in high salt concentration showed that the hexamer containing a methylphosphonate linkage with the Rp stereochemistry formed Z-DNA to the same extent as d(C-G)3, whereas the hexamer containing a methylphosphonate linkage with the Sp stereochemistry did not form Z-DNA. These results are consistent with a mechanism in which an interaction between the N-2 amino group of guanine and the prochiral Sp oxygen of deoxyguanosine 5'-phosphate kinetically controls the formation of Z-DNA. A water bridge between the N-2 amino group ofguanine and the 3'-phosphate oxygen of deoxyguanylic acid has been implicated in the stabilization of Z-DNA. To probe the importance of this water bridge, two additional analogs of the hexamer d(C-G)3 were synthesized. These analogs contained a methylphosphonate linkage, of distinct stereochemistry, that replaced the first deoxyguanosine 3'-phosphate. The CD spectra showed that the hexamer containing a methylphosphonate linkage of the Rp stereochemistry underwent the transition to Z-DNA to the same extent as d(C-G)3, whereas the hexamer containing a methylphosphonate linkage of the Sp stereochemistry underwent the transition to Z-DNA to a 35% lesser extent. Thus the water bridge involving the prochiral Sp oxygen provides modest stabilization energy for Z-DNA. These studies, therefore, suggest that the B-Z-DNA transition is regulated both thermodynamically and kinetically through hydrogen-bond interactions involving phosphate oxygens and the N-2 amino group of guanine.Pohl and Jovin (1) showed that the alternating copolymer poly[d(C-G)] underwent a salt-induced conformational change. The new species thus formed was left-handed Z-

The crystal structure of d[Cp(CH3)G] has been determined as part of a project to study the mechanism of the B----Z transition in DNA. The asymmetric unit contains two dinucleotides and the equivalent of 7.5 water molecules, partially disordered over 12 definable positions. The two symmetry-independent dinucleotides form a duplex with Watson-Crick base-pairing and a right-handed helical sense. Comparison with previously determined structures of the B and A conformation showed that this duplex is closer to B than to A but significantly different from B. It corresponds to a stretched out helix with a 4 A rise per base pair and a helical twist of 32 degrees. This structure may serve as a model for the bending of DNA in certain situations. The configuration at the methyl phosphonate is RP, and a mechanism, based on this assignment, is presented for the B----Z transition in DNA.