141
5 Molecular Structures, Relative Stability, and Proton Affinities of Nucleotides
study by Schneider et al. [39] clearly demonstrated differences in backbone torsion angles for A, BI, BII, ZI, and ZII types of DNA. In A-DNA north conformation of SU is observed while BI-, and BII-DNA contain south conformation of
SU. In Z-DNA both south and north conformations of sugar are observed. The
А, BI, and ВII forms of DNA include conformers of nucleotides only with the
anti-orientation of a base with respect to the SU. In the case of Z-DNA purine
nucleotides have a syn-orientation of a base [2, 39].
Fig. 5.3  The sugar–phosphate backbone
Table 5.1  The average values of torsion angles in different forms of DNA (The data were obtained
through the analysis of 118 naked (noncomplexed) DNA structures, see Ref. [39])
DNA form
Torsion angles of backbone (deg.)
α
β
γ
δ
ε
ζ
χ
Canonic A-DNA (AI)
295
173
54
82
206
285
201
AII-form
146
192
183
85
197
289
203
Canonic B-DNA (BI)
299
179
48
133
182
263
250
BII-form
293
143
46
143
251
168
278
ZI-form, Y-R step
66
186
54
147
264
76
205
ZI-form, R-Y step
210
233
177
96
242
292
63
ZII-form, R-Y step
169
162
179
95
187
187
58
Y pyrimidine, R purine