Table 2. Selected dihedral angles (in degrees) for the BP-1 and BP-2 complexes.

Angle*	BP-1	BP-2
χ	219.5	268.1
α′	156.8	-85.1
β′	107.0	-47.2
N6-C10-C9-O9	37.4	66.2
O9-C9-C8-O8	-54.4	-57.6
O8-C8-C7-O7	-84.8	-83.4

In both BP-1 and BP-2, the partially saturated ring assumes a distorted half-chair conformation. In BP-1, the C10-N⁶ bond linking the hydrocarbon to the base makes an angle of ≈50° with the plane of the aromatic rings, corresponding to an orientation for this bond that is about midway between axial and equatorial; whereas, in BP-2, this bond is pseudoequatorial and makes an ≈25° angle with the aromatic system. In both structures, the 7- and 8-hydroxyl groups are close to pseudoequatorial, and the 9-hydroxyl group is pseudoaxial. This orientation of the 9-hydroxyl group closely resembles that proposed in the NMR solution structure of a DNA duplex containing a cis opened (10R) BP tetrahydroepoxide dA adduct (PDB accession code 1N8C, Fig. 3A) that lacked the 7- and 8-hydroxyl groups on the tetrahydro ring (6).

^*Angles are defined as follows: χ = O4′-C1′-N9-C4; α′ = N1-C6-N⁶-C10(BP); β′ = C6-N⁶-C10(BP)-C9(BP).