Figure 5.
Fig. 5. Structure-based reaction mechanism that resolves
the apparent orthogonal paradox for electron transpositions by
altering the substrate stereochemistry. (A) A simplified
valence-bond representation of the glycosylic bond dissociation
hides the paradox that the three electron pairs to be transposed
are involved in orthogonal orbitals. (B) In the normal
anti-conformation of deoxyuridine, the *-orbital
involved in the anomeric effect and the -orbital of
the C2==O bond are orthogonal to one another, thus preventing
orbital overlap. (C) Severe distortions of the deoxyribose and
the glycosylic bond in the strained conformation of deoxyuridine
enforced by the UDG active center align the pairs of atomic
orbitals participating in each electron transposition, thereby
electronically coupling the anomeric and - [Arom] effects
to promote bond cleavage.
*-orbital
involved in the anomeric effect and the 
-orbital of
the C2==O bond are orthogonal to one another, thus preventing
orbital overlap. (C) Severe distortions of the deoxyribose and
the glycosylic bond in the strained conformation of deoxyuridine
enforced by the UDG active center align the pairs of atomic
orbitals participating in each electron transposition, thereby
electronically coupling the anomeric and