Scheme 2. Mechanism of action of enediyne anticancer antibiotics: DNA cleavage initiated by (a) C4' or (b) C5' hydrogen atom abstraction.

Less than 20% of the strand breaks result from hydrogen atom abstraction at C4' - ( Scheme 2 ) and C1' - ( Scheme 3 ). NCS chromophore effects primarily single-stranded DNA cuts by the C(6) radical at C5' - ( Scheme 2 ) of deoxyribose, whereas those double stranded lesions which are observed involve additional hydrogen abstraction by the C2- radical from C1' - or C4' - of the deoxyribose on the complementary strand.

Scheme 3. Mechanism of action of enediyne anticancer antibiotics: DNA cleavage initiated by C1'- hydrogen atom abstraction.

Below is an example of DNA cleavage shown by natural enediyne Calicheamicin ( Scheme 4 ).

Scheme 4. Mechanism of DNA cleavage by calicheamicin.