Biophysical Studies on the Effect of the 13 Position Substitution of the Anticancer Alkaloid Berberine on Its DNA Binding

The structural effects and thermodynamics of the DNA binding of six berberine analogues with alkyl chains of varying length and a terminal phenyl group at the C-13 position were investigated. All the analogues bound DNA noncooperatively in contrast to the cooperative binding of berberine. The binding affinity was higher and the effect of the chain length was only up to (CH2)(3), after which the binding affinity decreased slightly. Intercalative binding with strong stabilization of the DNA helix was revealed. Binding resulted in the weakening of the base stacking with moderate conformational changes within the B-form. The binding was entropy driven in each case, the entropy contribution to the free energy increasing with the chain length up to the threshold (CH2)(3). The complexation was dominated by nonpolyelectrolytic forces in each case; polyelectrolytic forces contributed only a quarter to the total free energy at 50 mM [Na+]. Overall, the phenylalkyl substitution at the C-13 position considerably enhanced the DNA binding and was highest for the analogue with (CH2)(3). Structural and thermodynamic data on the DNA binding aspects of the substituted berberines are presented in comparison with berberine.