DNA binding properties of the marine sponge pigment fascaplysin

Association of fascaplysin with double-stranded calf thymus DNA was investigated by means of isothermal titration calorimetry, absorption spectroscopy, and circular dichroism. The UV spectroscopic data could be well interpreted in terms of a two-site model for the binding of fascaplysin to DNA revealing affinity constants of K1 = 2.5 x 10(6) M(-1) and K2 = 7.5 x 10(4) M(-1) (base pairs of DNA). Based on the typical change observed in the absorption and circular dichroism spectra, intercalation of fascaplysin is regarded as the major binding mode. The calorimetric titration curves showed an exothermic reaction which was exhausted at a 2:1 base pair/drug; ratio. This finding is in agreement with an intercalation model comprising nearest neighbor exclusion. In addition, significantly weaker non-intercalative DNA interactions can be observed at high drug concentration. By comparison of all these data with the binding behavior of known intercalating agents, it is concluded that fascaplysin intercalates into DNA.