DNA binding properties of the marine sponge pigment fascaplysin

Association of fascaplysin with double-stranded calf thymus DNA was investi gated by means of isothermal titration calorimetry absorption spectroscopy, and circular dichroism. The UV spectroscopic data could be well interprete d in terms of a two-site model for the binding of fascaplysin to DNA reveal ing affinity constants of K-1 = 2.5 x 10(6) M-1 and K-2 = 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 exothe rmic reaction which was exhausted at a 2:1 base pair/drug; ratio. This find ing is in agreement with an intercalation model comprising nearest neighbor exclusion. In addition, significantly weaker non-intercalative DNA interac tions can be observed at high drug concentration. By comparison of all thes e data with the binding behavior of known intercalating agents, it is concl uded that fascaplysin intercalates into DNA. (C) 2001 Elsevier Science Ltd. All rights reserved.