Spectroscopic investigations on the interactions of potent platinum(II) anticancer agents with bovine serum albumin

The interactions of three platinum(II)-based anticancer complexes [(5,6-dimethyl-1,10-phenanthroline) (1S,2S-diaminocyclohexane)platinum(II)]2+, [(5,6-dimethyl- 1,10-phenanthroline)(1R,2R-diaminocyclohexane)platinum( II)]2+, and [(5,6-dimethyl-1,10-phenanthroline)(1,2- diaminoethane)platinum(II)]2+ (56MEEN) with BSA have been examined by circular dichroism (CD), fluorescence and 1H pulsed gradient spin–echo (PGSE) diffusion NMR spectroscopy. The number of association constants and sites differed depending upon the spectroscopic method. This may be because each technique monitors different types of interaction/s and/or as a consequence of the different concentration ranges required for each technique. The titration of BSA with the achiral 56MEEN as monitored by CD indicates a reduction in the α-helical nature of the albumin, with the association constant calculated to be ~5×106 M−1 for one site. Due to the chiral nature of the other two complexes, their association with albumin was not monitored using CD but was examined using fluorescence and PGSE diffusion NMR. Titration of BSA with any of the three metal complexes resulted in quenching of fluorescence, with the number of association sites calculated to be ~1.1, with an association constant of ~2×105 M−1. PGSE diffusion NMR provided insights into interactions occurring with the BSA in its entirety, rather than with individual regions. Metal complex binding sites were estimated (~10 equivalent) from the diffusion data, with the average association constant for all sites ~102–103M−1. These experiments highlight the information that can be elucidated from complementary spectroscopic techniques and demonstrate the usefulness of PGSE diffusion NMR in monitoring multiple weak binding sites, which is of great importance in studying drug-biomolecule interactions.