Spectroscopic investigations on the binding of an iodinated chlorin p-copper complex to human serum albumin.

The insertion of suitable metals or high Z elements in tumour-avid tetrapyrrole compounds is a promising approach to obtain potential agents for multimodal cancer therapeutics and tumour imaging. Using chlorin p, a chlorophyll derivative, we synthesized a novel iodinated chlorin p-copper complex (ICp-Cu) that can be applied to the photodynamic therapy and photon activation therapy of cancer. In the present study, we investigated the interaction of ICp-Cu with human serum albumin (HSA) using UV-Vis absorption and fluorescence spectroscopy. The addition of HSA to ICp-Cu at physiological pH led to a ∼7 nm red shift in its Soret and Q band absorption. The binding constant (K) and the number of binding sites (n) of ICp-Cu obtained from the quenching of the intrinsic fluorescence of HSA were 2.9 × 10 M and 1.2 respectively. The distance between the Trp-214 residue and ICp-Cu computed from Förster non-radiative energy transfer (FRET) theory was 3.1 nm. The emission of the ICp-Cu fluorescence when excited at the protein's Trp absorption (295 nm) further substantiated the FRET between the Trp residue and ICp-Cu. Synchronous spectroscopy revealed that the quenching of protein Trp fluorescence was higher than that of Tyr with no significant shift in peak position. Results suggested that HSA acts as a carrier protein for ICp-Cu with a high probability that the binding of ICp-Cu occurred at subdomain IIA and the binding had no effect on the conformation of HSA.