Abstract
Porphyrins and their metal derivatives are strong protein binders. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA and protein. The presence of several highโaffinity binding sites on human serum albumin (HSA) makes it possible target for many organic and inorganic molecules. Chlorophyll a and chlorophyllin (a foodโgrade derivative of chlorophyll), the ubiquitous green plant pigment widely consumed by humans, are potent inhibitors of experimental carcinogenesis and interact with protein and DNA in many ways. This study was designed to examine the interaction of HSA with chlorophyll (Chl) and chlorophyllin (Chln) in aqueous solution at physiological conditions. Fourier transform infrared, UVโvisible, and CD spectroscopic methods were used to determine the pigment binding mode, the binding constant, and the effects of porphyrin complexation on protein secondary structure. Spectroscopic results showed that chlorophyll and chlorophyllin are located along the polypeptide chains with no specific interaction. Stronger protein association was observed for Chl than for Chln, with overall binding constants of K~Chl~ = 2.9 ร 10^4^M^โ1^ and K~Chln~ = 7.0 ร 10^3^M^โ1^. The protein conformation was altered (infrared data) with reduction of ฮฑโhelix from 55% (free HSA) to 41โ40% and increase of ฮฒโstructure from 22% (free HSA) to 29โ35% in the pigmentโprotein complexes. Using the CDSSTR program (CD data) also showed major reduction of ฮฑโhelix from 66% (free HSA) to 58 and 55% upon complexation with Chl and Chln, respectively. ยฉ 2005 Wiley Periodicals, Inc. Biopolymers, 2005