Effect of macromolecular binding and self-association on the photophysical and photochemical properties of triarylmethane dyes

In response to transmembrane potentials, which are negative on the inner side of energized mitochondria, extensively conjugated cationic molecules (dyes) with appropriate lipophilic/hydrophilic character naturally accumulate inside these subcellular compartments. Because enhanced mitochondrial membrane potential is a prevalent cancer cell phenotype, the selective destruction of tumor cells via mitochondrial targeting is currently under investigation in this laboratory as a novel therapeutic strategy for photochemotherapy of neoplastic diseases. Cationic triarylmethane dyes (TAM+) represent a class of photosensitizers whose phototoxic effects develop at least in part at the mitochondrial level. The aim of this project was to determine how the molecular structure of TAM+ dyes affects their noncovalent binding to model host biopolymers (hexokinase, DNA, and low density lipoprotein), and explore the effects of these noncovalent interactions on photosensitization efficacy (or drug potency).; Depending upon the degree of biopolymer loading, TAM+ dyes were found to bind to hexokinase (HK) and DNA both as monomers and as aggregates. Laser irradiation of TAM+-HK and TAM+-DNA complexes has indicated that photosensitization efficacy decreases upon increasing the degree of dye aggregation. While dye aggregates were extensively destroyed (photobleached) upon laser irradiation, little damage was inflicted by them on the host macromolecules. On the other hand, dye monomers were less susceptible to photobleaching and more effective in promoting the destruction of HK and DNA. TAM+ dyes were also found to bind to low density lipoprotein (LDL) in aqueous media. The dominant mechanism of TAM+ binding to LDL has been characterized as the partitioning of these dyes into the lipid core of the host lipoprotein. No evidence has been found for the formation of TAM+ aggregates in the presence of LDL. Accordingly, in LDL-TAM+ complexes the degree of photoinduced macromolecular damage was found to increase with increasing drug concentration.; Biopolymer-assisted dye aggregation can thus be expected to control, at least in part, the photosensitization efficiency of TAM+ dyes in complex biological environments. Our findings indicate that considerations on how molecular structure affects dye aggregation are of relevance for the rational design of new photosensitizers specifically tailored for the selective destruction of tumor cells via mitochondrial targeting.