Spectroscopic study of a novel bis(heptamethine cyanine) dye and its interaction with human serum albumin.

A newly synthesized near-infrared (NIR) bis(heptamethine cyanine) dye 7 was evaluated for its utility as a non-covalent label for proteins. This dye forms inter- and intramolecular H-aggregates in polar solvents, even at very low concentrations. The intramolecular dimeric form of the dye can be described as a clam-shell complex with two interacting hydrophobic carbocyanine moieties. In this intramolecular H-aggregate, the chromophore has a low extinction coefficient and low fluorescence quantum yield. In aqueous solution, in the absence of human serum albumin (HSA), dye 7 has characteristic absorption bands at 792 and 435 nm, and its fluorescent emission is significantly diminished in comparison to that in methanol or when compared to its monomeric equivalent 5. Dye 7 seems to be more advantageous than its monomeric counterpart 5 as a non-covalent label for biomolecules. Upon addition of HSA, the H and D bands are decreased and the monomeric band is increased, with concomitant increase in fluorescence intensity, suggesting that clam-shell H-aggregates open up in the complex with HSA. The binding stoichiometry is 1:1. The main advantage of this dimeric dye as a non-covalent label is that the free dye has negligible fluorescence.