A study of intramolecular H-complexes of novel bis(heptamethine cyanine) dyes

Near-infrared (NIR) bis(heptamethine cyanine) (BHmC) dyes containing a flexible polymethylene linker between the two cyanine subunits are a novel class of compounds with versatile spectroscopic properties. The first bis-cyanine of this type is BHmC-10 (with a decamethylene bridge) that has been reported by us recently [G. Patonay, J.S. Kim, R. Kodagahally, L. Strekowski, Appl. Spectrosc., in press]. As part of this work, additional bis-cyanines BHmC-4, BHmC-6, and BHmC-8 were synthesized and their spectral properties were evaluated for the dyes free in solution and in the presence of human serum albumin (HSA). These bis-cyanines undergo H-type aggregation, mainly H-type intramolecular complexation between the two cyanine subunits, when free in aqueous solution. This H-type interaction in phosphate buffer (pH 7.2) is characterized by hypsochromic (H) absorption at 700 nm, low extinction coefficient, and low fluorescence quantum yield. By contrast, an analogous monomeric cyanine exhibits strong fluorescence under similar conditions. Upon binding with HSA, the fluorescence of BHmC-6 changes negligibly, that for BHmC-8 shows a slight increase, and the fluorescence of BHmC-4 is greatly increased. It is suggested that BHmC-4 binds with HSA in the open form exclusively, while the H-type intramolecular interaction in BHmC-6 is mostly retained in the complex with HSA. Bis-cyanine BHmC-4 may be of significant bioanalytical utility due to its negligible fluorescence in aqueous solution and a strong increase in fluorescence upon binding with a protein.