Cooperative self-assembly of cyanines on carboxymethylamylose and other anionic scaffolds as tools for fluorescence-based biochemical sensing

Abstract

We recently found that certain cyanines form tight complexes with carboxymethylamylose (CMA) in aqueous solutions and that in these complexes the cyanine exists as a strongly fluorescent and stable J-aggregate. Cyanine dyes are characterized by their ability to form J-aggregates showing very narrow absorption and fluorescence spectra relative to the monomer. Although they have found uses in sensing applications, the practicability has been limited in many cases due to the low quantum efficiencies for J-aggregate fluorescence. The CMA-cyanine complex is formed by a cooperative self-assembly in which both components undergo conformational changes during the association. The CMA exists as a random coil in solution prior to complex formation; helix formation is prevented due to repulsion of the charges on the carboxymethylated glucose units. The cyanine exists as a nonfluorescent monomer in the same solutions. A helical atomic force microscopy image and large induced circular dichroism (CD) spectra of the cyanine J-aggregate indicate that the self-assembly is a superhelix scaffold of CMA decorated with J-aggregates of the cyanine. Similar behavior was also observed with carboxymethylated cellulose (CMC). Enzymatic disruption of the helical structures (e.g., by the use of amylase to disrupt the structure of CMA helix) leads to the disappearance of the J-aggregate-associated fluorescence. The photophysical behavior and applications of this complex for sensing are discussed.