Structural effects of biologically relevant rhodamines on spectroscopy of fluorescence fluctuations

Exciton coupling in pi-pi complexes between the indole ring and other pi systems is known to enhance the efficiency of energy and electron transfer. Rhodamines' xanthylium rings allow the formation of weakly or nonfluorescent complexes with the amino acid tryptophan. Thus, because of the short distance of the participating electronic clouds, intrinsic electron transfer-induced fluorescence quenching occurs. In solution, the rate constant of electron transfer is known to be limited by collision interactions at the contact distance. By contrast, in protein local environments tryptophan residues can be either exposed or buried in hydrophobic regions. Herein, I report on the properties of aromatic derivatized rhodamines, among which is one with a bound phenylalanine amino acid group. Encompassed is the spectroscopic and kinetic information in bulk and at the single-molecule levels both in free solution and in the presence of human serum albumin. Spectroscopic characteristics are focused with special emphasis on enhanced fluorescence that is addressed considering optimized geometries and electronic spectra. The importance of the probes associated with peptides and metal ions both in condensed phase or interfaces and as substrates with proteins is put into perspective.