Diffusion of organic dyes in bovine serum albumin solution studied by fluorescence correlation spectroscopy

The understanding of the transport of drugs and naturally occurring molecules in living cells and tissues requires a thorough knowledge of the diffusion behaviour of the molecular systems in these media. In this work, we studied the translational diffusion of three fluorescent molecules, electrically neutral coumarin 102 (C102), cationic rhodamine 6G (R6G) and anionic fluorescein (FL) in phosphate-buffered (pH 7) aqueous solutions of bovine serum albumin (BSA) protein in the absence and presence of common salt and urea using fluorescence correlation spectroscopy (FCS) by monitoring the fluorescence intensity fluctuations in a small confocal observation volume. The diffusion due to both free and BSA-bound molecules is observed in the case of the C102-BSA system. While no exchange between the bound and free states of the molecule is observed in this case, a rapid exchange between the two states is observed in the case of electrically charged hydrophilic dyes R6G and FL. This molecular picture, which is the first of its kind, is a reflection of a weaker binding of R6G and FL compared to C102 with the protein molecule. The binding sites of the probe molecules in BSA were identified based on the urea-induced change of diffusion of the probes in BSA.