Intramolecular charge transfer as probing reaction: Fluorescence monitoring of protein-surfactant interaction

Ethyl p-(dimethylamino)cinnamate (EDAC) has been used as a fluorescence probe for monitoring the interaction between a model water-soluble protein, bovine serum albumin (BSA), and an anionic surfactant, sodium dodecyl sulfate (SDS). The probe EDAC undergoes intramolecular charge transfer (ICT) in the excited state in water and other polar solvents. The emission from the ICT state becomes more intense and blue-shifted due to reduced polarity in the hydrophobic environments of BSA and SDS micelles relative to that in pure water. The intensity of the ICT emission from EDAC increases with surfactant concentration and reaches a maximum at the critical micelle concentration of SDS, which can be employed as a simple technique for following micellization. Analysis of the fluorescence spectra of the probe provide evidences in favor of surfactant-induced protein uncoiling due to massive binding of the SDS molecules to BSA in the cooperative binding region of the binding curve, describing protein (BSA)-surfactant (SDS) interaction. The polarity of the BSA-SDS aggregate formed is intermediate between that of hydrophobic regions of BSA and SDS micelles as sensed by the intramolecular charge-transfer (ICT) probe, EDAC.