Amino acid-based cationic gemini surfactant–protein interactions

• (C12Cys)2, a new gemini surfactant, forms stable suspensions with and without BSA.
• BSA–(C12Cys)2 represents an oppositely charged biopolymer–surfactant system.
• The tensiometric profile is consistent with a cooperative binding process.
• (C12Cys)2 quenches the intrinsic fluorescence of BSA by a static quenching process.

A novel cationic amino acid-based gemini surfactant derived from cysteine, (C12Cys)2, has been synthesized and both its supramolecular behaviour and its interaction with the model protein bovine serum albumin (BSA) have been characterized under physiological mimetic conditions (PBS, pH 7.4). Surface tension measurements were used to obtain important system parameters, such as critical micelle concentration (CMC), critical aggregation concentration (CAC), protein saturation point (PSP), maximum surface excess concentration (Γmax), minimum surface area per molecule (Amin) at the air/solution interface and the degree of surfactant binding to protein (α). Formation of a protein–surfactant complex was confirmed by UV–vis and fluorescence spectroscopy. Fluorescence quenching measurements allowed determination of the Stern–Volmer quenching constant (KSV), surfactant–protein binding constant (Ka) and number of binding sites (n). UV–vis measurements and the calculated value for the bimolecular quenching constant (kq) suggest that (C12Cys)2 quenches BSA intrinsic fluorescence by a static quenching mechanism.

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