Photoinduced Electron Transfer in a Protein-Surfactant Complex: Probing the Interaction of SDS with BSA Anjan Chakraborty, Debabrata Seth, Palash Setua, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, WB, India ReceiVed: March 14, 2006; In Final Form: June 21, 2006 Photoinduced fluorescence quenching electron transfer from N,N-dimethyl aniline to different 7-amino coumarin dyes has been investigated in sodium dodecyl sulfate (SDS) micelles and in bovine serum albumin (BSA)- SDS protein-surfactant complexes using steady state and picosecond time resolved fluorescence spectroscopy. The electron transfer rate has been found to be slower in BSA-SDS protein-surfactant complexes compared to that in SDS micelles. This observation has been explained with the help of the “necklace-and-bead” structure formed by the protein-surfactant complex due to coiling of protein molecules around the micelles. In the correlation of free energy change to the fluorescence quenching electron transfer rate, we have observed that coumarin 151 deviates from the normal Marcus region, showing retardation in the electron transfer rate at higher negative free energy region. We endeavored to establish that the retardation in the fluorescence quenching electron transfer rate for coumarin 151 at higher free energy region is a result of slower rotational relaxation and slower translational diffusion of coumarin 151 (C-151) compared to its analogues coumarin 152 and coumarin 481 in micelles and in protein-surfactant complexes. The slower rotational relaxation and translational diffusion of C-151 are supposed to be arising from the different location of coumarin 151 compared to coumarin 152 and coumarin 481. 1. Introduction The interaction of proteins with surfactants has received a great deal of interest for many years due to its application in a great variety of industrial, biological, and cosmetics systems. 1-7 The globular protein bovine serum albumin (BSA) has the important role of interacting with cell membrane surfactant. BSA functions biologically as a career for fatty acid anions and other simple amphiphiles in a blood stream. It has a molecular weight of 66 411 g mol -1 and contains 583 amino acids in a single polypeptide chain. The protein contains 17 disulfide bridges and one free -SH group, which can cause it to form a covalently linked dimer. At neutral pH, it undergoes conformational changes. The interior of the protein is almost hydrophobic, while both the charged amino acid residues and apolar patches cover the interface. 8-10 It is known in general that anionic surfactants interact strongly with the proteins and form protein-surfactant complexes. 4-7 This leads to the unfolding of proteins. The binding isotherm of BSA with surfactant is well studied. 4-6 It consists of four regions with increasing surfactant concentration. At the initial region, surfactant binds to the specific high-energy region of the protein. The concentration of the surfactant is the lowest at this region. The second region is the noncooperative interaction. The third region corresponds to the massive increase in binding due to the cooperative ligand interaction. The unfolding of proteins is believed to start in this region. Here, the “necklace- and-bead” structure of BSA-surfactant begins to form. The last region is associated with a growth in protein bound micelles, and further binding of the surfactant to the protein does not occur. 1-7 Several techniques such as X-ray crystallography, 11 NMR, 4,12,13 light scattering, 14-17 and small angle neutron scattering (SANS) 18,19 have been used to unravel protein- surfactant interaction. Different photophysical 20-22 and dynam- ical 23-26 studies have also been employed to probe the protein- surfactant interaction. In this work, we are going to explore the fluorescence quenching electron transfer (ET) dynamics in sodium dodecyl sulfate (SDS) micelles and in BSA-SDS protein-surfactant complexes using steady state and picosecond time resolved fluorescence spectroscopy. We have used several coumarin dyes as the electron acceptors and N,N-dimethylaniline (DMA) as the donor (Scheme 1). Several groups 4-7,15,16 have studied the interactions between BSA and SDS. Takeda et al. 15 reported * To whom correspondence should be addressed. E-mail: nilmoni@chem.iitkgp.ernet.in. Fax: 91-3222-255303. SCHEME 1: Structures of the Coumarin Dyes and Aromatic Amine 16607 J. Phys. Chem. B 2006, 110, 16607-16617 10.1021/jp0615860 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/29/2006