Protein Secondary Structure Controlled with Light and Photoresponsive Surfactants

Abstract

The interaction of a light-responsive azobenzene surfactant with bovine serum albumin (BSA) has been investigated as a means to examine photoreversible changes in protein secondary structure. The cationic azobenzene surfactant undergoes a reversible photoisomeriztion upon exposure to the appropriate wavelength of light, with the visible-light (trans) form being more hydrophobic and, thus, inducing a greater degree of protein unfolding than the UV-light (cis) form. Fourier transform infrared (FT-IR) spectroscopy is used to provide quantitative information on the secondary structure elements in the protein (alpha-helices, beta-strands, beta-turns, and unordered domains). Comparing the secondary structure changes induced by light illumination in the presence of the photoresponsive surfactant with previous measurements of the tertiary structure of BSA obtained from small-angle neutron scattering (SANS) allows the three discrete conformation changes in BSA to be fully characterized. At low surfactant concentrations, an alpha-helix --> beta-structure rearrangement is observed as the tertiary structure of BSA changes from a heart-shaped to a distorted heart-shaped conformation. Intermediate surfactant concentrations lead to a dramatic decrease in the alpha-helix fraction in favor of unordered structures, which is accompanied by an unfolding of the C-terminal portion of the protein as evidenced from SANS. Further increases in photosurfactant concentration lead to a beta --> unordered transition with the protein adopting a highly elongated conformation in solution. Each of these protein conformational changes can be precisely and reversibly controlled with light illumination, as revealed through FT-IR spectra collected during repeated visible-light <--> UV-light cycles.

Spatial and temporal control of surfactant systems

“…This proposition has led to a series of recent investigations in which the interactions of azobenzene-based surfactants with DNA 32, 88, 89 or proteins 5, 46, 47, 90, 91 have been reported.…”

“…Complementary studies with trans -azoTAB have also provided evidence that trans -azoTAB can bind more strongly than cis -azoTAB to hydrophobic domains of some proteins, and lead to light-induced changes in the secondary, 46 tertiary, 5, 90 and quaternary structure 91 (i.e., degree of unfolding of proteins). For bovine serum albumin, a series of structural transitions was indentified, and addition of trans -azoTAB led to transitions between the structural states denoted as N, N d , F and E. The concentrations required for trans -azoTAB to cause the N to N d , N d to F, and F to E were 0.55mM, 0.8mM and 2.83mM, respectively.…”

“…This observation led to the demonstration that it was possible to control transitions in protein conformation reversibly by illumination of azoTAB with light at the appropriate wavelength (Figure 18). 5, 46 It was suggested by the authors that the capability to use light sensitive surfactants to control protein folding states could be exploited to characterize partially folded proteins in non-native conformations. It is noteworthy, however, that the effects of azoTAB on protein conformation are different for different proteins.…”

Emulsion formed in bovine serum album/anionic surfactant/H2O system under acidic condition

“…The protein folding/unfolding is very important in biological, chemical, and industrial applications [1][2][3]. For example, proteins are not static entities and instead regularly undergo conformational changes to intermediately folded states during the course of activity, particularly upon interaction with ligands or substrates.…”

“…So far, extensive efforts have been devoted to probe conformational changes in proteins with pH, temperature, and chemical denaturants . Surfactant is a kind of denaturant of protein and can provide specific insight into protein folding/unfolding processes [25,26] and the interaction of protein with surfactants has received a great deal of interest for many years [3,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In general, interactions between protein and surfactant are dependent on both Coulombic interactions and the hydrophobicities of the protein and surfactant.…”