The photoresponsive interaction of light-sensitive azobenzene surfactants with bovine serum albumin (BSA) at neutral pH has been investigated as a means to control protein folding with light irradiation. The cationic azobenzene surfactant undergoes a reversible photoisomerization upon exposure to the appropriate wavelength of light, with the visible-light (trans) form of the surfactant being more hydrophobic than the UV-light (cis) form. As a consequence, the trans form exhibits enhanced interaction with the protein compared to the cis form of the surfactant, allowing photoreversible control of the protein folding/unfolding phenomena. Small-angle neutron-scattering (SANS) measurements are used to provide detailed information on the protein conformation in solution. A fitting of the protein shape to a lowresolution triaxial ellipsoid model indicates that three discrete forms of the protein exist in solution depending on the surfactant concentration, with lengths of approximately 90, 150, and 250 Å, respectively, consistent with additional dynamic light-scattering measurements. In addition, shape-reconstruction methods are applied to the SANS data to obtain relatively high-resolution conformation information. The results confirm that BSA adopts a heart-shaped structure in solution at low surfactant concentration, similar to the well-known X-ray crystallographic structure. At intermediate surfactant concentrations, protein elongation results as a consequence of the C-terminal portion separating from the rest of the molecule. Further increases in the surfactant concentration eventually lead to a highly elongated protein that nonetheless still exhibits some degree of folding that is consistent with the literature observations of a relatively high helical content in denatured BSA. The results clearly demonstrate that the visible-light form of the surfactant causes a greater degree of protein unfolding than the UV-light form, providing a means to control protein folding with light that, within the resolution of SANS, appears to be completely reversible.Protein folding is a remarkable process that affects nearly every aspect of biological function. The conformation of a protein in solution is generally a function of electrostatic, hydrogen-bonding, van der Waals, and hydrophobic interactions among the amino acid residues that all typically favor a folded conformation, overcoming the entropic penalty associated with this folding of the protein into a compact state. For a given amino acid sequence, the protein will often adopt a unique structure in solution, termed the native state, whereby the charged and polar amino acid groups are exterior and exposed to water, while the nonpolar moieties generally reside in the interior of the folded structure, protected from unfavorable solvent interactions. Protein unfolding can be induced by a variety of external conditions such as changes in pH (ionization of nonpolar residues), temperature (complex interplay between enthalpic and entropic effects), and pressure (a conse...

The viscosity and gelation of mixtures of hydrophobically modified poly(acrylic acid) (HM-PAA) and a cationic photosensitive surfactant can be controlled reversibly by changing between UV and visible light irradiation of the sample. At the critical aggregation concentration (cac) of the surfactant, micellar aggregates form on the polymer and solubilize the alkyl side chains grafted on the HM-PAA, leading to physical cross-linking and gelation. The hydrophobic trans (visible light) form, with a planar azobenzene group in the surfactant tail, has a lower cac than the more polar cis (UV light) form, resulting in gelation at lower surfactant concentrations under visible light. Reversible viscosity changes of up to 2 orders of magnitude are observed upon exposure to UV or visible light. Observed viscosity maxima of 5.6 × 10 4 cPa for the trans form and 2.2 × 10 3 cPa for the cis form of the surfactant suggest that the transform surfactant micelles are more effective at solubilizing the alkyl side chains than are the more hydrophilic cis-form micelles. Steady-state fluorescence studies of the cationic probe crystal violet reveal an enhancement in binding of the crystal violet to sites adjacent to bound surfactant molecules as the surfactant concentration is initially increased prior to the cac, followed by a decrease in binding of crystal violet at concentrations above the cac due to a decrease in the number of available binding sites as the anionic polyelectrolyte wraps around the newly formed cationic micelles. The nearest-neighbor binding is greater in the trans form as opposed to the cis. Surface tension values decrease slowly with increasing surfactant concentration below the cac due to strong binding of the surfactant to the polyelectrolyte, followed by a large drop in the surface tension at the cac that results from release of bound surfactant upon micelle formation and subsequent wrapping of the surfactant aggregates by the polyelectrolyte. Dynamic viscoelastic measurements are typical of gel systems with G′ > G′′ above the cac and also indicate that approximately 25% of the polymer chains are elastically effective in the presence of the trans form, while only 7.5% of polymer chains participate in cross-linking in the presence of the cis form of the surfactant.

The self-assembly behavior of a light-sensitive azobenzene-based surfactant, both in pure surfactant solutions and in the presence of a hydrophobically modified, water-soluble polymer, has been investigated using small-angle neutron scattering (SANS), light scattering, and UV-vis absorption techniques. The surfactant undergoes reversible photoisomerization upon exposure to the appropriate wavelength of light, with the trans form predominant under visible light being more hydrophobic than the cis isomer under UV-light. As a result, the trans form exhibits a lower critical micelle concentration than does the cis form of the surfactant, allowing photoreversible control of micelle formation. The SANS measurements reveal that micelle formation in pure surfactant solutions with the trans surfactant proceeds as commonly observed in traditional alkyl-based surfactants. Fully developed micelles were observed with aggregation numbers >50, whereas the micelle shapes are consistent with triaxial ellipsoids with axes R(a), R(b), and R(c) approximately equal to 20, 30, and 30-35 A, respectively. In contrast, with the surfactant in the cis conformation disk-shaped premicellar aggregates were observed at low surfactant concentrations with aggregation numbers <10, thicknesses of 6-10 A, and radii of 10-20 A whereas elevated cis-azoTAB concentrations eventually gave rise to fully developed micelles akin to the trans micelles. This stark difference between the self-assembly behavior of the two azobenzene isomers is ascribed to the different geometries of the surfactant in the trans (planar) and cis (bent) conformation. In the presence of the hydrophobically modified polymer, however, both surfactant isomers resulted in well-developed micelles at the respective critical aggregation concentrations (cac's), presumably because of the effect of the dodecyl side chains attached to the polymer on the conformation of the mixed alkyl-azobenzene micelles.