Photocontrol of Protein Folding:  The Interaction of Photosensitive Surfactants with Bovine Serum Albumin

Abstract

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...

Adsorption and Conformation of Serum Albumin Protein on Gold Nanoparticles Investigated Using Dimensional Measurements and in Situ Spectroscopic Methods

“…Much more work has been reported regarding the conformation of free (unbound) BSA. 12,28,31,34,36,46,69 Relevant to the present study, Kun et al 69 employed DLS over various pH conditions and were able to demonstrate a larger hydrodynamic size for the E form than the N form. Li et al 70 examined the conformation of BSA as a function of pH adsorbed at a flat gold surface using force microscopy and small-angle neutron scattering; their findings indicated that the conformation of BSA conjugates is more compact when the pH is close to the isoelectric point of native BSA (4.7).…”

“…Using an equilateral triangular model for globular N-form BSA with 8 nm edges and 3 nm thickness, 31 A BSA ≈ 25.9 nm 2 (details of calculation of A BSA are included in the Supporting Information). N is the number of BSA molecules attached to the cross-sectional perimeter of the AuNPs (πd p0,m ) and N = N m πd p0 .…”

“…This result indicates possibly less R-helix presenting in the BSA conjugates at acidic pH relative to neutral pH, and a possible explanation may be the reduction of intermolecular electrostatic interaction, similar to the free BSA in different pH environments. 12,28,31,34,36,46,69 Secondary structures of proteins are important to proteins functionality, in particular its biological activity. For instance, altering the secondary structure of proteins can significantly impact the corresponding enzyme activity.…”

Chemically Induced Unfolding of Bovine Serum Albumin by Urea and Sodium Dodecyl Sulfate: A Spectral Study with the Polarity‐Sensitive Charge‐Transfer Fluorescent Probe (E)‐3‐(4‐Methylaminophenyl)acrylic Acid Methyl Ester

“…[26][27][28][29][30][31] In the native coiled state, most of the hydrophobic moieties are buried deep in the protein interior. Ionic surfactants like SDS interact electrostatically with the polar amino acids residing on the exterior of the protein in the native state.…”

General hydrophobic interaction potential for surfactant/lipid bilayers from direct force measurements between light-modulated bilayers

“…Light-responsive moieties have also been used in a number of recent studies (3)(4)(5)(6) to actively control surfactant architecture, utilizing functional groups such as azobenzene and stilbene, which reversibly isomerize from a straight trans conformation in visible light to a bent cis form upon UV illumination. Azobenzene-functionalized surfactants can reversibly modify surface tension at the air-water interface (3), form catanionic vesicles that can be reversibly disrupted and reformed for gene, DNA, and drug delivery applications (5-7), and reversibly control protein folding (4,8). In this work, a photoresponsive azobenzene moiety has been incorporated into a conventional single-chain surfactant as a model light-responsive surfactant system.…”