Folding control and unfolding free energy of yeast iso-1-cytochrome c bound to layered zirconium phosphate materials monitored by surface plasmon resonance.

The free energy change (Delta G degrees ) for the unfolding of immobilized yeast iso-1-cytochrome c (Cyt c) at nanoassemblies was measured by surface plasmon resonance (SPR) spectroscopy. Data show that SPR is sensitive to protein conformational changes, and protein solid interface exerts a major influence on bound protein stability. First, Cyt c was self-assembled on the Au film via the single thiol of Cys-102. Then, crystalline sheets of layered alpha-Zr(O(3)POH)(2).H(2)O (alpha-ZrP) or Zr(O(3)PCH(2)CH(2)COOH)(2).xH(2)O (alpha-ZrCEP) were adsorbed to construct alpha-ZrP/Cyt c/Au or alpha-ZrCEP/Cyt c/Au nanoassemblies. The construction of each layer was monitored by SPR, in real time, and the assemblies were further characterized by atomic force microscopy and electrochemical studies. Thermodynamic stability of the protein nanoassembly was assessed by urea-induced unfolding. Surprisingly, unfolding is reversible in all cases studied here. Stability of Cyt c in alpha-ZrP/Cyt c/Au increased by approximately 4.3 kJ/mol when compared to the unfolding free energy of Cyt c/Au assembly. In contrast, the protein stability decreased by approximately 1.5 kJ/mol for alpha-ZrCEP/Cyt c/Au layer. Thus, OH-decorated surfaces stabilized the protein whereas COOH-decorated surfaces destabilized it. These data quantitate the role of specific functional groups of the inorganic layers in controlling bound protein stability.

Surface plasmon resonance study of vesicle rupture by virus-mimetic attack.

Frank and coworkers [N. J. Cho, S. J. Cho, K. H. Cheong, J. S. Glenn and C. W. Frank, J. Am. Chem. Soc., 2007, 129, 10050] investigated what happens when lipid vesicles made of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), which serves as a mimic for cell membranes, are exposed to the amphipathic helix peptide, PEP1, which is of the same type found in hepatitis C virus. Using atomic force field microscopy and quartz crystal microbalance measurements they presented evidence that the vesicle is transformed into a lipid bilayer. We use surface plasmon resonance (SPR) microscopy to follow this process in real time. We find an induction period (intermediate state) of approximately 10-min duration between the time of membrane binding and membrane rupture. The SPR data support the interpretation that a lipid bilayer is formed and allow us to put forward a mechanism for the vesicle-rupture event. As a side benefit, we demonstrate how to build two-dimensional lipid patterns on a gold surface using this vesicle-rupture process.

Direct observation of a cooperative mechanism in the adsorption of heavy metal ions to thiolated surface by in-situ surface plasmon resonance measurements.

S-type adsorption kinetics, obtained by surface plasmon resonance spectroscopy measurements, suggest that the rate of Pt2+ adsorption on 1,6-hexanedithiol (HDT) on gold increases until the surface coverage reaches ca. 17%, after which, the adsorption profile of Pt2+ follows Langmuirian behavior for the surface coverage.

Gold nanoparticles as a colorimetric sensor for protein conformational changes.

Spherical gold nanoparticles and flat gold films are prepared in which yeast iso-1-cytochrome c (Cyt c) is covalently bound to the gold surface by a thiol group in the cystein 102 residue. Upon exposure to solutions of different pH, bound Cyt c unfolds at low pH and refolds at high pH. This conformational change causes measurable shifts in the color of the coated nanoparticle solutions detected by UV-VIS absorption spectroscopy and in the refractive index (RI) of the flat gold films detected by surface plasmon resonance (SPR) spectroscopy. Both experiments demonstrate the same trend with pH, suggesting the use of protein-covered gold nanoparticles as a simple colorimetric sensor for conformational change.

Evanescent-wave cavity ring-down investigation of polymer/solvent interactions.

Evanescent-wave cavity ring-down spectroscopy (EW-CRDS) is used to measure interfacial phenomena when methanol or water is placed in contact with a film of poly(dimethylsiloxane) (PDMS), which is attached to the face of a fused-silica prism that constitutes part of a ring cavity. In the first few minutes after contact, the uptake of methanol is slower than that of water, but after this initial period the methanol diffuses more rapidly in the film than water does. Bulk weight-gain measurements confirm this result and yield diffusion coefficients of (25.1 +/- 0.7) x 10(-7) cm(2)/s for methanol in PDMS and (7 +/- 2) x 10(-7) cm(2)/s for water in PDMS. The interfacial optical losses found in the EW-CRDS measurements result primarily from scatter. In particular, we find that delamination of the film from the fused-silica substrate dominates the optical losses in the case of methanol. This conclusion is confirmed by separate surface plasmon resonance experiments.

Undulatory delamination of thin polymer films on gold surfaces.

Using two-dimensional surface plasmon resonance measurements, we have observed the formation of traveling waves in the delamination of thin films of polydimethylsilane (PDMS) exposed to methanol. Films were spin-coated on a gold surface and the methanol was added to the top surface. The stress-induced instability caused by the swelling of the PDMS thin film when its edge is pinned to the gold surface leads to wrinkle formation and propagation at the interface. The periodic pattern is thought to be the result of an Asaro-Tiller-Grinfeld (ATG) instability.

Denaturation and renaturation of self-assembled yeast iso-1-cytochrome c on Au.

We have made surface plasmon resonance (SPR) measurements of yeast iso-1-cytochrome c (Cyt c) on a gold surface. Angle-resolved SPR curves are recorded as a function of urea concentration before and after self-assembly of the Cyt c. Exposure to a urea solution causes denaturation of Cyt c, which shifts the minimum in the SPR curve to a larger angle and decreases the signal amplitude. The Gibbs free energy change for denaturation of the protein on Au is calculated from the change of the SPR signal amplitude with urea concentration. We find that (1) Cyt c can be reversibly denatured and renatured, depending on the urea concentration, and (2) the Gibbs free energy change for denaturation of Cyt c on Au surface in water, DeltaG degrees (water), is 1.5 kcal/mol, which is approximately 4 times less than that in bulk solution.

In-situ analysis of stepwise self-assembled 1,6-hexanedithiol multilayers by surface plasmon resonance measurements.

1,6-Hexandithiol (HDT) forms 6.9 +/- 1.0 A thick defect-free monolayers on gold substrates if the solution is purged by argon during the adsorption while long term (> 1000 min) exposure of the substrate to alcoholic HDT results in the stepwise formation of multilayers in the absence of argon purging.