Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current-voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications.

This paper reports on the microstructures formed in aqueous solutions containing mixtures of sodium dodecyl sulfate (SDS) and a photosensitive, bolaform surfactant, bis(trimethylammoniumhexyloxy)azobenzene dibromide (BTHA). By using quasi-elastic light scattering and small-angle neutron scattering, we determined that aqueous solutions containing SDS and the trans isomer of BTHA (0.1 wt % total surfactant, 15 mol % BTHA, 85 mol % SDS) form vesicles with average hydrodynamic diameters of 1350 +/- 50 angstroms and bilayer thicknesses of 35 +/- 2 angstroms. The measured bilayer thickness is consistent with a model of the vesicle bilayer in which the trans isomer of BTHA spans the bilayer. Upon illumination with UV light, the BTHA underwent photoisomerization to produce a cis-rich photostationary state (80% cis isomer). We measured this photoisomerization to drive the reorganization of vesicles into cylindrical aggregates with cross-sectional radii of 19 +/- 3 angstroms and average hydrodynamic diameters of 240 +/- 50 angstroms. Equilibration of the cis-rich solution in the dark at 25 degrees C for 12 h or illumination of the solution with visible light leads to the recovery of the trans-rich photostationary state of the solution and the reformation of vesicles, thus demonstrating the potential utility of this system as the basis of a tunable fluid.

Even though the structures of cytochrome c oxidase (CcO) from different sources have been determined by X-ray crystallography in both the reduced and oxidized redox states, information about redox-induced structure-function relationships is still very limited. In the current work, redox-dependent structural changes are determined for CcO reconstituted in a protein-tethered bilayer lipid membrane by surface-enhanced infrared absorption spectroscopy in the ATR mode. Significantly, the redox changes in the enzyme are attained by direct wiring of CcO to a gold electrode, ensuring that sequential intra-protein electron transfer occurs by a directed pathway that is natural to the system. The characteristics of CcO were observed to be dramatically altered after the reconstituted enzyme was allowed to turn over in the presence of O 2. The data suggest that the enzyme is initially in an "inactive" state, but that direct electron transfer in the presence of O 2 converts the enzyme to an "activated" form which returns to the inactive conformation when the enzyme remains idle under anaerobic conditions. Potentiometric titrations are performed and reduced-minus-fully oxidized and oxidized-minus-fully reduced absorbance spectra are recorded at decreasing and increasing potentials, respectively, applied to the electrode in a regular succession. The two sets of difference spectra show mirror symmetry, however, they markedly differ from those measured in the presence of redox mediators. Plots of band area of individual bands obtained by Fourier self deconvolution vs. applied potential show a sigmoid dependence as expected for a redox process. However, the sigmoid curves do not coincide but are displaced depending on the direction of the potential change. In other words, these curves show hysteresis, which is an indication of cooperativity and non-equilibrium states for electron transfer and/or conformational changes of the protein. This is discussed in terms of known concepts of molecular hysteresis. © 2010 The Royal Society of Chemistry

Responsive polymeric brushes of poly(methacrylic acid) (PMAA) were grafted from silicon surfaces using controlled surface-initiated atom-transfer radical polymerization (SI-ATRP). The growth kinetics of PMAA was investigated with respect to the composition of the ATRP medium by grafting the polymer in mixtures of water and methanol with different ratios. The dissociation behavior of the polymer layers was characterized by FTIR titration after incubating the polymer-grafted substrates in PBS buffer solutions with different pH values. PMAA layers show a strong pH-dependent behavior with an effective pK(a) of the bulk polymer brush of 6.5 ± 0.2, which is independent of the polymer brush thickness and methanol content of the ATRP grafting medium. The pH-induced swelling and collapse of the grafted polymer layers were quantified in real time by in situ ellipsometry in liquid environment. Switching between polymer conformations at pH values of 4 and 8 is rapid and reversible, and it is characterized by swelling factors (maximum thickness/minimum thickness) that increase with decreasing the methanol content of the SI-ATRP medium.