Stability Of bR Research Articles

The Role of Graphene Monolayers in Enhancing the Yield of Bacteriorhodopsin Photostates for Optical Memory Applications

Bacteriorhodopsin (bR) is a photoactive protein that has gained increasing importance as a tool for optical memory storage due to its remarkable photochemical and thermal stability. The two stable photostates (bR and Q) obtained during the bR photocycle are appropriate to designate the binary bit 0 and 1, respectively. Such devices, however, have limited success due to a low quantum yield of the Q state. Many studies have used genetic and chemical modification as optimization strategies to increase the yield of the Q state. Nonetheless, this compromises the overall photochemical stability of bR. This paper introduces a unique way of stabilizing the conformations of bacteriorhodopsin and, thereby, the bR and Q photostates through adsorption onto graphene. All-atom molecular dynamics (MD) simulations with NAMD and CHARMM force fields have been used here to understand the interactive events at the interface of the retinal chromophore within bR and a single-layer graphene sheet. Based on the stable RMSD (~4.5 Å), secondary structure, interactive van der Waals energies (~3000 kcal/mol) and electrostatic energies (~2000 kcal/mol), it is found that the adsorption of bR onto graphene can stabilize its photochemical behavior. Furthermore, the optimal adsorption distance for bR is found to be ~4.25 Å from the surface of graphene, which is regulated by a number of interfacial water molecules and their hydrogen bonds. The conformations of the key amino acids around the retinal chromophore that are responsible for the proton transport are also found to be dependent on the adsorption of bR onto graphene. The quantity and lifetime of the salt bridges also indicate that more salt bridges were formed in the absence of graphene, whereas more were broken in the presence of it due to conformational changes. Finally, the analysis on the retinal dihedrals (C11 = C12-C13 = C14, C12-C13 = C14-C15, C13 = C14-C15 = NZ and C14-C15 = NZ-CE) show that bacteriorhodopsin in the presence of graphene exhibits increased stability and larger dihedral energy values.

Bacteriorhodopsin/TiO2 nanotube arrays hybrid system for enhanced photoelectrochemical water splitting

In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO2 nanotube array hybrid electrode system. Photoanode materials composed of ∼ 7μm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamide electrolytes containing NH4F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO2 surface was found to depend on the pretreatment process of the TiO2 films. Our results demonstrate the opportunity to fabricate fairly stable bR/TiO2 hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm2), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm2 which is a ∼ 50% increase over that measured for pure TiO2 nanotubes (0.43 mA/cm2) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm2. To the best of our knowledge, this is the first report on the use of bR/TiO2 hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.

The Contribution of a Covalently Bound Cofactor to the Folding and Thermodynamic Stability of an Integral Membrane Protein

The factors controlling the stability, folding, and dynamics of integral membrane proteins are not fully understood. The high stability of the membrane protein bacteriorhodopsin (bR), an archetypal member of the rhodopsin photoreceptor family, has been ascribed to its covalently bound retinal cofactor. We investigate here the role of this cofactor in the thermodynamic stability and folding kinetics of bR. Multiple spectroscopic probes were used to determine the kinetics and energetics of protein folding in mixed lipid/detergent micelles in the presence and absence of retinal. The presence of retinal increases extrapolated values for the overall unfolding free energy from 6.3 ± 0.4 kcal mol − 1 to 23.4 ± 1.5 kcal mol − 1 at zero denaturant, suggesting that the cofactor contributes 17.1 kcal mol − 1 towards the overall stability of bR. In addition, the cooperativity of equilibrium unfolding curves is markedly reduced in the absence of retinal with overall m-values decreasing from 31.0 ± 2.0 kcal mol − 1 to 10.9 ± 1.0 kcal mol − 1 , indicating that the folded state of the apoprotein is less compact than the equivalent for the holoprotein. This change in the denaturant response means that the difference in the unfolding free energy at a denaturant concentration midway between the two unfolding curves is only ca 3–6 kcal mol − 1 . Kinetic data show that the decrease in stability upon removal of retinal is associated with an increase in the apparent intrinsic rate constant of unfolding, k u H2O, from ~1 × 10 − 16 s − 1 to ~1 × 10 − 4 s − 1 at 25 °C. This correlates with a decrease in the unfolding activation energy by 16.3 kcal mol − 1 in the apoprotein, extrapolated to zero SDS. These results suggest that changes in bR stability induced by retinal binding are mediated solely by changes in the activation barrier for unfolding. The results are consistent with a model in which bR is kinetically stabilized via a very slow rate of unfolding arising from protein–retinal interactions that increase the rigidity and compactness of the polypeptide chain.

Synthesis and stability of Br 2, ICI and IBr intercalated pitch-based graphite fibers

This work presents a further study of the intercalation of halogens in pitch-based fiber and the stability of the resultant intercalation compounds. P-100 fibers were intercalated with purified IBr at 50 °C to produce high electrical conductivity graphite intercalation compounds (GICs). After intercalation and subsequent equilibration in ambient atmosphere, the fibers average a five-fold conductivity,enhancement over the pristine fiber, and after nine weeks in air, the conductivity, σ, degrades by only 1%. The intercalation of ICl in P-100 was performed in vacuum-sealed vessels at 50 °C, 20° and 0 °C. Both the lowest equilibrium resistivity and its smallest ambient gain were observed for fibers reacted at 20 °C. P-100 brominated at 68 °C in vacuum-sealed vessels showed no loss in electrical and stability properties over those reacted at 20 °C in the presence of air. Energy dispersive spectroscopy (EDS) results confirm the existance of excess bromine and chlorine in the iodine interhalide GICs, which is predicted by the oxidation mechanism proposed for this class of intercalation reactions.