The photocurrent response of human cones is fast and monophasic

Background  

The precise form of the light response of human cone photoreceptors in vivo has not been established with certainty. To investigate the response shape we compare the predictions of a recent model of transduction in primate cone photoreceptors with measurements extracted from human cones using the paired-flash electroretinogram method. As a check, we also compare the predictions with previous single-cell measurements of ground squirrel cone responses.     

Complexes of native ubiquitin and dodecyl sulfate illustrate the nature of hydrophobic and electrostatic interactions in the binding of proteins and surfactants

A previous study, using capillary electrophoresis (CE) [J. Am. Chem. Soc. 2008, 130, 17384-17393], reported that six discrete complexes of ubiquitin (UBI) and sodium dodecyl sulfate (SDS) form at different concentrations of SDS along the pathway to unfolding of UBI in solutions of SDS. One complex (which formed between 0.8 and 1.8 mM SDS) consisted of native UBI associated with approximately 11 molecules of SDS. The current study used CE and (15)N/(13)C-(1)H heteronuclear single quantum coherence (HSQC) NMR spectroscopy to identify residues in folded UBI that associate specifically with SDS at 0.8-1.8 mM SDS, and to correlate these associations with established biophysical and structural properties of this well-characterized protein. The ability of the surface charge and hydrophobicity of folded UBI to affect the association with SDS (at concentrations below the CMC) was studied, using CE, by converting lys-ε-NH(3)(+) to lys-ε-NHCOCH(3) groups. According to CE, the acetylation of lysine residues inhibited the binding of 11 SDS ([SDS] < 2 mM) and decreased the number of complexes of composition UBI-(NHAc)(8)·SDS(n) that formed on the pathway of unfolding of UBI-(NHAc)(8) in SDS. A comparison of (15)N-(1)H HSQC spectra at 0 mM and 1 mM SDS with calculated electrostatic surface potentials of folded UBI (e.g., solutions to the nonlinear Poisson-Boltzmann (PB) equation) suggested, however, that SDS binds preferentially to native UBI at hydrophobic residues that are formally neutral (i.e., Leu and Ile), but that have positive electrostatic surface potential (as predicted from solutions to nonlinear PB equations); SDS did not uniformly interact with residues that have formal positive charge (e.g., Lys or Arg). Cationic functional groups, therefore, promote the binding of SDS to folded UBI because these groups exert long-range effects on the positive electrostatic surface potential (which extend beyond their own van der Waals radii, as predicted from PB theory), and not because cationic groups are necessarily the site of ionic interactions with sulfate groups. Moreover, SDS associated with residues in native UBI without regard to their location in α-helix or β-sheet structure (although residues in hydrogen-bonded loops did not bind SDS). No correlation was observed between the association of an amino acid with SDS and the solvent accessibility of the residue or its rate of amide H/D exchange. This study establishes a few (of perhaps several) factors that control the simultaneous molecular recognition of multiple anionic amphiphiles by a folded cytosolic protein.     

Doping,Schottky barrier and pn junction formation in amorphous germanium and silicon by rf sputtering

A method is described for preparing doped specimens of amorphous germanium and amorphous silicon by rf sputtering. We present conductivity and thermoelectric power measurements which demonstrate that doping has been achieved, and sketch some photoconductive properties of the material. To demonstrate device potential, we have fabricated Schottky barriers and pn junctions and present iV characteristics and a photo-voltage spectrum.     

The metal-insulator (tunnel)-silicon-thyristor thresholding transducer in optical systems

We consider in this paper the system properties of the optical metal-insulator (tunnel)-silicon-thyristor (MIST) device when operating as a threshold transducer. Measured values of switching speed, noise performance and sensitivity for devices operated in the monostable mode are reported. Improvements in these parameter values are also predicted for operation in the bistable mode. For our non-optimized experimental devices, operation at 2 Mbit s^–1 with a–36 dBm receiver sensitivity is shown to be possible, thereby illustrating the suitability of the MIST in its present form for data bus applications. Areas for further improvement and development are outlined.     

A synthetic trivalent hapten that aggregates anti-2,4-DNP IgG into bicyclic trimers

This paper describes the synthesis of the trivalent hapten molecule 1, containing three 2,4-dinitrophenyl (2,4-DNP) groups, and the use of this molecule to aggregate three molecules of anti-2,4-DNP IgG into a complex with 3:2 stoichiometry (IgG312). The equilibrium product IgG312 was generated in approximately 90% yield upon mixing IgG and 1; during incubation, thermodynamically unstable, high-molecular-weight aggregates (>104 nm in diameter) form first and convert subsequently to IgG312. The thermodynamics and the kinetics of the formation of aggregates were studied using size-exclusion high-performance liquid chromatography (SE-HPLC), dynamic light scattering (DLS), and analytical ultracentrifugation (AUC). An analytical model based on multiple species in equilibrium was developed and used to interpret the SE-HPLC data. The aggregate IgG312 was more stable thermodynamically and kinetically than monomeric aggregates of this IgG with monomeric derivatives of 2,4-DNP; this stability suggests potential applications of these aggregates in biotechnology.