Photoinduced surface potential change of bacteriorhodopsin mutant D96N measured by scanning surface potential microscopy

We report the direct measurement of photoinduced surface potential differences of wild-type (WT) and mutant D96N bacteriorhodopsin (BR) membranes at pH 7 and 10.5. Atomic force microscopy (AFM) and scanning surface potential microscopy (SSPM) were used to measure the BR membrane with the extracellular side facing up. We present AFM and SSPM images of WT and mutant D96N in which the light-dark transition occurred in the mid-scan of a single BR membrane. Photosteady-state populations of the M state were generated to facilitate measurement in each sample. The photoinduced surface potential of D96N is 63 mV (peak to valley) at pH 10.5 and is 48 mV at pH 7. The photoinduced surface potential of WT is 37 mV at pH 10.5 and approximately 0 at pH 7. Signal magnitudes are proportional to the amount of M produced at each pH. The results indicated that the surface potentials were generated by photoformation of surface charges on the extracellular side of the membrane. Higher surface potential correlated with a longer lifetime of the charges. A mechanistic basis for these signals is proposed, and it is concluded that they represent a steady-state measurement of the B2 photovoltage.     

A Variant of the Topkis—Veinott Method for Solving Inequality Constrained Optimization Problems

In this paper we give a variant of the Topkis—Veinott method for solving inequality constrained optimization problems. This method uses a linearly constrained positive semidefinite quadratic problem to generate a feasible descent direction at each iteration. Under mild assumptions, the algorithm is shown to be globally convergent in the sense that every accumulation point of the sequence generated by the algorithm is a Fritz—John point of the problem. We introduce a Fritz—John (FJ) function, an FJ1 strong second-order sufficiency condition (FJ1-SSOSC), and an FJ2 strong second-order sufficiency condition (FJ2-SSOSC), and then show, without any constraint qualification (CQ), that (i) if an FJ point z satisfies the FJ1-SSOSC, then there exists a neighborhood N(z) of z such that, for any FJ point y ∈ N(z) \ {z } , f ₀ (y) ≠ f ₀ (z) , where f ₀ is the objective function of the problem; (ii) if an FJ point z satisfies the FJ2-SSOSC, then z is a strict local minimum of the problem. The result (i) implies that the entire iteration point sequence generated by the method converges to an FJ point. We also show that if the parameters are chosen large enough, a unit step length can be accepted by the proposed algorithm. Accepted 21 September 1998     

Erratum to: Energy distribution of electrons in an out-of-equilibrium metallic wire

We have measured with a tunnel probe the energy distribution function of quasiparticles in the middle of a 1.5 μm-long copper wire placed between two reservoirs at different potentials. We find a distribution function close to half the sum of the Fermi distributions in the reservoirs. The deviation from this behavior is not yet understood.     

A Spectroscopic and Theoretical Investigation of a Free‐Base meso‐Trithienylcorrole

The unique optical properties of free‐base meso‐tris(5‐methylthien‐2‐yl)corrole were compared to those of the widely investigated meso‐triphenyl‐substituted analogue. A combination of spectroscopic and computational experiments was undertaken to elucidate the relationship between structural features of the neutral, mono‐anionic and mono‐cationic forms of the corroles and their corresponding optical properties. A general bathochromic shift was measured for the thienyl‐substituted corrole. The experimental spectra are supported by excited state calculations. A systematic series of ground state minimizations were performed to determine energy minima for the flexible and solvent‐sensitive molecules. Trithienylcorrole was found to have a more nonplanar macrocycle in conjunction with a high degree of π‐overlap with the meso‐substituents. Both structural features contribute to their bathochromically shifted optical spectra. The configurational character of the thienyl‐substituted corrole is shown to have a larger degree of molecular orbital mixing and doubly excited character, which suggest a more complex electronic structure that does not fully adhere to the Gouterman four‐orbital model. The reactivity of the thienyl groups, particularly with respect to their ability to be (electro)‐polymerized, combined with the tight coupling of the meso‐thienyl groups with the corrole chromophore elucidated in this work, recommends the meso‐thienylcorroles as building blocks in, for instance, organic semiconductor devices.     

Phase coherence of conduction electrons below the Kondo temperature

We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature T falls below T_{K}. The contribution to tau_{varphi};{-1} from the Fe impurities is clearly visible over the temperature range 40 mK-10 K. Below T_{K}, tau_{varphi};{-1} falls rapidly until T/T_{K} approximately 0.1, in agreement with recent theoretical calculations. At lower T tau_{varphi};{-1} deviates from theory with a flatter T-dependence. Understanding this anomalous dephasing for T/T_{K}<0.1 may require theoretical models with larger spin and number of channels.     

Bounding separable recourse functions with limited distribution information

The recourse function in a stochastic program with recourse can be approximated by separable functions of the original random variables or linear transformations of them. The resulting bound then involves summing simple integrals. These integrals may themselves be difficult to compute or may require more information about the random variables than is available. In this paper, we show that a special class of functions has an easily computable bound that achieves the best upper bound when only first and second moment constraints are available.This research has been partially supported by the National Science Foundation under Grants ECS-8304065 and ECS-8815101, by the Office of Naval Research Grant N00014-86-K-0628 and by the National Research Council under a Research Associateship at the Naval Postgraduate School, Monterey, California.     

Computational complexity of Van der Heyden's variable dimension algorithm and Dantzig-Cottle's principal pivoting method for solving LCP's

We show that Van der Heyden's variable dimension algorithm and Dantzig and Cottle's principal pivoting method require 2ⁿ–1 pivot steps to solve a class of linear complementarity problems of ordern. Murty and Fathi have previously shown that the computational effort required to solve a linear complementarity problem of ordern by Lemke's complementary pivot algorithm or by Murty's Bard-type algorithm is not bounded above by a polynomial inn. Our study shows that the variable dimension algorithm and the principal pivoting method have similar worst case computational requirements.