EXCITON TRANSFER OUT OF OPEN PHOTOSYSTEM II REACTION CENTERS

Abstract A simple photochemical model of photosystem II consisting of antenna chlorophyll and a reaction center was used to examine the phenomenon of exciton detrapping, i.e. the transfer of excitation energy from open reaction centers back to the antenna. η, the ratio of the probability of detrapping when the reaction centers are all open, Ψ_t(o) to the probability when the centers are closed, Ψ_t(x) was used as a variable parameter to examine the various pathways of energy dissipation in a system in which P, the yield of photochemistry, and R, the ratio of the maximum to the minimum yields of fluorescence, were assumed to be known (e.g. R= 4.0 and P= 0.90). It is shown that η must fall within a range of values between 0 and R (1 –P) and that, for given values of R and P, Ψ_t(o) and the ratio of the rate constant for photochemistry at the reaction center, k_p, to the rate constant for energy transfer back to the antenna, k_t, can be determined for any assumed value of η. Even though detrapping occurs at open reaction centers, it is the magnitude of the yield of nonradiative decay at closed reaction centers, Ψ_a(x) which sets the upper limit on η. Equations for the overall yields of fluorescence and nonradiative decay in the antenna chlorophyll and of nonradiative decay at the reaction center chlorophyll, under conditions of both open and closed reaction centers, were derived in conventional probability terms and in terms of R, P and η. As η increases within its range of permissible values, energy dissipation in the antenna decreases and nonradiative decay at the reaction center increases. The determination of a specific value of η or of the ratio k_pk_t would require additional information such as the value of the maximum yield of fluorescence and the ratio of the rate constants for fluorescence and nonradiative decay in the antenna chlorophyll. The characteristics of a system in which there is no nonradiative decay in the reaction center (i.e. k_d= 0), in which case R (1 –P) = 1.0, were also examined. In this case the yield of detrapping has no influence on energy dissipation in the system. Finally, the question of heterogeneity in PSII was considered. It is suggested that Ψ_d(x) may be greater in PSII_β than in PSII_α so that the probability of detrapping could be greater in the PSII_α fraction.     

Analysis and Experiments for a Computational Model of a Heat Bath

A question of some interest in computational statistical mechanics is whether macroscopic quantities can be accurately computed without detailed resolution of the fastest scales in the problem. To address this question a simple model for a distinguished particle immersed in a heat bath is studied (due to Ford and Kac). The model yields a Hamiltonian system of dimension 2N+2 for the distinguished particle and the degrees of freedom describing the bath. It is proven that, in the limit of an infinite number of particles in the heat bath (N), the motion of the distinguished particle is governed by a stochastic differential equation (SDE) of dimension 2. Numerical experiments are then conducted on the Hamiltonian system of dimension 2N+2 (N1) to investigate whether the motion of the distinguished particle is accurately computed (i.e., whether it is close to the solution of the SDE) when the time step is small relative to the natural time scale of the distinguished particle, but the product of the fastest frequency in the heat bath and the time step is not small—the underresolved regime in which many computations are performed. It is shown that certain methods accurately compute the limiting behavior of the distinguished particle, while others do not. Those that do not are shown to compute a different, incorrect, macroscopic limit.     

Dynamic Hierarchical Packing of Wireless Switches Using a Seed, Repair and Replace Genetic Algorithm

Suppose that items of equipment are to be added to a supply station (e.g., new switch modules are to be added to a telecommunications switch) over time to meet growing demand requirements. Both supply and demand have multiple components: an item of equipment supplies different amounts of several resources, and demand may be expressed in terms of the vector of resources required. There are several different types of equipment to choose among, each type supplying known amounts of each resource per unit of equipment. The supply station is organized into bays, shelves, or other capacitated containers so that when the cumulative amount of equipment added exceeds the holding capacity of the installed containers, new containers must be added, creating a relatively large jump in cumulative costs. Thus, it is desirable to sequentially pack items of equipment into the available containers, by choosing which types of equipment to install when, so as to minimize the total cost of covering demand in each period. We discuss an instance of this problem arising from wireless telephony and describe the performance of a conventional branch-and-bound optimization algorithm for solving it. The branch-and-bound approach works well on small instances of the problem, and has been used successfully in practical planning. However, it can take CPU-days to run, thus preventing development of a useful interactive planning tool. Therefore, we introduce a novel seed, repair, and replace genetic algorithm (SRR-GA) for solving dynamic packing problems of this type. We contrast its performance with the branch-and-bound algorithm's on both hand-generated and randomly-created dynamic packing problems, finding that the SRR-GA is two to three orders of magnitude faster and produces solutions of equal or better quality on practical problems. Variations of the dynamic packing problem and of the SRR-GA for solving it are mentioned, and the paper concludes by suggesting other potential applications of the SRR-GA to hard combinatorial optimization problems.     

Analytic solutions of simple flows and analysis of nonslip boundary conditions for the lattice Boltzmann BGK model

In this paper we analytically solve the velocity of the lattice Boltzmann BGK equation (LBGK) for several simple flows. The analysis provides a framework to theoretically analyze various boundary conditions. In particular, the analysis is used to derive the slip velocities generated by various schemes for the nonslip boundary condition. We find that the slip velocity is zero as long as fe=0 at boundaries, no matter what combination of distributions is chosen. The schemes proposed by Nobleet al. and by Inamuroet al. yield the correct zeroslip velocity, while some other schemes, such as the bounce-back scheme and the equilibrium distribution scheme, would inevitably generate a nonzero slip velocity. The bounce-back scheme with the wall located halfway between a flow node and a bounce-back node is also studied for the simple flows considered and is shown to produce results of second-order accuracy. The momentum exchange at boundaries seems to be highly related to the slip velocity at boundaries. To be specific, the slip velocity is zero only when the momentum dissipated by boundaries is equal to the stress provided by fluids.     

Length‐Independent Charge Transport in Chimeric Molecular Wires

Advanced molecular electronic components remain vital for the next generation of miniaturized integrated circuits. Thus, much research effort has been devoted to the discovery of lossless molecular wires, for which the charge transport rate or conductivity is not attenuated with length in the tunneling regime. Herein, we report the synthesis and electrochemical interrogation of DNA‐like molecular wires. We determine that the rate of electron transfer through these constructs is independent of their length and propose a plausible mechanism to explain our findings. The reported approach holds relevance for the development of high‐performance molecular electronic components and the fundamental study of charge transport phenomena in organic semiconductors.     

Perspectives of approximate dynamic programming

Approximate dynamic programming has evolved, initially independently, within operations research, computer science and the engineering controls community, all searching for practical tools for solving sequential stochastic optimization problems. More so than other communities, operations research continued to develop the theory behind the basic model introduced by Bellman with discrete states and actions, even while authors as early as Bellman himself recognized its limits due to the “curse of dimensionality” inherent in discrete state spaces. In response to these limitations, subcommunities in computer science, control theory and operations research have developed a variety of methods for solving different classes of stochastic, dynamic optimization problems, creating the appearance of a jungle of competing approaches. In this article, we show that there is actually a common theme to these strategies, and underpinning the entire field remains the fundamental algorithmic strategies of value and policy iteration that were first introduced in the 1950’s and 60’s.     

Nanotexaphyrin: One‐Pot Synthesis of a Manganese Texaphyrin‐Phospholipid Nanoparticle for Magnetic Resonance Imaging

The discovery and synthesis of novel multifunctional organic building blocks for nanoparticles is challenging. Texaphyrin macrocycles are capable and multifunctional chelators. However, they remain elusive as building blocks for nanoparticles because of the difficulty associated with synthesis of texaphyrin constructs capable of self‐assembly. A novel manganese (Mn)‐texaphyrin‐phospholipid building block is described, along with its one‐pot synthesis and self‐assembly into a Mn‐nanotexaphyrin. This nanoparticle possesses strong resilience to manganese dissociation, structural stability, in vivo bio‐safety, and structure‐dependent T₁ and T₂ relaxivities. Magnetic resonance imaging (MRI) contrast enhanced visualization of lymphatic drainage is demonstrated with respect to proximal lymph nodes on the head and neck VX‐2 tumors of a rabbit. Synthesis of 17 additional metallo‐texaphyrin building blocks suggests that this novel one‐pot synthetic procedure for nanotexaphyrins may lead to a wide range of applications in the field of nanomedicines.     

A method for the calculation of normal-mode vibrational frequencies using symmetry coordinates. Application to the calculation of secondary deuterium isotope effects on carbocations

A method is introduced for the calculation of normal-mode vibrational frequencies of polyatomic molecules based on numerical differencing of analytical gradients in symmetry coordinates. This procedure requires a number of gradient evaluations equal to the largest number of symmetry coordinates belonging to any single irreducible representation of the molecular point group (plus a single gradient evaluation at the equilibrium configuration), which is fewer than the 3N-6 (N atoms) gradient evaluations needed for schemes based on Cartesian or internal coordinates. While the proposed method will not generally be as efficient as procedures which involve the direct calculation of energy second derivatives analytically (as are now available for single-determinant wavefunctions) it appears to be equally accurate, and it should be the method of choice for frequency calculations involving multideterminant wavefunctions for which analytical second-derivative algorithms have yet to be developed. The method is illustrated by the calculation of equilibrium secondary deuterium-isotope effects on a number of reactions involving simple carbocations.