Contact effects on electronic transport in donor-bridge-acceptor complexes interacting with a thermal bath

A model for electron transfer in donor-bridge-acceptor complexes with electronic coupling to nuclear bridge modes is studied using the Redfield formulation. We demonstrate that the transport mechanism through the molecular bridge is controlled by the location of the electronic-nuclear coupling term along the bridge. As the electronic-nuclear coupling term is shifted from the donor/acceptor-bridge contact sites into the bridge, the mechanism changes from kinetic transport (incoherent, thermally activated, and bridge-length independent) to coherent tunneling oscillations. This study joins earlier works aiming to explore the factors which control the mechanism of electronic transport through molecular bridges and molecular wires.     

Subtle differences in structural transitions between poly-L- and poly-D-amino acids of equal length in water

Mirror-image asymmetric molecules, i.e., chiral isomers or enantiomers, are classically considered as chemically identical. Recent studies, however, have indicated that parity violation by the nuclear weak force induces a tiny energy difference between chiral isomers. Upon combination with a massive amplification process, expansion of this difference to a detectable macroscopic level may be achieved. Yet, experimental tests of this possibility, where one enantiomer is compared to the other in solution, are hampered by the possible presence of undetectable impurities. In this study we have overcome this problem by comparing structural and dynamic features of synthetic D- and L-polyglutamic acid and polylysine molecules each of 24 identical residues. In these water-soluble polypeptides helix formation is an intramolecular autocatalytic process amplified by each turn, which is actually unaffected by low level of putative impurities in the solvent. The helix and random coil configurations and their transition were determined in this study by circular dichroism (CD) and isothermal titration calorimetry (ITC) in water and deuterium oxide. Distinct differences in structure and transition energies between the enantiomeric polypeptides were detected by both CD and ITC when dissolved in water. Intriguingly, these differences were by and large abolished in deuterium oxide. Our findings suggest that deviation from physical invariance between the D- and L-polyamino acids is induced in part by different hydration in water which is eliminated in deuterium oxide. Based on the recent findings by Tikhonov and Volkov (V. I. Tikhonov and A. A. Volkov, Science 2002, 296, 2363) we suggest that ortho-H(2)O, which constitutes 75% of bulk H(2)O, has a preferential affinity to L-enantiomers. Differential hydration of enantiomers may have played a role in the selection of L-amino acids by early forms of life.     

Colloidal Hybrid Nanostructures: A New Type of Functional Materials

One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor–metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.     

Some characteristics of multibeam holography

Holograms recorded with a number of reference beams are sensitive to alignment when reconstructed. Interference effects due to misalignment are evaluated, and as a result optimal recording parameters can be suggested.     

An eigenvalue problem for the numerical range of a bounded linear operator

LetX be a complex Lebesgue space with a unique duality mapJ fromX toX ^*, the conjugate space ofX. LetA be a bounded linear operator onX. In this paper we obtain a non-linear eigenvalue problem for (A)=sup{Re: W(A} whereW(A)={J(x)A(x)) : x=1}, under the assumption that (A) and the convex hull ofW(A) for some linear operatorsA onl _p , 2<p<.     

Atomic multipoles and perpendicular electrostatic forces in diatomic and planar molecules

Current methods for assigning atomic multipoles focus on reproduction of the molecular electrostatic potential. Another aspect of electrostatic interaction, which is usually overlooked, is the forces that an external electric field exerts on the nuclei of a molecule. In a self-consistent theory, both the electrostatic potential and force should be accounted for. However, in general it is not easy to meet this requirement for the force. For planar molecules, though, a formal solution is available in terms of atomic multipoles that are extracted from the molecular multipolar tensors. These Force-Related (FR) atomic multipoles are discussed in detail for some typical diatomics and planar polyatomics, and are shown to provide a solid uniform framework for treating both aspects of the electrostatics. In contrast, the commonly used potential-derived charges (i.e., the atomic charges obtained by fitting the electrostatic potential) can yield large deviations with respect to electrostatic forces on the nuclei, even when the electrostatic potential is very well reproduced.     

Krein's strings,the symmetric moment problem,and extending a real positive definite function

The symmetric moment problem is to find a possibly unique, positive symmetric measure that will produce a given sequence of moments {M_n}. Let us assume that the (Hankel) condition for existence of a solution is satisfied, and let σ_n be the unique measure, supported on n points, whose first 2n moments agree with M₀,…,M_2n−1. It is known that σ_2n ⇒ σ₀ (weak convergence) and σ_2n+1 ⇒ σ_∞, where σ₀ and σ_∞ are solutions to the full moment problem. Moreover, σ₀ = σ_∞ if and only if the problem has a unique solution. In this paper we present an analogue of this theorem for Krein's problem of extending to ℝ a real, even positive definite function originally defined on [−T,T] where T < ∞. Our proof relies on the machinery of Krein's strings. As we show, these strings help explain the connection between the moment and the extension problems. © 1999 John Wiley & Sons, Inc.     

The realization graph of a degree sequence with majorization gap 1 is Hamiltonian

It is known that the degree sequences of threshold graphs are characterized by the property that they are not majorized strictly by any degree sequence. Consequently every degree sequence d can be transformed into a threshold sequence by repeated operations consisting of subtracting I from a degree and adding 1 to a larger or equal degree. The minimum number of these operations required to transform d into a threshold sequence is called the majorization gap of d. A realization of a degree sequence d of length n is a graph on the vertices 1, …, n, where the degree of vertex i is d_i. The realization graph %plane1D;4A2;(d) of a degree sequence d has as vertices the realizations of d, and two realizations are neighbors in %plane1D;4A2;(d) if one can be obtained from the other by deleting two existing edges [a, b], [c, d] and adding two new edges [a, d]; [b, c] for some distinct vertices a, b, c, d. It is known that %plane1D;4A2;(d) is connected. We show that if d has a majorization gap of 1, then %plane1D;4A2;(d) is Hamiltonian.