We consider circular currents in molecular wires with loop substructures studied within simple tight-binding models. Previous studies of this issue have focused on specific molecular structures. Here we address several general issues. First we consider the quantitative definition of a circular current and adopt a definition that identified the circular component of a loop current as the sole source of the magnetic field induced in the loop. The latter may be associated with the field at the loop center, with the magnetic moment associated with this field or with the total magnetic flux threading the loop. We show that all three definitions yield an identical measure of the loop current. Secondly, we study dephasing effects on the loop current and the associated induced magnetic field. Finally, we consider circular currents in several molecular structures: benzene, azulene, naphthalene and anthracene and show that circular currents occur generically in such structures and can be, in certain voltage ranges, considerably larger than the net current through the molecule, and are furthermore quite persistent to normal thermal dephasing.Comment: 25 pages, 14 figure

Research toward renewable and sustainable energy has identified specific terpenes capable of supplementing or replacing current petroleum-derived fuels. Despite being naturally produced and stored by many plants, there are few examples of commercial recovery of terpenes from plants because of low yields. Plant terpene biosynthesis is regulated at multiple levels, leading to wide variability in terpene content and chemistry. Advances in the plant molecular toolkit, including annotated genomes, high-throughput omics profiling, and genome editing, have begun to elucidate plant terpene metabolism, and such information is useful for bioengineering metabolic pathways for specific terpenes. We review here the status of terpenes as a specialty biofuel and discuss the potential of plants as a viable agronomic solution for future terpene-derived biofuels.

While mesoscopic conducting loops are sensitive to external magnetic fields, as is pronouncedly exemplified by observations of the Aharonov-Bohm (AB) effect in such structures, the small radius of molecular rings implies that the field needed to observe the AB periodicity is unrealistically large. In this paper we study the effect of magnetic field on electronic transport in molecular conduction junctions involving ring molecules, aiming to identify conditions where magnetic field dependence can be realistically observed. We consider electronic conduction of molecular ring structures modeled both within the tightbinding (Hückel) model and as continuous rings. We also show that much of the qualitative behavior of conduction in these models can be rationalized in terms of a much simpler junction model based on a two-state molecular bridge. Dephasing in these models is affected by two common tools: the Büttiker probe method and coherence damping within a density matrix formulation. We show that current through benzene ring can be controlled by moderate fields provided that several conditions are satisfied: (a) conduction must be dominated by degenerate (in the free molecule) molecular electronic resonances, associated with multiple pathways as is often the case with ring molecules; (b) molecular-leads electronic coupling must be weak so as to affect relatively distinct conduction resonances; (c) molecular binding to the leads must be asymmetric (e.g., for benzene, connection in the meta or ortho, but not para, configurations) and, (d) dephasing has to be small. When these conditions are satisfied, considerable sensitivity to an imposed magnetic field normal to the molecular ring plane is found in benzene and other aromatic molecules. Interestingly, in symmetric junctions (e.g. para connected benzene) the transmission coefficient can show sensitivity to magnetic field that is not reflected in the current-voltage characteristic. The analog of this behavior is also found in the continuous ring and the two level models. 2Although sensitivity to magnetic field is suppressed by dephasing, quantitative estimates indicate that magnetic field control can be observed in suitable molecular conduction junctions.3

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The mechanism of action of antimicrobial peptides is traditionally attributed to the formation of pores in the lipid cell membranes of pathogens, which requires a substantial peptide to lipid ratio. However, using incoherent neutron scattering, we show that even at a concentration too low for pore formation, an archetypal antimicrobial peptide, melittin, disrupts the regular phase behavior of the microscopic dynamics in a phospholipid membrane, dimyristoylphosphatidylcholine (DMPC). At the same time, another antimicrobial peptide, alamethicin, does not exert a similar effect on the DMPC microscopic dynamics. The melittin-altered lateral motion of DMPC at physiological temperature no longer resembles the fluid-phase behavior characteristic of functional membranes of the living cells. The disruptive effect demonstrated by melittin even at low concentrations reveals a new mechanism of antimicrobial action relevant in more realistic scenarios, when peptide concentration is not as high as would be required for pore formation, which may facilitate treatment with antimicrobial peptides.

We consider the generation of pure spin currents by electric field driving. First, we discuss the possibility of spin pumping by electric field within a simple two-level bridge model. Then, we apply the scheme to study spin transport in single-and doublestranded DNA junctions. Within a physically relevant range of parameters we show the possibility of generating pure spin currents in DNA, even in the absence of spin−orbit coupling.

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