This paper presents a comparison of the theoretical and experimental current-voltage (I-V) characteristics of a self-assembled monolayer of a, a 0-xylyl dithiol molecules on a gold substrate measured with a scanning tunneling microscope probe. Good quantitative agreement is obtained with the tip-molecule distance as the only "fitting parameter." Several other thiol-coupled molecules that we have studied also show similar agreement. The conceptual picture presented in this paper could be useful for the interpretation of I-V measurements on molecular monolayers in general.

A relatively simple and straightforward procedure for characterizing molecular wires is to measure the conductance spectrum by forming a self-assembled ordered monolayer (SAM) on a metallic surface and using a high scanning-tunneling microscope resolution (STM) tip as the other contact. We find that the conductance spectrum (dI/dV vs. V) can be understood fairly well in terms of a relatively simple model, provided the spatial profile of the electrostatic potential under bias is properly accounted for. The effect of the potential profile is particularly striking and can convert a symmetric conductor into a rectifier and vice versa. The purpose of this paper is to (1) describe the theoretical model in detail, (2) identify the important parameters that influence the spectra and show how these parameters can be deduced directly from the conductance spectrum, and (3) compare the theoretical prediction with experimentally measured conductance spectra for xylyl dithiol and phenyl dithiol.

In this paper, we present a method for computing the resistance of molecular wires and illustrate it with a systematic theoretical study of a particular class of organic molecules. These molecules consist of one or more benzene rings with a thiol͑-SH͒ group at the ends. This end group can attach readily to metallic surfaces, thus allowing the molecule to function as a nanoelectronic interconnect. The conduction through these molecules at low bias occurs by tunneling, leading to resistances that are typically several tens of megaohms. The resistance goes up exponentially with the number of rings and is sensitive to the relative orientation of the rings and the bonding between them. The Green-function-based method presented here provides a powerful tool for accurate modeling of the semi-infinite contacts that are used to measure molecular resistance.

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The metabolism of Krebs cycle intermediates is of fundamental importance for eukaryotic cells. In the kidney, these intermediates are transported actively into epithelial cells. Because citrate is a potent inhibitor for calcium stone formation, excessive uptake results in nephrolithiasis due to hypocitraturia. We report the cloning and characterization of a rat kidney dicarboxylate transporter (SDCT1). In situ hybridization revealed that SDCT1 mRNA is localized in S3 segments of kidney proximal tubules and in enterocytes lining the intestinal villi. Signals were also detected in lung bronchioli, the epididymis, and liver. When expressed in Xenopus oocytes, SDCT1 mediated electrogenic, sodium-dependent transport of most Krebs cycle intermediates (K m ‫؍‬ 20-60 M), including citrate, succinate, ␣-ketoglutarate, and oxaloacetate. Of note, the acidic amino acids L-and D-glutamate and aspartate were also transported, although with lower affinity (K m ‫؍‬ 2-18 mM). Transport of citrate was pH-sensitive. At pH 7.5, the K m for citrate was high (0.64 mM), whereas at pH 5.5, the K m was low (57 M). This is consistent with the concept that the ؊2 form of citrate is the transported species. In addition, maximal currents at pH 5.5 were 70% higher than those at pH 7.5, and our data show that the ؊3 form acts as a competitive inhibitor. Simultaneous measurements of substrate-evoked currents and tracer uptakes under voltage-clamp condition, as well as a thermodynamic approach, gave a Na ؉ to citrate or a Na ؉ to succinate stoichiometry of 3 to 1. SDCT1-mediated currents were inhibited by phloretin. This plant glycoside also inhibited the SDCT1-specific sodium leak in the absence of substrate, indicating that at least one Na ؉ binds to the transporter before the substrate. The data presented provide new insights into the biophysical characteristics and physiological implications of a cloned dicarboxylate transporter.In kidney proximal tubules, reabsorption of Krebs cycle intermediates such as citrate, succinate, ␣-ketoglutarate, malate, and fumarate has been shown to be accomplished by Na ϩ -coupled transporters (1-9). Numerous studies have been performed in intact proximal tubules (10), isolated brush border membrane vesicles (BBMV) 1 (1-5, 11, 12), and basolateral membrane vesicles (BLMV) (12-14), mostly using citrate or succinate as substrates. In BBMV, succinate uptake was found to be mediated with low affinity (K m ϭ ϳ1 mM) (5,8,12,15). Studies on the pH dependence suggested that citrate is transported in its protonated divalent form (Cit Ϫ2 ) (1, 2, 12), whereas succinate is transported either in its deprotonated (Ϫ2) or protonated (Ϫ1) form (11). In addition, it was shown that the Ϫ3 form of citrate (trivalent form, Cit Ϫ3 ) inhibits transport of Cit Ϫ2 (11). Radiotracer studies revealed that the cotransport process exhibits a stoichiometry of 2-3 sodium ions/dicarboxylate molecule (2,8,16). On the other hand, experiments with a voltage-sensitive dye showed that the cotransport was electrogenic (14, 17), which favors a 3...

In this paper we show that (1) for graphene tubules an axial magnetic field can induce a semimetalsemiconductor transition periodically at zero bias. This is an Aharonov-Bohm-type effect and is generally true for thin cylindrical conductors; (2) the Landauer approach can be used to estimate the conductance of individual tubules measured by scanning tunneling probes as a function of applied bias, temperature, tubule geometric factors, and magnetic field. Numerical calculations are presented showing that the conductance modulation should be observable even at room temperature.Recently graphene tubules have been produced using an arc-discharge evaporation method similar to that used for fullerene synthesis. ' A monolayer graphene tubule

Double-pulse flash photolysis experiments on solutions of carbonmonoxymyoglobin (MbCO) are used to determine the time scale for protein conformational averaging. The interconversion times for transitions between the "open" and "closed" subpopulations of MbCO are found to be 10(-6)-10(-4)s, depending on solvent composition and temperature. In aqueous solution at 273 K, the interconversion rate is found to be 1.4 x 10(6)s. Since the interconversion rate is comparable to or slower than the geminate rebinding rate, we describe the geminate phase of the kinetics as a superposition of contributions from the open and closed states. Although geminate kinetics remain intrinsically nonexponential for both open and closed states near room temperature, we find that substates within these two subpopulations interconvert more rapidly than the geminate rebinding. These observations cannot be explained by a superposition of contributions from a quasicontinuous conformational distribution (Steinbach et al., 1991) and are probably due to the long-time tail of the relaxation of the protein (Tian et al., 1992). Bimolecular rebinding takes place at a statistically averaged rate, since the interconversion and relaxation rates are faster than the bimolecular kinetics. The geminate and bimolecular kinetics are analyzed quantitatively as a function of pH using this approach and the spectroscopically determined populations of the open and closed states. The analysis accounts for the observed kinetics and also successfully predicts the kinetic response observed in the double-pulse experiments. In aqueous solution at 273 K, the geminate amplitudes and rates are found to be I(0)g = 32% and k(0)g = 1.3 x 10(7)s(-1) for the open state and I(1)g = 9.3% and k(1)g = 1.4 x 10(6)s(-1) for the closed state. In 75% glycerol solution at 264 K, the dominant component of the geminate rebinding is characterized by I(0)g1 = 89% and k(0)g1 = 3.1 x 10(6)s(-1) for the open state and I(1)g1 = 26% and k(1)g1 = 3.1 x 10(6)s(-1) for the closed state. The fact that the interconversion rate is comparable to the geminate rate of the closed state in aqueous solution is consistent with the idea that the open state provides an important pathway for ligand escape from (or entry to) the heme pocket (Tian et al., 1993). The increased viscosity of 75% glycerol solution delays the closed--> open interconversion until the end of the geminate phase, which forces the ligand to find alternative pathways to the solution. This observation, in conjunction with the near equivalence of the geminate rates for the open and closed states in 75% glycerol solution, suggests that the solvent composition fundamentally alters the protein-ligand dynamics.