Conductance of a biomolecular wire

Abstract

Carotenoids (Car) act as ''wires'' that discharge unwanted electrons in the reaction center of higher plants. One step in this ''side-path'' electron conduction is thought to be mediated by Car oxidation. We have carried out direct measurements of the conductance of single-Car molecules under potential control in a membrane-mimicking environment, and we found that when Car are oxidized conductance is enhanced and the electronic decay constant (␤) is decreased. However, the neutral molecule may already be conductive enough to account for observed electron transfer rates.carotenoid ͉ molecular electronics ͉ photosynthesis ͉ potential control ͉ single molecule P hotosynthetic systems are natural photoelectronic devices, integrating electronic and photonic elements in a protein scaffold. Understanding the role played by the different components is important both for understanding photosynthetic processes as well as for using them in artificial molecular electronic devices. Carotenoids (Car) in photosynthesis contribute to light harvesting, structural stabilization, and protection from photooxidation. In photosystem II reaction centers, Car participate in the ''side-path'' electron donation reactions for reduction of P 680 ϩ as part of the photoprotective system. The role of the Car is thought to be that of an electron carrier (1, 2). It is believed that ␤-carotene in photosystem II is oxidized under illumination (1, 2). To investigate whether Car oxidation is a prerequisite for electron transport in photosystem II photoprotection, we measured the conductivity and the length-dependence of the tunneling rate for single-Car molecules under potential control. Measuring electron transport under potential control allows us to simulate the redox environment of the Car under natural conditions and to relate our electronic measurements to its function in vivo.Single (neutral) Car polyenes are much more conductive than alkanes of equivalent length because of their conjugated -electron system (3-5). We and others (6-9) have shown that the conductivity of redox-active single molecules can be regulated by changes in the molecule's oxidation state. Here, we have extended our experimental methods to work in a water-and oxygen-free environment that mimics the conditions found in cell membranes. In this way, we hope to understand one of the roles of Car in photosynthesis and to explore their role as gatable electronic components. Finding such components remains a challenge to date (10).Single-molecule electron transport measurements under potential control were carried out by following the method developed by Xu and Tao (6, 11), for which a scanning tunneling microscope (STM) is used with a partially insulated probe to enable measurements in a conductive electrolyte, which is required for maintaining potential control. A scheme of the experimental setup is shown in Fig. 1 (see also the supporting information, which is published on the PNAS web site). Photosystem II is a transmembrane protein complex, so the surrounding environment o...

Molecular length dictates the nature of charge carriers in single-molecule junctions of oxidized oligothiophenes

“…We therefore used a through-bond distance of 5.4 Å n -1 as opposed to a through-space distance of 3.8 Å n −1 to convert the measured β into a decay constant of 0.2 Å -1 . Interestingly, this value compares well with β determined for oligoene systems 35,36 , and shows that transport across TDO oligomers is similar to that across alkene oligomers.…”

Electric double layer effect on observable characteristics of the tunnel current through a bridged electrochemical contact

“…45 As noted in Sec. I, reports of experimental work on in situ tunneling through a redox molecule have appeared recently 32,38 showing a strong tunneling current maximum close to the equilibrium potential. The dependence of the position of the maximum on the bias voltage was studied in Refs.…”

“…20 This expectation has been supported by a number of recent experimental studies. [31][32][33][34][35][36][37][38] Not only the maximum itself has been clearly observed but the dependence of maximum current and the position of the maximum on the bias voltage has also been studied 32,33 and analyzed. 32 Closer examination of Tao's data shows that the current maximum for this system is also close to the equilibrium potential.…”

Temperature-Dependent In-Plane Structure Formation of an X-Shaped Bolapolyphile within Lipid Bilayers

“…charge carrier conducting rods, into lipid membranes is possible due to the fact that  interactions can stabilize the trans-membrane orientation of the bolapolyphiles in the bilayer. This is of wider interest for molecular electronics in well-organized hybrid-and superstructures [43][44][45][46][47] , especially for the addressing of cells by external electrical signals as required in devices coupling sensors, stimulators or chips with living cells or protocells. [48][49] …”