Increasing interest in photoactivated proteins as natural replacement of standard inorganic materials in photocells drives to the compared analysis of bacteriorhodopsin and proteorhodopsin, two widely diffused proteins belonging to the family of opsins. These proteins share similar behaviours but exhibit relevant differences in the sequential chain of the amino acids constituting their tertiary structure. The use of an impedance network analogue to model the protein main features provides a microscopic interpretation of a set of experiments on their photoconductance properties. In particular, this model links the protein electrical responses to the tertiary structure and to the interactions among neighbouring amino acids. The same model is also used to predict the small-signal response in terms of the Nyquist plot. Interesting enough, these rhodopsins are found to behave like a wide gap semiconductor with intrinsic conductivities. ------------------- The growing demand of eco-friendly fuels is orienting the research toward the production of photovoltaic devices based on new kinds of active matter. In particular, organic and biological materials should guarantee better performances when compared with standard solid-state materials, like semiconductors, oxides, etc Therefore, much interest arose on photoactive proteins, i.e. biomaterials able to convert, in vivo, visible light in energy useful for the cell survival. Among these proteins, the best known is bacteriorhodopsin (bR), which uses green light to pump protons outside the cellular membrane. This protein, with some lipids, constitutes the so-called purple membrane of the Archean microorganism Halobacterium salinarum. Purple membrane appears as a film of about 5 nm thickness, with the proteins close packed in hexagons. Another photoactivated protein is recently receiving constant monitoring: the proteorhodopsin (pR), found in an uncultivated marine bacterium (SAR-86 group), which shows features more complex than bR and of wider interest in technology and ecology. For example, different kinds of pRs were found in marine bacteria, usually adapted to different wavelengths, at different ocean depths. Furthermore, it seems quite easy to produce protein mutants, able to react to different wavelengths and also to be engineered for producing light flashes. Among pR peculiarities we also recall the possibility of inward or outward proton pumping with respect to the value of the environmental pH concentration. Moreover, a substantial improvement of current generation can be observed during illumination in electrochemical chambers containing bacteria in which pR was expressed. Finally, pR displays promising technological applications: it is abundant in nature, easy to express in different proteins, it works with different wavelengths (different mutants) and thus it is a perfect dye for optoelectronic devices. In spite of these large potentialities, till now a few attention has been devoted to identify pR tertiary (3D) structure which, in a first attempt, is assumed similar to that of bR, with seven transmembrane helices [8]. On the other hand, the knowledge of the protein topology is a relevant information for investigating the protein features, like, for example, the differences among mutants, the electrical properties, etc
Posted on: Wed, 26 Jun 2013 21:04:23 +0000
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