Electrochromic probing

Electrochromic probing [7] relies on the detection by UV spectroscopy of the reorientation of suitable electrochromic probes, usually azo or other dye molecules, dissolved in the polymer matrix and then oriented in an electric field. The rate and kinetics of the reorientation after release of the electric field is related to the probe molecule size and to the available free volume size. 

Free volume of a polymer is widely recognized to be a factor of crucial importance for the gas transport in polymeric membranes. The techniques of spin probes [58-60], photo- and electrochromic probes [49, 52, 60], inverse gas chromatography [41, 62-64] and positron annihilation [50-51,55, 65-66] were employed in the investigations of the free volume of glassy polymers. Below we report the results provided by low temperature nitrogen adsorption and Xe NMR spectrometry techniques [67]. 

The aim of this Chapter is to review a method by which fluorescence properties of organic dyes can, in general, be predicted and understood at a microscopic (nm scale) by interfacing quantum methods with classical molecular dynamics (MD) methods. Some review of our extensive applications [1] of this method to the widely exploited intrinsic fluorescence probe in proteins, the amino acid tryptophan (Trp) will be followed by a discussion of electrochromic membrane voltagesensing dyes. 

It is possible, however, that the electrochromic response of some styrylpyridi-nium probes, for example, RH421 (see Fig. 2), is enhanced by a reorientation of the dye molecule as a whole within the membrane. There is a steep gradient in polarity on going from the aqueous environment across the lipid headgroup region and into the hydrocarbon interior of a lipid membrane. Therefore, any small reorientation of a probe within the membrane is likely to lead to a change in its local polarity and hence a solvatochromic shift of its fluorescence excitation spectrum. Such a... 

L. M. Loew, S. Scully, L. Simpson, and A. S. Waggoner, Evidence for a charge-shift electrochromic mechanism in a probe of membrane potential, Nature 281, 497 (1979). 

Other electrochromic membrane probes of note include, me-rocyanine 540 used to study the transmembrane potential in liver mitochondria although some problems are evident (5, 30, 31). Overall, measurements of transmembrane potential differences in living cells are well established with the methods fairly reliable and robust. Many of these membrane potential measurement strategies are generic and hold for plasma membranes of neurones and other excitable tissues (6). 

Loew EM. Design and characterization of electrochromic membrane probes. J. Biochem Biophys Methods. 1982 6 243-260. Waggoner A. Optical probes of membrane potential. J. Membr. Biol. 1976 27 317-334. 

Thylakoid membrane as a Molecular Voltmeter and Ammeter. The antibiotic valinomycin was found in 1964 by Moore and Pressman to affect ion transport across the mitochondrial membrane. Subsequently, Pressman and coworkers proposed that valinomycin is an ionophore and acts as a mobile ion carrier across the lipid membrane [cf. sectionlll.B. above]. Since the light-induced FIAC in thylakoid membrane appears to be an electrochromic band shift in pigments produced by an electric field, Boeck and Witt °, Junge and Schmid used this unique effect to probe the effect of compounds such as the ionophore, valinmycin, on the voltage and current density generated across the membrane. 

In this paper, recent results of studies performed on the imide derivatives of the trimeric aniline are presented. Films of TANI-polyimide were also prepared and their electrochemical and electrochromic properties were probed. It is shown from this work that well behaved polymeric materials incorporating aniline trimers can be prepared, these trimers can be addressed electrochemically within such polymeric systems and the behavior of the trimer within the polymer does not differ significantly from that within the model compounds. 

The electrochromic shift of the carotenoids was used as internal probe of the surface potential changes induced by addition of non-permeable ions. Experimental data could be adjusted to theoretical curves by introducing three experimental improvements ... 

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