Chlorophyll BLMs

Because BLM made of pure lipid or oxidized cholesterol In common salt solutions are nonconducting, the physical properties of BLM are with one exception similar to those of a liquid hydrocarbon layer of equivalent th ckness. The Interfaclal tension of BLM Is less than 5 dynes cm, which Is approximately one order of magnitude lower than that of the hydrocarbon/water Interface. This low Interfaclal tension Is due to the presence of polar groups at the Interface. BLM have negligible permeability for Ions and most polar molecules. Permeability to water Is comparable to that of biological membranes. The permeability to water of Chlorophyll BLM, as determined by an osmotic flew method. Is 50 pm s, which Is in-the range of phospholipid BLM but six times larger than that of oxidized cholesterol BLM. 

Photoenergy Transduction. Pigmented BLM elicit large photopotentials under asymmetric conditions, particularly when the membrane Is Interposed between two solutions containing different redox couples. For example, the presence of FeCl on one side and ascorbic acid on the other side resulted In a photopotential of more than 150 mV across a chlorophyll-BLM (66). A variety of redox compounds affecting the photoresponse of the chlorophyll-BLM have been investigated (67,68). 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

Microscopic observations afforded the calculation of the physical area of BLM, which, in combination with electrical measurements, led to values of BLM capacitances per unit area. Typical BLMs prepared from DODAC (both in the presence and in the absence of chlorophyll-a) had areas of 5.7 x 10-3 cm and 0.7 pF/cm capacitances. These values agreed well with those determined for BLMs prepared from phospholipids (capacitance 0.7-1.3 pF/cm ) and from single-chain surfactants (capacitance = 0.3-0.6 pF/cm ). 

Pigmented Bilayer Lipid Membranes, Chlorophyll molecules have been incorporated into two types of artificial bllayer lipid membrane systems for the study of photoenergy transduction. The first consists of a planar bllayer lipid membrane (BLM) separating two aqueous solutions where photovoltaic effects can be Induced. The second system comprises liposomes which are Ideally suited for studies of photo-induced permeability, spectroscopy and chemical reactions. For more complete technical details, two pertinent publications are available (42,43). 

BLMs are made of pure lipid, for example phosphatidyl choline, or oxidized cholesterol In common salt solutions. Chlorophyll molecules situated at the blface in the Chl-BLM are tightly compressed together, and the average area occupied per porphyrin group Is much less than 75 ( ). We may expect that the... 

In a novel experiment, Koyama et al. (57) obtained a spectrum of carotenoid BLM by resonance Raman spectroscopy—a major advance in BLM spectroscopy. For efficient charge transfer, the orientation of chlorophyll molecules at the membrane-solution interface is important. Brasseur et al. (58) developed a procedure for conformation analysis to define the position of chlorophyll in BLM. They found that the porphyrin ring is orientated at an angle of 45 5° to the plane of the BLM, which is in excellent agreement with the value reported previously (44). 

The observed Volta potential does not change at a sufficiently large buffer capacity of the solution (from 5 to 100 mM Tris-HCI) [47,48]. This indicates that the potential shift at the octane/water interface results from electron transfer across the interface and not from the pH change in the boundary layer. During a redox reaction on BLM containing chlorophyll, a layer adjacent to the membrane is formed with a proton concentration different from that in the bulk phase [7, 38]. Boguslavsky et al. [47,48]... 

The action spectrum of the photoresponse was first reported by Tien s group [5] and was confirmed by Ilani and Berns [4] as being similar to the absorption spectrum of chlorophyll. The implication is that chlorophyll is an essential component of the photosensitive BLM. Mangel et al. [6] have demonstrated that completely synthetic membranes consisting of purified phosphatidyl choline, carotenoid and chlorophyll a have photovoltaic properties similar to those reported for chloroplast extract BLM. More important, it was demonstrated that all three components are needed to produce the sustained photoresponse. The intensity of the photocurrent produced was also reported to be a function of chlorophyll and carotenoid concentration in the membrane-forming solution. 

The photopotentials obtained on some of these new BLM systems are indeed exciting in that they are larger by at least a factor of three (the highest values hitherto reported) than those of chlorophyll-containing BLMs (see Figure 4). If a pigmented BLM separating two different redox... 

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