Capacitance phospholipid bilayer

A variety of methods have been developed to study exocytosis. Neurotransmitter and hormone release can be measured by the electrical effects of released neurotransmitter or hormone on postsynaptic membrane receptors, such as the neuromuscular junction (NMJ see below), and directly by biochemical assay. Another direct measure of exocytosis is the increase in membrane area due to the incorporation of the secretory granule or vesicle membrane into the plasma membrane. This can be measured by increases in membrane capacitance (Cm). Cm is directly proportional to membrane area and is defined as Cm = QAJV, where Cm is the membrane capacitance in farads (F), Q is the charge across the membrane in coulombs (C), V is voltage (V) and Am is the area of the plasma membrane (cm2). The specific capacitance, Q/V, is the amount of charge that must be deposited across 1 cm2 of membrane to change the potential by IV. The specific capacitance, mainly determined by the thickness and dielectric constant of the phospholipid bilayer membrane, is approximately 1 pF/cm2 for intracellular organelles and the plasma membrane. Therefore, the increase in plasma membrane area due to exocytosis is proportional to the increase in Cm. 

Procarione and Kaufmann (49) studied the electrical properties of phospholipid bilayers between metal contacts. They observed, for example, irregularities in current and capacitance vs. temperature data which may be the result of phase transitions in the lipid bilayer. They also observed that both temperature-independent (tunneling) and temperature-dependent conduction processes with an activation energy of 0.65 eV were important. 

Figure 1. The agreement of the spectral density of voltage fluctuations from valinomycin-modified phospholipid bilayers at equilibrium conditions (13, 14) with the Nyquist relation 1. An aqueous 0.01-M KCl solution at 33 °C was used in the experiments. Bilayer direct current resistances and valinomycin solution concentrations were 0.52-Mfl and 1.5 X 10 8 M (l), 0.19 Mfl and 5 X 10 8 M (2), and 0.055 Mi2 and 1.5 X 10 7 M (3). Solid lines are drawn in accordance with relation 1 for the impedance of a parallel resistance-capacitance (RC) circuit using foregoing resistance values and a value of membrane geometrical capacitance.

Techniques for using a silicon-based light addressable potentiometric sensor (LAPS) to measure the electrical properties of phospholipid bilayer membranes were developed. Membrane conductance, capacitance, and potential could all be measured when the membrane was painted on an aperture between the silicon surface and a controlling electrode. The sensor was tested by observing changes in membrane properties on the addition of simple ion carriers and channels. 

The methods described in the present paper provide a means to measure phospholipid bilayer conductance, capacitance, and trans-membrane potentials in conjunction with the LAPS. It is quite plausible that the stability of this bilayer system can be significantly improved for many purposes by decreasing the distance between the bilayer and the silicon by micromachining the silicon. Studies in this direction will be reported elsewhere. 

The importance of phospholipid carbonyls for the transmembrane ion conductance is further exemplified by the observation that the ester carbonyls contribute significantly to the capacitance of phospholipid bilayers with a reciprocal relationship in the 0.1-50-Hz range. The noise spectrum of an axon is dominated by capacitance and is of the/ type in a range covering the above frequencies. " ... 

Hybrid phospholipid bilayers consisting of an outer phospholipid monolayer on a thiol SAMs (formed by liposome or vesicle fusion or by the Langmuir-Blodgett technique) have been prepared by this approach. These bilayers exhibit extremely low capacitance values, so that they can be used in sensor devices to test ions and lipophilic molecules. Thiolipids can also be used as an alternative to directly form the hybrid bilayer on the metal surface (Figure 3). ... 

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