The biophysical chemistry of membranes and ion-conducting channels through membranes is an interdisciplinary area which studies biological and biochemical systems using the techniques and analytical approaches of physical chemistry. The membrane systems are particularly exciting since they include phenomena which are not generally observed with homogeneous chemical systems. Studies of such phenomena require new instrumentation and techniques in the area of electorchemistry and laser spectroscopy.
The electrochemistry of conducting ion channels in membranes centers on the mechanisms of ion permeation through membranes and ion channels and ion block of membrane channel conduction by recording the ionic currents channels under a variety of different experimental conditions. Interest has centered on phenomena which are inconsistent with simple models of ion permeation or blockage. A good example is the action of thallous ion in the gramacidin channel. When thallous ion is the only ion present in the solutions on either side of the membrane, th ions permeate protein ion channels (gramicidin) which are added to the ion-permeable membrane. However, when the thallous ion is present at low concentration with sodium ions, it acts as a channel-blocking ion for both thallous and sodium ions. Our experiments and analyses seek a self-consistent mechanism for such anomalous behavior.
Electrochemistry gives only the net flow of ions through the channel but very little information about the kinetics of ion motion within the channel. We have developed the technique of laser Doppler velocimetry for a detailed study of ion velocities within membrane channels. The technique detects the small Doppler frequency shift of monochromatic laser light scattered from the moving ions in the channel, which can then be converted into a detailed velocity distribution for these moving ions. These data permit the experimental determination of the ion permeation mechanism.
Ion binding to a membrane surface or a channel molecule is studied using a second laser spectorscopic technique, environment-sensitive laser excitation spectroscopy. Europium ion, which substitutes for calcium ion in many biological systems, has an extreemly narrow optical absorption line which is sensitive to the local environment of the ion. A narrow band laser can selectively excite ions in a specific local environment. This permits us to determine all the different binding sites for the ion. For example, a protein which bound calcium or europium at a single site would produce a spectrum of only two absorption lines in the 579-nm region, one for the ions in a water environment and one for the ions at the protein binding site. Because the ions at a specific site can be selectively excited using the laser, the properties of this site can be studied in detail. Using the technique, it is possible to determine the number of residual waters on the bound ion, the symmetry of the local environment and the distance between ion-binding sites.
The experimental studies of membranes and membrane channels are complemented with theoretical studies using kinetics and statistical mechanics. For example, we have explained the very long delay in the opening of membrane channels for potassium under certain conditions using kinetic Ising models which predict that the rate of change of conformation of one molecule will depend on the conformation of its neighbors. Such analysis directs physical chemical analysis to a study of unique behavior for a biological membrane system.
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Images displaying different representations of the lipophilic surface of Gramicidin A
"Illustrating kinetic principles by ion motions in thin membranes," M. Starzak, J. Chem. Ed. in press.
"Structure of an Eu3+ gramicidin complex in methanol/water solutions. Studied by laser excitation spectroscopy," W. P. Grygiel and M. Starzak, Biophys. Chem. 60 (1991)39.
"Thermodynamic and kinetic properties of ions in narrow, multistate channels. 1. Stationary state distributions and thermodynamics," M. Starzak, J. Phys. Chem. 99 (1995)4278.
"Kinetics of permeation and gating in membrane channels," M. Starzak, Prog. Surface Sci. 46 (1994)61.
"Ion velocity distributions in gramicidin channels determined with laser Doppler velocimetry," F. Macias and M. Starzak, Biochim. Biophys. Acta 1153 (1993)331.
"Mathematical Methods in Chemistry and Physics," M. Starzak, Plenum Publishing Co. New York, 1988.
"Physical Chemistry," M. Starzak, Wm. C. Brown (in press).
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