Effects of surface charge on the conductance of the gramicidin channel.

The electric conductance of the cation-permeable gramicidin channel in negatively charged phosphatidylserine membranes has been studied. At low electrolyte concentrations the single-channel conductance is much larger in the negatively charged membrane than in a neutral membrane. This enhancement of conductance is in agreement with theoretical expectations, although a complete description of the salt concentration dependence of conductance was not possible.

[Primary shape change of platelets in vitro (author's transl)].

Studies with interference contrast microscopy reveal that platelets undergo a typical shape change within 30--60' after venepuncture, i.e. swelling, formation of large tentacles, tiny protrusions and vesicles at the platelet surface.

l/f noise in black lipid membranes induced by ionic channels formed by chemically dimerized gramicidin A.

The noise behavior of lipid bilayer membranes, doped with a chemically dimerized gramicidin A, was investigated. In contrast to normal gramicidin A, which generates a Lorentzian type power spectrum due to the formation and disappearance of conducting dimers, the current power spectrum density Sm(f) obtained with this gramicidin A derivative showed over several orders of magnitude a clear l/f behavior. The intensity of this l/f component was analyzed as a function of the membrane-applied voltage, membrane resistance, electrolyte concentration, and composition.

Ion channels formed by chemical analogs of gramicidin A.

Channel-forming peptides such as gramicidin A offer the opportunity to study the relationship between chemical structure and transport properties of an ion channel. This article summarizes a number of recent experiments with chemical analogs and derivatives of gramicidin A using artificial lipid bilayer membranes. The introduction of negative charges near the channel mouth leads to an increase in the cation transport rate.

Structure of the gramicidin A channel: discrimination between the piL,D and the beta helix by electrical measurements with lipid bilayer membranes.

Measurements with different chemically modified gramicidins in lipid bilayer membranes were used to discriminate between the dimeric pi(L,D) helix proposed by Urry and the dimeric parallel or antiparallel helices proposed by Veatch and Blout. Evidence for the pi(L,D) helix was obtained on the basis of the different actions of a negatively charged O-pyromellitylgramicidin and a negatively charged N-pyromellityldesformylgramicidin on lipid bilayer membranes. O-Pyromellitylgramicidin forms ionic channels in lipid membranes when it is applied to both sides of the membrane.

Rhodopsin and other proteins in artificial lipid membranes.

Some basic aspects of incorporation of hydrophobic peptides and proteins in artificial lipid membranes are discussed. As examples valinomycin as a carrier model and gramicidin A as a channel former in lipid vesicles and in planar lipid membranes are presented. In the second part of the lecture some examples of incorporation of membrane proteins into lipid vesicles and planar lipid membranes are reported.

The action of a carbonsuboxide dimerized gramicidin A on lipid bilayer membranes.

Gramicidin A was dimerized with carbonsuboxide as bifunctional reagent. The effect of the resulting malonyl-bis-desformylgramicidin on lipid bilayer membranes was investigated and compared with the effect of the monomer gramicidin. It was found that the single channel conductance and the ion selectivity are very similar to the behaviour of the monomer molecule, whereas the channel forming kinetics and the life time of the single channel of the malonyl-bis-desformylgramicidin differ strongly from the behaviour of the monomer gramicidin.

Formation of ion channels by a negatively charged analog of gramicidin A.

O-pyromellitylgramicidin is a derivative of gramicidin in which three carboxyl groups are introduced at the terminal hydroxyl end of the peptide. Experiments with artificial lipid membranes indicate that this negatively charged analog forms ion-permeable channels in a way similar to that of gramicidin. If O-pyromellitylgramicidin is added to only one aqueous solution, the membrane conductance remains small, but increases by several orders of magnitude if the same amount is also added to the other side.

Raman spectroscopic investigation of the interaction of gramicidin A with dipalmitoyl phosphatidylcholine liposomes.

The interaction of gramicidin A with dipalmitoyl phosphatidylcholine liposomes is investigated by Laser-Raman spectroscopy. As revealed by the methylene C-H stretching mode the phase transition of the hydrocarbon chains near 40 degree C is eliminated in the presence of gramicidin A. Liposomes prepared from a mixture of lecithin and cholesterol seem to be unaffected by gramicidin A and show only the normal broadened phase transition.

Influence of membrane thickness and ion concentration on the properties of the gramicidin a channel. Autocorrelation, spectral power density, relaxation and single-channel studies.

The properties of the gramicidin A channel in membranes made from a series of monoglycerides have been studied. In agreement with previous studies, the dissociation rate constant kD of the dimeric channel was found to increase strongly with increasing chain length of the monoglyceride, corresponding to a decrease of the mean life-time of the channel. The value of kD, however, was not strictly correlated with the membrane thickness, as seen from a comparison of membranes with different solvent content.

Voltage-induced thickness changes of lipid bilayer membranes and the effect of an electrin field on gramicidin A channel formation.

The thickness changes of black lipid membranes of different composition after a voltage jump were investigated. In a second series of electrical relaxation experiments the kinetics of channel formation by gramicidin A were measured. The time course of the membrane current was compared with the time course of the thickness change of the membranes.

Single-channel parameters of gramicidin A,B, and C.

The single-channel conductance lambda and the mean channel lifetime gamma of natural and synthetic gramicidins A, B, and C has been studied. Significant differences in delta were found between gramicidin A and B; both gramicidins differ only in one amino acid (tryptophan replaced by phenylaline). The distribution of lambda is narrow in glycerylmonooleate membranes but considerably broader in dioleoyl phosphatidylcholine and dioleoyl phosphatidylethanolamine membranes.

[Frequently recurring cyclic megakaryocytopenic-thrombopenic purpura].

An unusual case of acquired frequently remitting megakaryocytopenic-thrombocytopenic purpura in a 54 years old man is reported. The patient could be followed up anamnestically for 11 years. Regularly megakaryocytopenic-thrombocytopenic periods were followed by complete remissions.

Correlation analysis of electrical noise in lipid bilayer membranes: kinetics of gramicidin A channels.

If a membrane contains ion-conducting channels which form and disappear in a random fashion, an electric current which is passed through the membrane under constant voltage shows statistical fluctuations. Information on the kinetics of channel formation and on the conductance of the single channel may be obtained by analyzing the electrical noise generated in a membrane containing a great number of channels. For this purpose the autocorrelation function of the current noise is measured at different concentrations of the channel-forming substance.