Herman E. Ries

Electron microscope and pressure—area studies on a gramicidin and its binary mixtures with cerebronic acid, cholesterol and valinomycin☆

Valine-gramicidin A (VGA), cerebronic acid, cholesterol and valinomycin differ widely in hydrocarbon and polar groups, and represent molecules important in biomembrane structure and function. The pressure-area isotherm for VGA, a linear pentadecapeptide, gives a molecular area of 386 A2 indicating an initial horizontal orientation which is presumably followed by a vertically oriented helical structure to give a high collapse pressure, 63 dyn cm−1. Cerebronic (2 -hydroxy-tetracosanoic) acid gives a small molecular area, 26 A2, indicating vertical orientation and a most unusual stable maximum pressure, 68 dyn cm−1. Cholesterol, in spite of its bulky hydrocarbon structure and weak polar group, also orients vertically to give a molecular area of 39 A2 and a collapse pressure of 43 dyn cm−1. The isotherm for valinomycin, a cyclic dodecadepsipeptide, suggests an initial horizontal orientation giving a molecular area of 296 A2, followed by a liquid-like collapse at 23 dyn cm−1. The equimolar mixture of VGA and cerebronic acid shows a small condensation interaction at low pressures and somewhat less interaction in the high pressure region. Similar effects are observed for the mixture of VGA and cholesterol, except that cholesterol has a strengthening effect at high pressures. Valinomycin, however, has a surprisingly pronounced condensing effect on the VGA monolayer at low pressures where both molecules are horizontally oriented, but above 29 dyn cm−1 the VGA completely dominates. Electron micrographs of the collapsed films support the interpretation of the isotherms.

Monomolecular Films of Molecules Which Lie Flat on the Surface of Water I. Surface Pressures and Potentials of Films of Long Molecules: Polymers of ω‐Hydroxy Decanoic Acid

Film pressures and surface potentials have been measured for a series of linear polymers of ω‐hydroxy decanoic acid, ranging in molecular weight from 780 to 25,200. The molecules are oriented parallel to the surface. An improved form of the apparatus of Harkins and Fischer was used. The chief modification in the procedure was the use of two radioactive polonium electrodes over the film. The pressure‐area relations show that the area per molecule in the condensed films is nearly proportional to the molecular weight and that the compressibility of the films is large. The film density is found to be lowest for the polymers of smallest molecular weight. The films are not very sensitive to changes in hydrogen ion concentration, but on changing from an acid to an alkaline substrate the films in the expanded state give higher molecular areas at given pressures, and the pressures for film collapse are considerably higher. Dimensions of the molecules are calculated and a complete table of mean values is included. ...

Interaction of cholesterol, cerebronic acid, valinomycin, and related compounds in monolayers of binary mixtures

Abstract Pressure—area isotherms and electron-microscope studies of thin-film mixtures of biologically important compounds shed light on interaction effects in biomembranes. At relatively low surface pressures most of the binary mixtures under study give isotherms at smaller areas than the calculated averages based on simple additivity of the component isotherms. This is a condensing or cooperative type of interaction in contrast to an expanding or interfering type of interaction. In spite of such cooperative interaction, a subsequent plateau in the isotherms for most mixtures demonstrates a squeezing out (collapse) of the weaker component. Generally the isotherms then rise steeply to final collapse at a pressure higher than that of the calculated average and frequently higher than that of the stronger component. The latter effect demonstrates considerable interaction. Electron micrographs of transferred films support the interaction indicated at collapse. A most unusual interaction effect is observed with an equimolar mixture of two widely different structures, cholesterol and cerebronic (2-hydroxytetracosanoic) acid. Relatively weak cooperative interaction is observed in the low pressure region. However, pronounced condensation takes place at intermediate pressures. Moreover, an inflection point at about 23 dynes per centimeter appears to be completely unrelated to the component isotherms which show no inflection in this region. No similar effect has been observed in a wide variety of mixed monolayer experiments. Thus marked interaction is indicated for the monolayer mixture of cholesterol and cerebronic acid. A plateau at 48 dynes per centimeter undoubtedly indicates the ejection of cholesterol. Electron micrographs of transferred samples demonstrate that the long flat ribbons of cholesterol and the tall narrow ridges of cerebronic acid have been altered by the interaction. Equimolar binary mixtures of cholesterol and valinomycin, cerebronic acid and valinomycin, and related compounds demonstrate different effects.