Interfacial energies and surface-tension forces involved in the preparation of thin, flat crystals of biological macromolecules for high-resolution electron microscopy

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Abstract
It is generally agreed that surface‐tension forces and the direct interaction between the specimen and either the air‐water interface or the water‐substrate interface can influence significantly the preparation of biological materials for electron microscopy. Even so, there is relatively little systematic information available that would make it possible to control surface‐tension forces and interfacial energies in a quantitative fashion. The main objective in undertaking the present work has been to understand somewhat better the factors that influence the degree of specimen flatness of large, monolayer crystals of biological macromolecules. However, the data obtained in our work should be useful in understanding the preparation of specimens of biological macromolecules in general. Data collection by electron diffraction and electron microscopy at high resolution and high tilt angles requires thin crystals of biological macromolecules that are flat to at least 1°, and perhaps less than 0·2°, over areas as large as 1 μm2 or more. In addition to determining empirically by electron diffraction experiments whether sufficiently flat specimens can be prepared on various types of modified or unmodified carbon support films, we have begun to use other techniques to characterize both the surfaces involved and the interaction of our specimen with these surfaces. In the specific case of large, monolayer crystals of bacteriorhodopsin prepared as glucose‐embedded specimens on hydrophobic carbon films, it was concluded that the initial interfacial interaction involves adsorption of the specimen to the air‐water interface rather than adsorption of the specimen to the substrate. Surface‐tension forces at the air‐water interface and an apparently repulsive interaction between the specimen and the hydrophobic carbon seem to be major factors influencing the specimen flatness in this case. In the more general case it seems likely that interfacial interactions with either the substrate or the air‐water interface can be variously manipulated in the search to find desirable conditions of specimen preparation.