SPECIMEN FLATNESS OF GLUCOSE-EMBEDDED BIOLOGICAL-MATERIALS FOR ELECTRON CRYSTALLOGRAPHY IS AFFECTED SIGNIFICANTLY BY THE CHOICE OF CARBON EVAPORATION STOCK

Imperfect specimen flatness can be a significant limitation in the app lication of electron crystallography to high-resolution structure anal ysis of biological macromolecules. We now report that the choice of so lid carbon stock that is used to make evaporated carbon films can have a very great effect on the preparation of flat specimens of glucose-e mbedded purple membrane. The degree of purity of the carbon does not s eem to be the controlling factor, and other likely factors such as the type of mica used as a substrate, the evaporation apparatus used (and its limiting vacuum), and the use of a continuous versus an interrupt ed evaporation protocol do not have a discernible influence. The physi cal or chemical basis for the observed differences in specimen flatnes s is still unknown; however, the important conclusion that we can comm unicate at this point is that the choice of evaporating material does have a major effect on the flatness of purple membrane, the specimen u sed here. The implication is that different sources of carbon stock sh ould be tried whenever difficulty is encountered in the preparation of suitably flat specimens of biological macromolecules.