ELECTRON-MICROSCOPY OF METAL-SHADOWED FIBRINOGEN MOLECULES DEPOSITED AT DIFFERENT CONCENTRATIONS

Summary. The hypothesis that fibrinogen molecules are elongated (47.5 nm) trinodular structures is founded upon electron microscopic analyses of air‐dried metal shadowed preparations in which collections of three discrete linearly aligned particles were commonly observed and were assumed to represent an intact molecule. In our study, human fibrinogen at concentrations ranging from 0.1 to 150 μg/ml in 0.1% ammonium acetate buffer, pH 9.3 ± 0.1, was deposited on carbon films, air‐dried, shadowed with platinum, and then examined by electron microscopy. When deposited at a concentration of 10 μg/ml or higher, most images showed considerable polydispersity, evidently due to overloading, that was characterized by multinodular shapes varying in length, width, and height; discrete particulate forms were also commonly observed. At a lower concentration in the range of 1 μg/ml, the degree of polydispersity was much less and the predominant form was a discrete particle with a mean diameter and height of 13.7 ± 1.4 nm and 5.4 ± 0.8 nm, respectively. Only rarely were three such particles aligned linearly so as to suggest a trinodular molecular shape. At still lower fibrinogen concentrations in the range of 0.1 μg/ml, the number of particles per unit of surface was considerably diminished, but virtually all forms observed were individual particles that were well separated (> 20 nm) from one another. Approximation of the volume of each particle indicates that an anhydrous molecule having the size of fibrinogen can be accommodated. These observations plus quantitative considerations of particle distribution do not support the conclusion that molecules of fibrinogen are elongated trinodular structures but suggest instead that the particulate forms reflect single molecules of fibrinogen and that multinodular forms represent collections of two or more molecules. Copyright © 1979, Wiley Blackwell. All rights reserved