PRELIMINARY STUDIES OF AN IMMUNOGLOBULIN-M BY NEAR-FIELD MICROSCOPIES

A human monoclonal IgM was studied using the Atomic Force Microscope ( AFM) after deposition onto freshly cleaved mica. Many images were obta ined with different samples showing homogeneous distributions of circu lar objects with a diameter of 75.4 +/- 7.9 nm. These objects were app roximately 1.2 +/- 0.2 nm high. The distribution was very similar to t hat observed with the electron microscope after preparation of shadowe d replicas. By changing repeatedly the AFM cantilever, we were able to find one which gave a series of images where the objects had reduced diameter (36.3 +/- 6.5 nm) and increased height (2.8 +/- 0.5 nm). Howe ver, we did not observe in this experimental situation any structural detail on an individual object. In particular, reducing the observatio n window resulted in a fast deterioration of the sample despite the fa irly low applied force (about 100 pN). The same immunoglobulin was als o studied using the scanning tunneling microscope (STM) after coating the samples with platinum and carbon. The STM was able to resolve the grains of the metal coating and the pentagonal shape of the IgM molecu le remained apparent under the grains.