Glassy cholesteric structure: thickness variation induced by electron radiation in transmission electron microscopy investigated by atomic force microscopy.

During the observation of glassy cholesteric liquid crystals in transmission electron microscopy (TEM), a new contrast is created or enhanced by electron radiation which has a direct relationship with the periodic microstructure of the specimen. In this paper, we investigate the variations of the sample thickness and mass density as possible causes of this irradiation contrast. By means of observations in atomic force microscopy (AFM) coupled to TEM, we compared the surface corrugations of non-irradiated and irradiated specimens. It is shown that the final contrast is the result of several processes. including fracture during ultramicrotomy and mass loss during irradiation. Mass loss acts as an etching, and hence results in a decrease of the sample thickness. The etching depends on the initial molecular orientation, thus evidencing the latent structure. An electron channelling mechanism is suggested to explain this behaviour.

Scanning tunneling microscopy of a liquid crystalline phase of poly((dA-dT).(dA-dT)) induced by a histone H1 peptide.

In this report, we present the first observations of uncoated poly((dA-dT).(dA-dT)) molecules organized in a liquid crystalline phase induced by the binding of a histone H1 peptide. The effect of the peptide on the polymer condensation is clearly illustrated on the large-scale STM images which reveal a well defined spacing between parallel DNA helices. High resolution images of rare isolated molecules of poly((dA-dT).(dA-dT)) exhibit two sets of helical pitch values, 6 and 7.5 nm. While the lower value can be correlated with the pitch of poly((dA-dT).(dA-dT)), the larger one may arise from peptide binding in the polymer minor groove.