2.4. Dynamic negative staining
Dynamic structural changes to macromolecules can be induced by a variety of treatments, such a temperature, chemical treatments (e.g. pH, salt and buffer ionic strength, temperature, drugs, enzyme substrates or inhibitors, reduction or oxygenation and toxin interaction) and illumination by a particular wavelength of light. Since the early days of electron microscopy it was apparent that experimentally induced dynamic structural changes of macromolecules could be assessed from negatively stained specimens (Simon et al., 1991). In general the minimum time period for such treatments is thought to be limited by the grid preparation time and drying time of the negative stain on a specimen grid (e.g. 1 or 2 min). However, recent investigations by Zhao and Craig (2003; 2008) have shown that time-resolved structural changes to myosin filaments can be trapped on a millisecond time scale by the rapid stabilizing interaction of uranyl acetate (Fig. 10); it is likely that other experimental systems could readily utilize this approach.