7.6 Adsorption of heterogeneously charged material

Chromatin consists of an assembly of highly positively charged histone protein together with negatively charged DNA and we have found in-situ imaging difficult on mica. Allen et al. used glass a substrate (Allen, Bradbury et al. 1996) for subsequent imaging in air. We have found that, with a suitable choice of buffer, glass is also an excellent substrate for imaging in-situ (S. Leuba, personal communication).

7.7 Future developments

The range of substrate materials explored to date is quite limited. Metals are known to denature many proteins on adsorption (Roscoe and Fuller 1992) while graphite is so hydrophobic that even the use of electrochemical potential control to induce a large positive surface charge does not appear to permit adsorption of negative molecules like DNA (unpublished work). Attractive though the notion of electrochemical control of an interface is (Lindsay, Tao et al. 1992; Lindsay and Tao 1993) it is limited by the chemical complexity of the sample. Chemical interactions often overwhelm electrostatic interactions. For example, few salt solutions absorb weakly enough to permit study of the (relatively dilute) biopolymers in the salt solutions. Tris buffer is easily oxidized hiding adsorption of DNA at positive electrodes.

Semiconductors probably deserve further study, and are promising as substrates for further functionalizing (Wagner, Spudich et al. 1997). Passivated metal surfaces (using, for example, functionalized alkane-thiol monolayers (McDermott, McDermott et al. 1995)) with template stripped gold (Wagner, Kernen et al. 1994) might permit fabrication of flat, appropriately functionalized surfaces. Chemical attachment of the DNA to an electrode surface overcomes some of the problems of adsorption, and the electrical behavior of DNA oligomers chemically attached to an electrode has been investigated (Kelley, Barton et al. 1998). Chemically-inert surfaces might facilitate electrochemical potential control of the surface charge over a substantial range.