Volume-templating mechanism

The most incisive studies of the problem at the molecular level are those from the Felsenfeld laboratory (see, for example. Refs. [75,76]). They have shown that at least under some circumstances, a polymerase can step around a nucleo-some, displacing it in cis, but not causing dissociation. It is not yet clear, however, if this mechanism is physiologically relevant and/or whether it is the only mechanism. There exist results in apparent conflict with this model (i.e.. Ref. [77]). That the in vivo process is certainly more complex than the in vitro models used to date is further indicated by the discovery of elongation factors that markedly increase transcriptional rates and suppress pausing (see, for example, Conaway and Conaway [78]). Thus, the question as to how transcription elongation occurs in a chromatin template remains at least partially unresolved. For a further discussion, see Jackson, this volume, p. 467. 

The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed. 

The present study focuses on the synthesis of mesostructured aluminas. In the first part of the article, the nature of the interactions between the surfactant and the inorganic framework within a sodium dodecylsulfate templated alumina is studied. The mechanism of formation of this material is also described. In the second part, the synthesis of high pore volume mesoporous alumina prepared under basic conditions is reported. 

The first two voliunes in this Templates in Chemistry series have focused on templates that control solution-phase reactions. Among the templates discussed in these two voliunes were convex and concave templates that mediate the formation of (macro)cyclic molecules and mechanically bound molecules with their intriguing intertwined topology. Also, three-dimensional templates that are used to imprint polymers and that organize compounds in the solid state for predestined reactions have been included in the earlier volumes. 

We must now consider the important question of the probability of these mechanisms taldng place in biomineralization. It is often implicitly assumed that the main function of the matrix is to act as a template for mineral development through the control of crystal orientation and structure, but as we have discussed previously this is not necessarily correct since the matrix may serve simply as an energetically favourable site for heterogeneous nucleation, or as a means of volume constraint, or as a framework for structural support. Thus the bulk mineral is often observed to be composed of crystallites interlaminated and virapped with organic sheets and it is often not at all clear what function the matrices serve. 

---