Experimental Techniques in High-Pressure Studies

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

Squires, Venier, and Aida (1983) describe an experimental technique they use to study the effect of solvent viscosity on the cisitrans ratio of stilbene irradiated in supercritical CO2. They use a dynamic flow technique similar to that described in chapter 4. In their system trau5-stilbene is coated onto glass beads, which are then packed into a high-pressure column. Supercritical CO2 flows through the column and solubilizes some of the trans-stilbene. The C02-stilbene phase is continuously irradiated with ultraviolet light as it flows through a quartz photoreactor at a fixed temperature and pressure. As the solvent viscosity increases, the photoisomerization of the cis isomer is inhibited while that of the trans isomer is facilitated. We should expect to see the cisitrans ratio of stilbene vary as the density of CO2 varies. This viscosity effect is clearly shown in figure 11.11. While there is a small effect of pressure on the... 

KI1 Kiran, E. and Zhuang, W., A new experimental method to study kinetics of phase separation in high-pressure polymer solutions. Mirltiple rapid pressure drop technique - MRPD, J. Supercrit. Fluids, 1, 1, 1994. 

A recent review and a book (1,2) on the state of the art in high pressure chemistry, indicate that the use of pressure as an experimental technique for the study of biological reactions is growing. Besides the use of pressure to simulate deep sea conditions (3), the effect of pressure on rates and equilibria is studied to obtain activation and reaction volumes. 

It is well known from thermodynamics that the properties of a macroscopic system can be fully described, if the equation of state is available, that is, V=f(p,T). However, in real physical experiments only one parameter is easily changeable the temperature at atmospheric pressure. High-pressure (HP) investigations require much more advanced experimental techniques and therefore such studies are less popular. 

During the past few decades, it has been shown that multi-nuclear NMR is a very powerful experimental technique to study alkali complexes particularly in non-aqueous solutions. The experimental method used throughout this work was 7Li NMR. This method possesses features that make it convenient for this kind of investigation. Particular attention was given to high-pressure 7Li NMR, which includes the first example reported for... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

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