Spectroscopically effective active

Figure 7. The Spectroscopically Effective Active Site of hemocyanln and tyrosinase.

Fig. 42. Spectroscopically effective active site representations of the coupled binuclear copper site (left) and the type 3 site in Rhus laccase (right) where OR and R represent endogenous protein bridges in the respective sites...

Our earlier research on the coupled binuclear copper proteins generated a series of protein derivatives in which the active site was systematically varied and subjected to a variety of spectroscopic probes. These studies developed a Spectroscopically Effective Model for the oxyhemocyanin active slte.(l) The coupled binuclear copper active site in tyrosinase was farther shown to be extremely similar to that of the hemocyanlns with differences in reactivity correlating to active site accessibility, and to the monophenol coordinating directly to the copper(II) of the oxytyroslnase site.(2) These studies have been presented in a number of reviews.(3) In the first part of this chapter, we summarize some of our more recent results related to the unique spectral features of oxyhemocyanin, and use... 

Figure 21. Comparison of the spectroscopically effective models for azide binding at the blnuclear copper active site in hemocyanln and the trlnuclear copper cluster site in laccase.

Substituted NH ions Whalley, 28> discussed the spectroscopic effects of orientional disorder about one axis (in contrast to the disorder about three axes as described by Whalley and Bertie 03) and Bertie and Whalley 129> in the a-phases of the methylammonium halides. In principle, all vibrations of an orientational disordered crystal are spectroscopically active, but if the disorder is only about one axis, some restrictions operate, the symmetric bands are sharp in the one-dimensional disordered case, but the bands due to asymmetric vibrations (E) are broad. Whalley use the infra-red results of Sandorfy et al. 130>131> 0f the CH3 -ammonium halides to illustrate the effect which is predicted from interionic coupling of the E-modes. No such effect is visible in the spectrum of the methoxyammonium ion CH3ONH3 reported by Nelson, 32>. 

Working with spectroscopic probes, one needs to know the solubilization site of the probe, which should be determined independently on the spectroscopic effect to be exploited. However, when micelles of a homologous series of surfactants are investigated, information on variation of solubilization sites may be obtained. Roelants et al. (177) concluded from activation energies of quenching processes that the quencher molecule iV-methyl-A7-decylaniline resides a little deeper in TTAC micelles than in CTAC micelles. 

Fig. 18. Experimental d orbital energy level diagram for resting metapyrocatechase, its substrate complex, and the enzyme-substrate-azide ternary system (top). The spectroscopically effective structural mechanism derived from this energy diagram for the Fe(II) active site in metapyrocatechase is also shown (bottom).

Much of the experimental work in chemistry deals with predicting or inferring properties of objects from measurements that are only indirectly related to the properties. For example, spectroscopic methods do not provide a measure of molecular stmcture directly, but, rather, indirecdy as a result of the effect of the relative location of atoms on the electronic environment in the molecule. That is, stmctural information is inferred from frequency shifts, band intensities, and fine stmcture. Many other types of properties are also studied by this indirect observation, eg, reactivity, elasticity, and permeabiHty, for which a priori theoretical models are unknown, imperfect, or too compHcated for practical use. Also, it is often desirable to predict a property even though that property is actually measurable. Examples are predicting the performance of a mechanical part by means of nondestmctive testing (qv) methods and predicting the biological activity of a pharmaceutical before it is synthesized. 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Enzymes that catalyze redox reactions are usually large molecules (molecular mass typically in the range 30-300 kDa), and the effects of the protein environment distant from the active site are not always well understood. However, the structures and reactions occurring at their active sites can be characterized by a combination of spectroscopic methods. X-ray crystallography, transient and steady-state solution kinetics, and electrochemistry. Catalytic states of enzyme active sites are usually better defined than active sites on metal surfaces. 

Up to the present time, the Mossbauer effect has been observed with nearly 100 nuclear transitions in about 80 nuclides distributed over 43 elements (cf. Fig. 1.1). Of course, as with many other spectroscopic methods, not all of these transitions are suitable for actual studies, for reasons which we shall discuss later. Nearly 20 elements have proved to be suitable for practical applications. It is the purpose of the present book to deal only with Mossbauer active transition elements (Fe, Ni, Zn, Tc, Ru, Hf, Ta, W, (Re), Os, Ir, Pt, Au, Hg). A great deal of space will be devoted to the spectroscopy of Fe, which is by far the most extensively used Mossbauer nuclide of all. We will not discuss the many thousands of reports on Fe... 

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