Native substrate distance

In order to show that the origin of this difference is not a function of the particular substrate analogue used, similar NMR relaxation studies have been performed with dimethyl sulfoxide (DMSO)1401 since the crystal structure of the enzyme-NADH-DMSO ternary complex is well resolved.1366 From the relaxation data, the distance between the methyl protons of DMSO and Co11 was calculated to be 8.9 0.9 A, again too great for direct coordination of the sulfoxide group to the metal ion. Since the cobalt enzyme appears to be functionally similar to the native enzyme, the difference is unlikely to be a direct result of substitution. One possibility is that there may actually be a difference between the solution and crystalline structure of the enzyme ternary complex, particularly since it is well established that the crystalline enzyme is 1000 times less active than in solution.1402... 

A complex and radically new situation evolves in the case of a direct, mediatorless, transport between the enzyme active center and the electrode. Apart from the problems mentioned above, some new fundamental questions arise, which have not been encountered either in electrochemistry or enzy-mology. In the case of preservation of the molecular integrity of the immobilized enzyme, electrochemical transformations of the substrate in this system take place at large (some 10-A) distances from the conductive phase. Therefore, it is necessary to investigate the mechanism of electron transfer and of the distribution of the potential jump (the structure of the electric double layer) in the electrode-enzyme-electrolyte system. The electrode becomes the donor or acceptor of electrons when the reaction proceeds at the enzyme active center. This implies a change in the functioning mechanism of the enzyme as compared to the native conditions. The chemical and electronic structure of the electrode surface must play an extremely important role in... 

Figure 3.21. A dynamic SIMS depth profile showing the volume fraction of a polystyrene of relative molecular mass 29000, with both ends carboxy terminated, in a matrix of deuterium-labelled, normally terminated polystyrene of relative molecular mass 500000, after annealing at 160 °C for 36 h. The volume fraction, derived from the intensity of the CH ion, is measured as a function of the distance from the substrate, which is a native-oxide-coated silicon wafer. After Zhao et al. (1991).

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