Proton-activated, 419 relaxation kinetics

In the following amplification of these generalizations, some attention will be given to controversial aspects of these statements. It is interesting that an area of scientific study such as proton transfer kinetics could be an active one for over 25 years, particularly because of relaxation techniques, and still be one for which it is difficult to make many generalizations that workers in the field can endorse without major reservations. 

If high temperatures eventually lead to an almost equal population of the ground and excited states of spectroscopically active structure elements, their absorption and emission may be quite weak, particularly if relaxation processes between these states are slow. The spectroscopic methods covered in Table 16-1 are numerous and not equally suited for the study of solid state kinetics. The number of methods increases considerably if we include particle radiation (electrons, neutrons, protons, atoms, or ions). We note that the output radiation is not necessarily of the same type as the input radiation (e.g., in photoelectron spectroscopy). Therefore, we have to restrict this discussion to some relevant methods and examples which demonstrate the applicability of in-situ spectroscopy to kinetic investigations at high temperature. Let us begin with nuclear spectroscopies in which nuclear energy levels are probed. Later we will turn to those methods in which electronic states are involved (e.g., UV, VIS, and IR spectroscopies). 

The Copper Site. In a crystal form of ECAO shown to contain catalytically-active protein (Parsons et al., 1995), the eopper is penta-coordinated in approximate square pyramidal eonfiguration by four basal (equatorial) ligands (His 524, His 526, His 689 and a water [We]) and an apical (axial) water (Wa). The presence of equatorial and axial waters had been first reported by Barker et al. (1979) from EPR, water proton relaxation and kinetic studies on pig plasma amine oxidase and the prediction of histidines and waters as the copper ligands came from EXAFS studies by Scott and Dooley (1985). The equatorial water (We) is labile and not always present. In the HP AO structure (Li et al., 1998) it is present in some, but not all, of the six independent subunits in the same erystal. A comprehensive discussion of the spectroscopic properties of the copper site in amine oxidases, including the exchange rates for the equatorial and axial waters, is given in the review by Knowles and Dooley (1994). 

The interactions of Mn2+ with the membrane-bound (Na++K+)-ATPase from sheep kidney medulla have been examined by kinetic and magnetic resonance techniques (80,81). EPR and water proton relaxation rate studies show that the enzyme binds Mn2+ at one tight binding site (Kd=0.88 /tM). Kinetic studies yield an activator constant for Mn2+ of 0.88 /M, identifying the one tight Mn2+ binding site as the active site of the ATPase. 

The thermodynamic and kinetic properties of the enzyme-Mn-a-n-xylose bridge complex (Table V) detected in the NMR experiment are consistent with its participation in the catalytic process. The inactive anomer of n-xylose binds to the Mn-enzyme as detected by changes in the water relaxation rate but in a manner which differs in structure from that of the active (a) substrate since no effect is observed on the relaxation rates of the C-1 proton of the form (3i). Hence, the enzyme selects the a-anomer of the substrate from the mutarotated mixture. 

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