Stopped-flow kinetics, rapid scanning techniques

The reactions of Ru -pac complexes with biologically important oxidants and reductants are reviewed here to illustrate the most probable mechanism of the interaction between the [Ru (pac)H20] complexes and redox-active cellular species, viz. thio-proteins and H202- The reactivity of the [Ru (edta) (H20)] complex with various oxygen atom donors (ROOH= H2O2, BuOOH, HSOs") that leads to the formation of the active intermediate [Ru (edta)(0)] /[Ru (edta)OH] species can be followed kinetically as a function of [ROOH] and temperature at a fixed pH of 5.1 using stopped-flow and rapid scan techniques, as reported recently 53,54). 

Kinetic experiments are performed in two different ways. In one an initial disequilibrinm exists between two or more reactants, which after being rapidly mixed, combine to react toward equilibrium see Rapid Scan, Stopped-Flow Kinetics). Ideally, the mixing time is short with respect to the timescale of the reaction or actually with respect to the formation of intermediates. In contrast, in the relaxation experiment, the reactants are together and in equilibrium, and the whole system is instantaneously displaced from equilibrium. Subsequently, the system relaxes to the same or a new equilibrium state. Table 1 suimnarizes the approximate time resolution of various commonly applied mixing and relaxation techniques. The table indicates the superiority of the relaxation methods with respect to time resolution, mainly due to the development of ultrafast lasers. Mixing liquids on the (sub)microsecond time scale appears to present an important experimental barrier. 

A major breakthrongh in time resolution ( 1 ms), dynamic range ( unlimited ) and reduction of sample amount came with the development by Britton Chance of the stopped-flow techniqne see Rapid Scan, Stopped-Flow Kinetics), which is stUl widely used today. The stopped-flow techniques finds a major application in the... 

The steady-state and rapid equilibrium kinetics do not give detailed information on the existence of multiple intermediates or on their lifetimes. Such information is provided by fast (or transient) kinetics. The methods can be divided in two categories rapid-mixing techniques (stopped-flow, rapid-scanning stopped-flow, quenched flow) which operate in a millisecond time scale and relaxation techniques (temperature jump, pressure jump) which monitor a transient reaction in a microsecond time scale. Most of the transient kinetic methods rely on spectrophotomet-rically observable substrate changes during the course of enzyme catalysis. 

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