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The Experimental Study of Fast Reactions (Relaxation Techniques (Flash…
The Experimental Study of Fast Reactions
Flow Techniques
Continuous-flow method
Two fluids are forcibly pumped into a chamber where they are rapidly mixed. The newly mixed fluid passes into a transparent tube of uniform diameter. The flow rates into the mixing chamber are kept constant so that the distance along the tube is proportional to the elapsed time after mixing.
Application
Micro Flow Cell, and microwave.
A type of assay used to easily measure the progress of a reaction at discrete time points and is commonly used for determining initial rates and inhibition values.
Stopped-flow method
Two fluids are forced into a mixing chamber as in the continuous- flow method. After a steady state is attained, the flow of the solutions is suddenly stopped and the concentration of a product or a reactant is determined spetrophotometrically as a function of time as the system approaches equilibrium.
A method that allows for the rate of a solution-based reaction to be determined in milliseconds, and with a very small volume of reactants.
Application
Use to investigate the mechanism of action of a DNA repair protein, investigate protein-protein interactions, ligand binding, electron transfer, fluorescence resonance energy transfer (FRET), and protein folding.
Relaxation Techniques
Flash photolysis method
A brief burst of light irradiates the system. If this light is absorbed, it can quickly change the temperature of the system and possibly its composition. The concentration of a reactant or product is measured spectrophotometrically as a function of time as the system relaxes to its new equilibrium.
The radiation can produce new species as well as change the temperature of the system.
Application
Flash lamp, photosynthesis
A method of investigating fast photochemical reactions by exposure to very brief, intense flashes of light and spectroscopically analyzing the resulting products.
Temperature-jump and pressure-jump method
A gaseous or liquid system is subjected to a rapid heating or a rapid change in pressure. A heating pulse can be delivered by a burst of microwave radiation or by the passage of a brief pulse of electric current if the system is electrically conductive. A rapid change in pressure can be achieved by rupturing a diaphragm. After the temperature or pressure change the system relaxes to its new equilibrium state. The concentration of a reactant or product is monitored spectroscopically or by some other rapid means.
Application
Fast proton transfer reactions, metal complex formations, and the formation of enzyme-substrate complexes.
TJ: Provides a means to follow fast reactions with half-lives of just a few microseconds.
PJ: Utilizes a sudden perturbation of chemical equilibria induced by a rapid increase or decrease in pressure
Shock-wave method
A reaction vessel is constructed with two chambers separated by a diaphragm that can be ruptured suddenly. On one side is a mixture of gaseous reactants and products at equilibrium at a fairly low pressure. On the other side is a "driver" gas at a higher pressure. When the diaphragm is ruptured the driver gas moves quickly into the low-pressure chamber. Collisions of the driver gas molecules with the other molecules produce a shock wave that propagates through the low-pressure gas and heats it. the reacting system will then relax to the equilibrium state for the new temperature. The concentration of a reactant or product is monitored spectrophotometrically during this relaxation.
Application
In the study of the earth's interior and the interiors of both the terrestrial and major plants.
Do not rely on mixing, but use the fact that equilibrium compositions can depend on temperature and pressure.
