Cryogenics and Temperature

Dependence of reaction rates on temperature is one of the most basic chemical phenomena originating from thermodynamic principles. General rule is that for each 10oC of temperature change, reaction rates change 2-4 fold. Higher organisms, including humans, regulate their temperature very carefully to maintain well-choreographed kinetic balance of underlying chemical reactions.

Reaction slows down many times at -40^oC in this cryogenic continuous flow Raman mixer.

In our our studies on biological systems and their models we take advantage of temperature dependence of reaction rates. By controlled cooling of sample we can slow chemical reaction down to the extend necessary for a particular experiment.

UV/Vis transmission cryostat allows to photolyze and measure samples in EPR tubes at temperatures above 90^oK.

At mild cryogenic conditions enzymatic reactions can be slowed down by a factor of several hundred times comparing to room temperature. On the other hand many solvents, including aqueous, remain liquid and chemical reactions still proceed. Reduced rates allow us to carry out more careful spectroscopic studies on transient species that occur in enzymatic catalysis, especially for precious biological samples. Cryogenic continuous Raman studies on highly oxidized species in enzyme TauD carried at -40oC is one application of this approach.

At deeper cryogenic temperatures (below 150-200 K) most solvents solidify or freeze. In addition to general decrease in reaction rates at lower temperature, freezing of solvent prevents molecular diffusion effectively halting most common reactions. Even most reactive species, such as radicals, can last days and months frozen at liquid nitrogen temperatures.

Janis liquid helium/nitrogen cryostat for IR/UV/Vis transmission measurements

Solid frozen samples cannot be manipulated using common laboratory methods. One of few tools that can be used on solid samples is transmission of electromagnetic field (light, heat, microwave). We use various cryoprotectants to freeze sample in completely transparent form called glass. Such glassy samples on can be studied using optical or infrared transmission as a probe. Furthermore, photo labile sample in solid glassy solvents can be photolyzed using visible and UV light.

We use careful temperature control on the upper end of temperature spectrum as well. Transmission infrared spectroscopy of biological aqueous samples is particularly demanding to temperature control due to effect of temperature on strong background water absorption. IR transmission measurements can be carried out across temperature range from liquid nitrogen temperatures (photolysis) to ambient temperature (electrochemistry). While stability of 0.5 K may be sufficient for solid glassy samples, ambient temperature experiments require even greater accuracy of temperature control - 0.05 K or better.