An ESR Study on the Effect of Hydration on the Dynamic Property of RNase A

A method is described for the measurement of dynamic property of RNase A by ESR under xeric conditions. The relationship between relative humidity and hydration degree of RNase A was determined by hydration isotherm. A solution of RNase A was allowed to react with a solution containing maleimide nitroxide label at 25 degrees, then was dialysed and lyophilized. The stable powder of RNase A - maleimide nitroxide label compound was put into the tubules, then was hydrated under different relative humidity for 11 days. After hydration, the tubules were closed and measured by ESR. The relationship between hydration value and A(max) was detected. The results showed that the lowest water content that could induce motion of RNase A by water is about 0.20 g of water per g of RNase A. That means the motion of RNase A molecule becomes detectable when there are 152 water molecules around one RNase A molecule.