| Electron spin-lattice relaxation measurements have been performed on various protein samples. A model is proposed which characterizes the protein structure by a single parameter, the fractal dimension, d. This structural parameter is shown to determine the temperature dependence of the Raman electron spin relaxation rate, which varies as T('3+2d). The value of d for 46 proteins are calculated, independently of the relaxation data, using x-ray structural data. The values of d compare extremely well, as typical values are: for ferredoxin, d = 1.34 (+OR-) 0.06 from relaxation data and 1.34 (+OR-) 0.05 from x-ray data; for ferricytochrome c, d = 1.67 (+OR-) 0.03 from relaxation data and 1.66 (+OR-) 0.05 from x-ray data. The data measured by the author and other workers clearly prove the validity of the fractal model. Changes in protein conformation with solvent changes is shown to be conviently monitored by relaxation measurements and explained using the fractal model. |
