Rotational correlation times of proteins determined by 111mCd PAC spectroscopy

Molecules in solution experience rotational diffusion. For large biomolecules such as proteins, the rotational correlation times, τc, are often on the nanosecond time scale. PAC isotopes exhibit lifetimes of the intermediate nuclear level on the time scale of nanoseconds, and the hyperfine interactions measured by PAC spectroscopy are therefore sensitive to dynamics on this time scale. With this work we have compiled experimentally determined rotational correlation times, τcexp, reported in the literature in the slow reorientation limit, and compared to calculated rotational correlations times, τccalc, using the simple Stoke-Einstein-Debye approximation. In a number of cases τcexp agrees with the calculated value within the experimental error, including proteins which are expected to be rigid. In several cases, however, τcexp is smaller than the theoretical value, presumably due to local dynamics at the PAC probe site. In support of this interpretation, τcexp < τccalc is observed for many enzymes, which are expected to display dynamics at the PAC probe site due to water coordination to the active site metal.