Modulation Methods and Slow Molecular Rotations

Abstract

The application of optical methods used with triplet — or other long-lived probes for measuring slow (i.e. usec-msec) Brownian rotational diffusion of membrane proteins has been fully reviewed in the past few years (Cherry, 1979? Jovin et al., 1981; Garland & Johnson, 1985). We have previously emphasised (Garland & Birmingham, 1985) the fact that studies of the rotational diffusion of proteins in membranes or cells have, in many respects, lagged behind studies of lateral diffusion. Nevertheless, measurements of lateral diffusion and of rotational diffusion are complementary to each other, and in many instances it would be highly desirable to kmnow D_R (the rotaional diffusion coefficient) as well as the more readily obtained D_L (lateral diffusion coefficient). The same is true for measurements of that fraction of a macromolecular population that does not rotate or translate on the time-scale of the experiment. For example, a finding that D_L for a membrane protein measured over a distance of a micron or more is exceeding slow (<^-13 cm² s^-1) does not indicate whetehr the protein is trapped by anchorage to some immobile structrure (in which case D_R would be low) or merely confined by a limiting boundary to some sub-micron location within which D_L over short distances is high (<10^-9 cm² s^-1.