A deriv. of the integral membrane protein, cytochrome b5, was prepd. in which the native heme group was replaced by the structurally similar Rh(III)-protoporphyrin IX. This metalloporphyrin had a finite triplet yield with a single exponential decay time of 22 ms in water. After insertion of the metalloporphyrin into the protein, its triplet-state decay became strongly nonexponential with >=3 equal amplitude components with time consts. varying over a range of 100. The derivatized protein was incorporated into unilamellar liposomes prepd. from dimyristoyllecithin, and the rotational diffusion of the protein in the lipid bilayer was studied at temps. above and below the lipid phase transition temp. via triplet absorbance anisotropy decay. The anisotropy decay curves were biphasic both above and below the lipid phase transition. The rotational diffusion const. was 2.4 * 105 s-1 at 35 Deg, and 1.1 * 104 s-1 at 10 Deg, both being calcd. from the fast decay component. The ratio of the limiting anisotropy to the initial anisotropy was 0.6 at both temps. This implies a cone of restricted motion of 34 Deg for the protein in the bilayer. [on SciFinder (R)]