High-field EPR-detected shifts of magnetic tensor components of spin label side chains reveal protein conformational changes: The proton entrance channel of bacteriorhodopsin

Continuous-wave high-field electron paramagnetic resonance (95 GHz, 3.4 T) is performed on a spin label side chain located at residue position 171 in the proton entrance channel of bacterior-hodopsin The conformational differences of three bacteriorhodopsin mutants, the single mutant F171C, the double mutant D96G/F171C, and the triple mutant D96G/F171C/F219L, are reflected in different g_xx and A_zz tensor component shifts of the nitroxide side chain. The most polar microenvironment is found in the single mutant, whereas the open proton entrance channel reported for the triple mutant allows a reorientation of the nitroxide group towards a microenvironment of lower polarity and/or reduced hydrogen bonding. The experimental data of the double mutant are explained by a light-independent equilibrium of two nitroxide orientations with different polarities of the local microenvironment. Upon illumination the spectrum of the single mutant revealsg _xx andA _zz tensor component shifts which resemble those determined for the triple mutant in the dark. This result provides strong evidence for a light-induced opening of the proton entrance channel of the single mutant similar to that found in the unilluminated triple mutant, in agreement with electron diffraction data.