15N T2' relaxation times of bacteriorhodopsin transmembrane amide nitrogens.

ABSTRACT

15N T(2)' relaxation times of bacteriorhodopsin (BR) amide nitrogens were determined in the temperature range from 40 to -60 degrees C using a Hahn echo pulse sequence and proton decoupling during the echo and detection times. Using oriented membrane samples, with their bilayer normal parallel to the external magnetic field, the (15)N amide nitrogens belonging to the transmembrane helices could be selected for the analysis. The experiments were performed on purple membrane fragments (in which BR is organized in a 2D crystalline network) and on BR reconstituted into dimyristoylphosphatidylcholine at a 1150 molar ratio (in which BR is in a freely diffusing monomeric state at 40 degrees C and in an aggregated state at 4 degrees C and below). The results are discussed in terms of helix dynamics, mosaic spread and resolution of the (15)N spectra for the various samples and experimental conditions.