INTERACTION OF TRYPTOPHAN-182 WITH THE RETINAL 9-METHYL GROUP IN THE L-INTERMEDIATE OF BACTERIORHODOPSIN

An intense indole N-H stretching vibrational band at 3486 cm(-1) in th e difference Fourier transform infrared spectrum is one of the charact eristic features of the L intermediate of bacteriorhodopsin [Maeda, Sa saki, Ohkita, simpson, and Herzfeld (1992) Biochemistry 31, 12543]. Th is band is now assigned to tryptophan-182. The Trp182-->Phe (W182F) pr otein shows specific features in the difference spectrum in the Visibl e region upon L formation, and exhibits great delay in the L-M convers ion. Fourier transform infrared difference spectra further indicate th at while the intensity of the C-methyl in-plane bending vibration at 1 009 cm(-1) is lost in the L intermediate of the wild type, its intensi ty remains high in the W182F protein. The intensity of the N-H stretch ing vibration upon L formation is diminished considerably in an artifi cial bacteriorhodopsin containing 9-desmethylretinal, It also exhibits delayed M formation. These results suggest that Trp182 interacts with the retinal side chain through the 9-methyl group, and thereby affect s the L-to-M conversion.