title: Water structural changes in the L and M photocycle intermediates of bacteriorhodopsin as revealed by time-resolved step-scan Fourier transform infrared (FTIR) spectroscopy.
abstract: In previous Fourier transform infrared (FTIR) studies of the photocycle intermediates of bacteriorhodopsin at cryogenic temperatures, water molecules were observed in the L intermediate, in the region surrounded by protein residues between the Schiff base and Asp96. In the M intermediate, the water molecules had moved away toward the Phe219-Thr46 region. To evaluate the relevance of this scheme at room temperature, time-resolved FTIR difference spectra of bacteriorhodopsin, including the water O-H stretching vibration frequency regions, were recorded in the micro- and millisecond time ranges. Vibrational changes of weakly hydrogen-bonded water molecules were observed in L, M, and N. In each of these intermediates, the depletion of a water O-H stretching vibration at 3645 cm-1, originating from the initial unphotolyzed bacteriorhodopsin, was observed as a trough in the difference spectrum. This vibration is due to the dangling O-H group of a water molecule, which interacts with Asp85, and its absence in each of these intermediates indicates that there is perturbation of this O-H group. The formation of M is accompanied by the appearance of water O-H stretching vibrations at 3670 and 3657 cm-1, the latter of which persists to N. The 3670 cm-1 band of M is due to water molecules present in the region surrounded by Thr46, Asp96, and Phe219. The formation of L at 298 K is accompanied by the perturbations of Asp96 and the Schiff base, although in different ways from what is observed at 170 K. Changes in a broad water vibrational feature, centered around 3610 cm-1, are kinetically correlated with the L-M transition. These results imply that, even at room temperature, water molecules interact with Asp96 and the Schiff base in L, although with a less rigid structure than at cryogenic temperatures.
authors.affiliation: Department of Biology, Center for Biotechnology and Interdisciplinary Studies, Room 2237, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, USA.
authors.forename: Joel E
authors.initials: JE
authors.lastname: Morgan
authors.forename: Ahmet S
authors.initials: AS
authors.lastname: Vakkasoglu
authors.forename: Robert B
authors.initials: RB
authors.lastname: Gennis
authors.forename: Akio
authors.initials: A
authors.lastname: Maeda
mesh_terms: D001436:Bacteriorhodopsins / Q000737:chemistry*; D019617:Halobacterium salinarum / Q000737:chemistry*; D008958:Models, Molecular; D010777:Photochemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013997:Time Factors; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D001436:Bacteriorhodopsins