TY - JOUR
T1 - Water structural changes in the L and M photocycle intermediates of bacteriorhodopsin as revealed by time-resolved step-scan Fourier transform infrared (FTIR) spectroscopy
AU - Morgan, Joel E.
AU - Vakkasoglu, Ahmet S.
AU - Gennis, Robert B.
AU - Maeda, Akio
PY - 2007/3/13
Y1 - 2007/3/13
N2 - 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.
AB - 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.
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U2 - 10.1021/bi0616596
DO - 10.1021/bi0616596
M3 - Article
C2 - 17300175
AN - SCOPUS:33947209952
SN - 0006-2960
VL - 46
SP - 2787
EP - 2796
JO - Biochemistry
JF - Biochemistry
IS - 10
ER -