Structure of the P{sup +{sub 700}}A{sup -{sub 1}} radical pair intermediate in photosystem I by high time resolution multifrequency electron paramagnetic resonance - analysis of quantum beat oscillations.
The geometry of the secondary radical pair, P{sup +}{sub 700} A{sup -}{sub 1}, in photosystem I (PSI) from the deuterated and {sup 15}N-substituted cyanobacterium Synechococcus lividus has been determined by high time resolution electron paramagnetic resonance (EPR), performed at three different microwave frequencies. Structural information is extracted from light-induced quantum beats observed in the transverse magnetization of P{sup +}{sub 700} A{sup -}{sub 1} at early times after laser excitation. A computer analysis of the two-dimensional Q-band experiment provides the orientation of the various magnetic tensors of P{sup +}{sub 700} A{sup -}{sub 1} with respect to a magnetic reference frame. The orientation of the cofactors of the primary donor in the g-tensor system of P{sup +}{sub 700} is then evaluated by analyzing time-dependent X-band EPR spectra, extracted from a two-dimensional data set. Finally, the cofactor arrangement of P{sup +}{sub 700} A{sup -}{sub 1} in the photosynthetic membrane is deduced from angular-dependent W-band spectra, observed for a magnetically aligned sample. Thus, the orientation of the g-tensor of P{sup +}{sub 700} with respect to a chlorophyll based reference system could be determined. The angle between the axis and the chlorophyll plane normal is found to be 29 {+-} 7{sup o}, while the g{sup Y}{sub 1} axis lies in the chlorophyll plane. In addition, a complete structural model for the reduced quinone acceptor, A{sup -}{sub 1}, is evaluated. In this model, the quinone plane of A{sup -}{sub 1} is found to be inclined by 68 {+-} 7{sup o} relative to the membrane plane, while the P{sup +}{sub 700} - A{sup -}{sub 1} axis makes an angle of 35 {+-} 6{sup o} with the membrane normal. All of these values refer to the charge separated state, P{sup +}{sub 700} A{sup -}{sub 1}, observed at low temperatures, where forward electron transfer to the iron-sulfur centers is partially blocked. Preliminary room temperature studies of P{sup +}{sub 700} A{sup -}{sub 1}, employing X-band quantum beat oscillations, indicate a different orientation of A{sup -}{sub 1} in its binding pocket. A comparison with crystallographic data provides information on the electron-transfer pathway in PSI. It appears that quantum beats represent excellent structural probes for the short-lived intermediates in the primary energy conversion steps of photosynthesis.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); FOR
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 943089
- Report Number(s):
- ANL/CHM/JA-37402; JACSAT; TRN: US201002%%586
- Journal Information:
- J. Am. Chem. Soc., Vol. 123, Issue 18 ; May 9, 2001; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- ENGLISH
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Related Subjects
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE
BENZOQUINONES
CHARGES
CHLOROPHYLL
CYANOBACTERIA
ELECTRON SPIN RESONANCE
ELECTRON TRANSFER
ENERGY CONVERSION
EXCITATION
GEOMETRY
LASERS
MAGNETIZATION
MEMBRANES
ORIENTATION
OSCILLATIONS
PHOTOSYNTHESIS
PHOTOSYNTHETIC MEMBRANES
RADICALS
SPECTRA
STRUCTURAL MODELS
TEMPERATURE RANGE 0065-0273 K
TENSORS
TIME RESOLUTION