95
Btochtmwa et Btophyswa Acta, 1040 (1992) 95-101 © 1992 Elsevier Soence Pubhshers B.V All rights reserved 0005-2728/92/$05.00
BBA Report
BBABIO 40266
The g = 2 multiline EPR signal of the S 2 state of the photosynthetic oxygen-evolving complex originates from a ground spin state R. David Britt
a G a r y A . L o r i g a n a K e n n e t h S a u e r b,c, M e l v i n P. K l e i n b and Jean-Luc Zimmermann
b,d
a Department of Chemtstry, Untcerszty of Cahforma, Darts, CA (USA) b Structural Btology Dwtston, Lawrence Berkeley Laboratory, Berkeley, CA (USA) c Department of Chemtstry, Unwerstty of Cahfornta, Berkeley, CA (USA) and d Section de Bto~nerg~ttque (URA CNRS 1290), Ddpartment de Btologte Cellulatre et Mol~culatre, Centre d'Etudes de Saclay, Glf-sur-Yvette (France) (Recewed 24 August 1992)
Key words" Photosystem II; Oxygen-evolving complex; Manganese; EPR, pulsed; Electron spin echo
The amplitude of the g = 2 Mn 'multiline' EPR signal of the S 2 state of the photosynthetic oxygen-evolving complex varies inversely with temperature, indicating that this signal arises from a ground spin state. Electron spin echo experiments at temperatures of 4.2 K and 1.4 K show such Curie-law behavior of the g = 2 multiline EPR signal, as do continuous-wave EPR experiments performed at a non-saturating microwave power in the range from 15.0 K to 4.2 K.
The S 2 state I of the oxygen-evolving complex (OEC) of Photosystem II (PS II) gives rise to Mn E P R signals in the g = 2 and g = 4 regions of the E P R spectrum [2-6]. The g = 2 'multiline' signal [7] displays 19 or more partially-resolved Mn hyperfine lines with an average splitting of approx. 80 G. The g = 4 signal shows no resolved hyperfine structure in non-oriented PS II membranes [8,9], but displays 16 or more Mn hyperfine lines with a splitting of approximately 36 G when stabilized by NHa-treatment of oriented membranes [10,11]. The two signals appear to arise from different spin states of a common exchange-coupled tetranuclear Mn cluster at the core of the oxygenevolving complex [10-13]. This view is supported by a recent theoretical study in which the S 2 multiline E P R signal could be simulated by an S = 1 / 2 spin state arising from a tetranuclear Mn cluster [14]. Continuous-wave (CW) E P R experiments investigating the temperature dependence of these signals have led to conflicting views as to the energy level positioning of the magnetic spin states from which they originate. Specifically, some CW E P R experiments have indicated an excited spin state origin for the g = 2
Correspondence to: R.D. Brttt, Department of Chemistry, Umverslty of California, Davis, CA 95616, USA
multiline signal [15,16], while others have indicated that this signal arises from a ground spin state [17,18]. Most CW E P R experiments on the temperature dependence of the g = 4 signal indicate a ground spin state origin [15-17]. However, a recent report [8] has suggested that, under certain conditions, this signal arises from an excited spin state. In this paper we introduce the use of the pulsed E P R technique of electron spin echo (ESE) spectroscopy to the measurement of the overall magnetic properties of the catalytic Mn cluster of the OEC. A two-pulse electron spin echo results from a transient electron spin magnetization coherence following microwave-driven rotation of the magnetization in a [Tr/2-~'-Tr-~'-ESE] sequence, where ~- is the time between the 7r/2 and ~- microwave pulses [19]. ESE spectroscopy allows one to measure an EPR spectrum by determining the ESE amplitude as a function of magnetic field. No field modulation is employed, because a gated integrator is utilized in this time-domain experiment to limit acquisition of background noise [20]. Thus the spectrum is not obtained as a derivative with respect to field, but as the direct absorption. This is advantageous for detecting broad, relatively featureless signals. Furthermore, it is a simple matter to do a single integration over field to obtain a quantitative measurement of the number of spins contributing to
96
the spectrum at a given temperature. CW EPR experiments on the temperature dependence of the multiline signal have typically used a simple peak-to-peak height
analysts to measure the signal amplitude [15,16], but this is subject to major errors if the linewidths also vary with temperature [17]. In principle, the CW EPR sig-
3 20
2 80 42K
240
200
-8 E
Btochtmwa et Btophyswa Acta, 1040 (1992) 95-101 © 1992 Elsevier Soence Pubhshers B.V All rights reserved 0005-2728/92/$05.00
BBA Report
BBABIO 40266
The g = 2 multiline EPR signal of the S 2 state of the photosynthetic oxygen-evolving complex originates from a ground spin state R. David Britt
a G a r y A . L o r i g a n a K e n n e t h S a u e r b,c, M e l v i n P. K l e i n b and Jean-Luc Zimmermann
b,d
a Department of Chemtstry, Untcerszty of Cahforma, Darts, CA (USA) b Structural Btology Dwtston, Lawrence Berkeley Laboratory, Berkeley, CA (USA) c Department of Chemtstry, Unwerstty of Cahfornta, Berkeley, CA (USA) and d Section de Bto~nerg~ttque (URA CNRS 1290), Ddpartment de Btologte Cellulatre et Mol~culatre, Centre d'Etudes de Saclay, Glf-sur-Yvette (France) (Recewed 24 August 1992)
Key words" Photosystem II; Oxygen-evolving complex; Manganese; EPR, pulsed; Electron spin echo
The amplitude of the g = 2 Mn 'multiline' EPR signal of the S 2 state of the photosynthetic oxygen-evolving complex varies inversely with temperature, indicating that this signal arises from a ground spin state. Electron spin echo experiments at temperatures of 4.2 K and 1.4 K show such Curie-law behavior of the g = 2 multiline EPR signal, as do continuous-wave EPR experiments performed at a non-saturating microwave power in the range from 15.0 K to 4.2 K.
The S 2 state I of the oxygen-evolving complex (OEC) of Photosystem II (PS II) gives rise to Mn E P R signals in the g = 2 and g = 4 regions of the E P R spectrum [2-6]. The g = 2 'multiline' signal [7] displays 19 or more partially-resolved Mn hyperfine lines with an average splitting of approx. 80 G. The g = 4 signal shows no resolved hyperfine structure in non-oriented PS II membranes [8,9], but displays 16 or more Mn hyperfine lines with a splitting of approximately 36 G when stabilized by NHa-treatment of oriented membranes [10,11]. The two signals appear to arise from different spin states of a common exchange-coupled tetranuclear Mn cluster at the core of the oxygenevolving complex [10-13]. This view is supported by a recent theoretical study in which the S 2 multiline E P R signal could be simulated by an S = 1 / 2 spin state arising from a tetranuclear Mn cluster [14]. Continuous-wave (CW) E P R experiments investigating the temperature dependence of these signals have led to conflicting views as to the energy level positioning of the magnetic spin states from which they originate. Specifically, some CW E P R experiments have indicated an excited spin state origin for the g = 2
Correspondence to: R.D. Brttt, Department of Chemistry, Umverslty of California, Davis, CA 95616, USA
multiline signal [15,16], while others have indicated that this signal arises from a ground spin state [17,18]. Most CW E P R experiments on the temperature dependence of the g = 4 signal indicate a ground spin state origin [15-17]. However, a recent report [8] has suggested that, under certain conditions, this signal arises from an excited spin state. In this paper we introduce the use of the pulsed E P R technique of electron spin echo (ESE) spectroscopy to the measurement of the overall magnetic properties of the catalytic Mn cluster of the OEC. A two-pulse electron spin echo results from a transient electron spin magnetization coherence following microwave-driven rotation of the magnetization in a [Tr/2-~'-Tr-~'-ESE] sequence, where ~- is the time between the 7r/2 and ~- microwave pulses [19]. ESE spectroscopy allows one to measure an EPR spectrum by determining the ESE amplitude as a function of magnetic field. No field modulation is employed, because a gated integrator is utilized in this time-domain experiment to limit acquisition of background noise [20]. Thus the spectrum is not obtained as a derivative with respect to field, but as the direct absorption. This is advantageous for detecting broad, relatively featureless signals. Furthermore, it is a simple matter to do a single integration over field to obtain a quantitative measurement of the number of spins contributing to
96
the spectrum at a given temperature. CW EPR experiments on the temperature dependence of the multiline signal have typically used a simple peak-to-peak height
analysts to measure the signal amplitude [15,16], but this is subject to major errors if the linewidths also vary with temperature [17]. In principle, the CW EPR sig-
3 20
2 80 42K
240
200
-8 E
