Time resolved EPR-Measurements on Light Induced Radical Pairs of Photosynthetic Model Systems
This present work reports on time-resolved measurements on light induced radical pairs of the photosynthetic model system N,N,N,N-tetraalkyle-p-phenylenediamine-zincphorpyrine-naphtoquinone (2-TAPD-ZnP-2-NQ). To characterise the radical pairs transient and pulsed EPR-measurements in X- and Q-Band are used. The core of the examinations lies in the verification of new coherent phenomena which can be observed on short living intermediates of the photosynthesis.
In the time development of the transversal magnetisation on TAPD+NQ- zero quantum coherence of the electrons and one quantum coherence of nuclei is observed in addition to the Torrey oscillations. Both coherence phenomena are based on the special generation of the radical pairs by a short laser pulse and confirm the validity of radical pair concept. The modulation depth of the nuclear coherence decreases drastically with the magnetic field. From the measured frequency of the observed nuclear coherence it is possible to determine the hyperfine coupling parameters of the methyle protons of TAPD+ .
On the other side it is also possible to observe the electron zero quantum coherence in the transient Q-band experiment. The phase and frequency of these quantum beats changes significantly across the powder spectra. These significant oscillations contain important information on the geometry of the radical pair TAPD+NQ-. Through simulation of the two dimensional Q- and X-band data set of TAPD+NQ- in the protonated and partly deuterated model system it was possible to verify a structure proposal of TAPD+NQ- based on ab-initio calculations.
Light induced electron zero quantum coherence and one quantum coherence of the nuclei are also detectable in a one pulse FID experiment in the X-band. The analysis of this experiment shows that the modulation depth of the individual coherence can be varied by selecting the length of the microwave pulse. This opens a new field of applications to determine the hyperfine coupling in spin correlated radical pairs.
The modulation frequency of the firstly detected nuclear ESEEM could be also used for determination of the hyperfine coupling. Due to the absence of a comprehensive theory quantitative statement are not possible at the moment. In future nuclear ESEEM will be an alternative method for the determination of hyperfine couplings in spin correlated radical pairs.