New techniques ESE and RYDMAR

In the last few years, pulsed EPR or electron spin echo (ESE) and reaction yield detected magnetic resonance (RYDMAR) techniques have been added to the arsenal of EPR techniques applied in photosynthesis. ESE combines high temporal resolution (currently 100 ns) with sensitivity to broad EPR signals, and it allows rapid and accurate determination of the spin-lattice and spin-spin relaxation times. 

In addition, it is often possible to determine hyperfine couplings from modulations in the echo amplitude or by combining the microwave pulses with radiofrequency pulses, thus performing a pulsed ENDOR experiment. Recent applications of ESE to photosynthesis are discussed in Refs. 119-123 and in Ref. R14. 

In RYDMAR (reviewed in Refs. R15 and R16) magnetic resonance is detected by monitoring the yield of a reaction product in the presence of microwaves resonant with the difference in energy between the levels of a coupled radical pair (which for two 5 = radicals has 1 singlet (5 = 0) and 3 triplet (5 = 1) energy levels). The yield of recombination and product formation is dependent on the relative population of the four levels, which is altered by microwave-induced transitions. 

The first successful RYDMAR experiment on reaction centers was carried out by Bowman et al. [124] using laser flashes and pulsed X-band microwaves of high intensity. Recently, a sensitive RYDMAR technique was developed by Mohl et al. [125] using a combination of continuous illumination with weak magnetic fields (100 to 200 G) and low-intensity microwave radiation at about 300 MHz. Typical spectra are displayed in Fig. 9. From a simulation of these spectra and from their variation with microwave intensity it was concluded that D(P BPh 20 G, 2/(P BPh ) = 10.1 0.5 G and the sum of the recombination rates to P, P and 

The various applications of EPR spectroscopy in photosynthesis that were discussed in this chapter merely serve to illustrate its potential, and are far from an exhaustive literature survey. EPR (and ENDOR) spectroscopy has helped to identify the structure of primary and secondary reactants, and it has proved to be one of the few tools that can be used to measure the interactions between the primary reactants, which are of course crucial to electron transport. Much of the latter results are still uncertain, and also the relationship between magnetic interactions and electron transfer integrals is still only approximate. Future work will certainly focus on these aspects. 

---