Affordable Access

Publisher Website

Millisecond time-resolved EPR of the spin-polarised triplet in the isolated Photosystem II reaction centre

Biochimica et Biophysica Acta (BBA) - Bioenergetics
Publication Date
DOI: 10.1016/0005-2728(90)90064-b
  • Photosystem Ii
  • Reaction Center
  • Esr
  • Millisecond Decay Kinetics
  • Microwave Power
  • Triplet State


Abstract The D 1/D 2-cytochrome b-559 PS II reaction centre was isolated using Triton X-100, and this detergent was subsequently exchanged for dodecyl β-maltoside. This preparation is stable in the dark, and at cryogenic temperatures in the light. The isolated PS II reaction centre showed a steady-stage Δm = 1 EPR spectrum of a chlorophyll triplet state, at temperatures under 20 K. Its polarisation pattern was AEEAAE, characteristic of the molecular triplet state of the primary donor ( 3P680) populated exclusively in the T 0 level via the 3RP. Increasing the temperature within the range 5–20 K caused a decrease in EPR signal intensity (the z peak being initially more sensitive than the x or y peaks), but the polarisation pattern remained unchanged. The millisecond time-resolved response of the intensity of the x, y and z peaks to a light excitation step pulse was measured, and the light-on and -off responses could all be described by a single exponential at low (≤ 30 μW) microwave power and low (≤ 100 mW) light intensity. The values of the decay rate constants, k x , k y and k z , extrapolated to zero microwave power, were compared with values reported for PS II preparations of larger antenna size. The value for k x , 605 s −1, was distinctly shorter than those previously reported, and tended towards that reported for biligated monomeric Chl a in methyltetrahydrofuran. k x was found to be extremely sensitive to microwave power. The value for k z was 185 s −1, and that for k y 965 s −1. A second exponential component was seen in the kinetics of the y and z peaks at high microwave power due to a non-negligible microwave-induced population of the T ±1 levels. The decay rate constants were all independent of the wavelength (458 or 514 nm) and intensity (50–300 mW) of the excitation light. They remained unchanged when the steady-state EPR signal intensity was reduced by 50% by increasing the temperature. This was taken to indicate that the effect of temperature was on the populating process. A theoretical model has been developed which can explain most of the effects of microwave power on the EPR kinetics.

There are no comments yet on this publication. Be the first to share your thoughts.