Distance determination in spin-correlated radical pairs in photosynthetic reaction centres by electron spin echo envelope modulation

Distances between radicals in photosynthetic reaction centres can be determ ined from the strong 'out-of-phase' electron spin echo envelope modulation (ESEEM) caused by the electron spin-spin dipolar interaction. The precision with which the dipolar coupling can be measured in such experiments and th e optimum strategy for sampling the time-domain spin echo signal are explor ed using the method of Cramer-Rao lower bounds. Consideration of the contri butions to the ESEEM from anisotropic hyperfine interactions suggests that the precision is much higher than hitherto estimated and that the accuracy may not be affected if nuclear modulations are ignored. (C) 1999 Elsevier S cience B.V. All rights reserved.