  Chemical reactions involving radical-ion pairs are ubiquitous in biology,
since not only are they at the basis of the photosynthetic reaction chain, but
are also assumed to underlie the biochemical magnetic compass used by avian
species for navigation. Recent experiments with magnetic-sensitive radical-ion
pair reactions provided strong evidence for the radical-ion-pair
magnetoreception mechanism, verifying the expected magnetic sensitivities and
chemical product yield changes. It is here shown that the theoretical
description of radical-ion-pair reactions used since the 70's cannot explain
the observed data, because it is based on phenomenological equations masking
quantum coherence effects. The fundamental density matrix equation derived here
from basic quantum measurement theory considerations naturally incorporates the
quantum Zeno effect and readily explains recent experimental observations on
low- and high-magnetic-field radical-ion-pair reactions.
