  Radical-ion-pair reactions, central for understanding the avian magnetic
compass and spin transport in photosynthetic reaction centers, were recently
shown to be a fruitful paradigm of the new synthesis of quantum information
science with biological processes. We show here that the master equation so far
constituting the theoretical foundation of spin chemistry violates fundamental
bounds for the entropy of quantum systems, in particular the Ozawa bound. In
contrast, a recently developed theory based on quantum measurements, quantum
coherence measures, and quantum retrodiction, thus exemplifying the paradigm of
quantum biology, satisfies the Ozawa bound as well as the Lanford-Robinson
bound on information extraction. By considering Groenewold information, the
quantum information extracted during the reaction, we reproduce the known and
unravel other magnetic-field effects not conveyed by reaction yields.
