  We realize arbitrary waveform-based control of spin-selective recombination
reactions of radical pairs in the low magnetic field regime. To this end, we
extend the Gradient Ascent Pulse Engineering (GRAPE) paradigm to allow for
optimizing reaction yields. This overcomes drawbacks of previously suggested
time-local optimization approaches for the reaction control of radical pairs,
which were limited to high biasing fields. We demonstrate how efficient
time-global optimization of the recombination yields can be realized by
gradient based methods augmented by time-blocking, sparse sampling of the
yield, and evaluation of the central single time-step propagators and their
Fr\'echet derivatives using iterated Trotter-Suzuki splittings. Results are
shown for both a toy model, previously used to demonstrate coherent control of
radical pair reactions in the simpler high-field scenario, and furthermore for
a realistic exciplex-forming donor-acceptor system comprising 16 nuclear spins.
This raises prospects for the spin-control of actual radical pair systems in
ambient magnetic fields, by suppressing or boosting radical reaction yields
using purpose-specific radio-frequency waveforms, paving the way for
reaction-yield-dependent quantum magnetometry and potentially applications of
quantum control to biochemical radical pair reactions. We demonstrate the
latter aspect for two radical pairs implicated in quantum biology.
