  We introduce a mathematically rigorous analysis of a generalized spin-boson
system for the treatment of a donor-acceptor (reactant-product) quantum system
coupled to a thermal quantum noise. The donor/acceptor probability dynamics
describes transport reactions in chemical processes in presence of a noisy
environment -- such as the electron transfer in a photosynthetic reaction
center. Besides being rigorous, our analysis has the advantages over previous
ones that (1) we include a general, non energy-conserving system-environment
interaction, and that (2) we allow for the donor or acceptor to consist of
multiple energy levels lying closely together. We establish explicit
expressions for the rates and the efficiency (final donor-acceptor population
difference) of the reaction. In particular, we show that the rate increases for
a multi-level acceptor, but the efficiency does not.
