  The essential biological processes that sustain life are catalyzed by protein
nano-engines, which maintain living systems in far-from-equilibrium ordered
states. To investigate energetic processes in proteins, we have analyzed the
system of generalized Davydov equations that govern the quantum dynamics of
multiple amide I exciton quanta propagating along the hydrogen-bonded peptide
groups in $\alpha$-helices. Computational simulations have confirmed the
generation of moving Davydov solitons by applied pulses of amide I energy for
protein $\alpha$-helices of varying length. The stability and mobility of these
solitons depended on the uniformity of dipole-dipole coupling between amide I
oscillators, and the isotropy of the exciton-phonon interaction. Davydov
solitons were also able to quantum tunnel through massive barriers, or to
quantum interfere at collision sites. The results presented here support a
nontrivial role of quantum effects in biological systems that lies beyond the
mechanistic support of covalent bonds as binding agents of macromolecular
structures. Quantum tunneling and interference of Davydov solitons provide
catalytically active macromolecular protein complexes with a physical mechanism
allowing highly efficient transport, delivery, and utilization of free energy,
besides the evolutionary mandate of biological order that supports the
existence of such genuine quantum phenomena, and may indeed demarcate the
quantum boundaries of life.
