Jennifer C. Brookes said:
Arguably, one of the most astonishing, and common, inferences from this examination is, counterintuitively, that the environment (the protein) in the PPC does not hinder any of these processes, but actually that it may help.
This review particularly focuses on an environment modeled using normal modes (bosonic baths). This assumes anharmonic, large amplitude, and long time-scale motions are on irrelevant time scales (with respect to the rates described here). If it were otherwise any quantum superposition state carrying information would collapse. It is found that, actually, normal mode vibrations, at least, do not decohere, but rather support and/or accelerate rates. In §2 Enzymes, the possibility of tunnelling being ‘stabilized by phonon emission’ is first introduced. In §3 Olfaction, tunnelling is conjectured to be assisted by phonon emission by an odorant. In both, it is possible that a vibrational mode may accept and/or promote the rate. Of course, there may also be ‘demoting’ modes. In §4, it is shown that a weak perturbative regime can model the excitonic energy transfer observed, but it is likely that the protein motions and so the mixing of electronic and vibrational wave functions is important [95,96]. Thus, the analysis goes well beyond that of vibrational modes and any semi-classical version of the golden rule. There is much exciting work in this area [97] and it is fascinating to consider whether these more ‘quantum’ models may be useful in the analysis of olfaction and enzyme reaction rates. It is less obvious how environmental vibrations contribute in magnetoreception; however, it has been experimentally shown that the environment does not disguise any effect [91], and it is likely that the D/A pair that encode the field are held at relative orientations optimal for the effect transduction (figure 13) which is of course determined by the host protein. As is the nuclear environment (the nuclear spin) which is key to coupling to the D/A states (equation (13)) for the asymmetry in reaction rates.
Intriguingly, although protein environments (e.g. enzymes) are more often thought of as insulating barriers (e.g. figure 2) and as ‘wet and noisy’ environments though not to be any way conducive to the survival of any ‘quantum effect’, it has been seen that protein motion may serve to promote key quantized events such as charge and energy transfer. Typically, proteins can facilitate transfer by: (i) reducing the effective tunnelling mass by solvent exclusion, (ii) enabling crossing by equalization of energy states reactants and products (i.e. moving the Born–Oppenheimer surfaces closer together; figure 1), and (iii) by reducing barrier widths. Perhaps, the most exciting is the idea that protein motion may support the persistence of coherent oscillations, seen in figure 11, for example, at ambient temperatures. The phenomena of vibronic coupling emerging in photosynthesis suggests the protein nuclear motion supports the efficient energy transfer in photosynthesis, these coherent effects have been observed in charge separation in natural systems [75,76]. Of course, this is exciting with respect to understanding the fundamental nature of one of the most essential processes in life, but beyond this, knowing the importance of vibrations in proteins allows for the manipulation of the effect: this can be tested and exploited in artificial systems [77–79].
