CPM Seminar

Do we live in Quantum World?

Dwayne Miller

Department of Chemistry
University of Toronto

The question posed in the title pertains to the relative importance of quantum effects in biological functions; a topic that has been debated since the very birth of quantum mechanics. The relevant length scale and time scale of the coherence of the constituent wave property under question is normally neglected in these discussions. One can not have quantum interference effects unless the functionally relevant motion is comparable to the de Broglie wavelength of the constituent matter waves and the time scale is comparable to known decoherence times. These two criteria are generally thought to be orders of magnitude too small and fast, respectively, to permit any manifestation of quantum effects in systems as large and complex as biological molecules. However, the relevant length scale of motion and time scales of passage through transition states is in fact comparable to these prerequisite length and time scales. It needs to be borne in mind that one of the central tenets in biology is that protein structures have evolved to optimize passage through the transition state region for a target function. To help resolve the prospect of quantum effects playing a role in biological response functions, we used coherent control protocols to manipulate vibration coherences in the photoisomerization of retinal in bacteriorhodopsin. This model system is representative of wide class of biological responses — from the primary events involved in vision to energy transduction. These studies were conducted under weak field conditions that access the same states involved in the biological response. It was possible to selectively enhance or suppress the quantum yield for isomerization by 20% in either direction. These observations illustrates that the wave properties of matter, as manifest by vibrational quantum coherences, can play a role in biological processes, to the point that they can even be manipulated, and opens this question to further inspection.