In response to “Spin in Quantum Biology” (Vol. 3, No. 2).
To the editors:
I first became aware of chiral induced spin selectivity (CISS) in 2014 when visiting Ron Naaman at the Weizmann Institute in Rehovot, Israel. As he was describing his recent experimental results, I recall thinking that the implications of CISS are enormous, and especially in biological systems. My mind was racing as I began to wonder whether this was the reason that long-range information transfer could be effective in a cell, whereas in human-made devices we have never been able to accomplish such efficiencies. I looked at Ron and said something like, “This is amazing. You’re gonna win a Nobel Prize for this!” Yes, CISS is that profound. Yet it remains unknown, or certainly unappreciated, by most scientists, and it is almost never mentioned by biologists. So when a scientific review article on CISS appeared in 2016,1 I immediately wrote an essay for Inference which described the effect while underscoring the implications for living systems.2
In this new essay, Naaman and his long-time colleague in the study of CISS, David Waldeck, both physical chemists, provide a general background on CISS, then they specifically address the consequences of this effect in biology. The authors begin by defining terms for the reader, noting, for example, that the spin of an electron is either spin-up or spin-down, while underscoring the importance of these quantities in electron transfer processes. Electron transfer is at the heart of most biological transformation, including the ubiquitous electron shuttles that take place in the membrane of the mitochondria—the powerhouse organelle of the cell and the core of cellular respiration. This is needed for the conversion of raw nutrients into energy to drive all biochemical processes.
Alarmingly, few biochemists take into account electron spin in their calculations and the interpretation of experimental data. This would be akin to working on a 2017 car engine while being familiar only with 1950s automotive technology. Nobody knew about electronic ignition, fuel injection, microprocessors, sensors, or actuators during the 1950s. Sure, the basics of combustion are the same, but there would be so much missing. Such is our knowledge of biochemistry; even the so-called experts neglect key aspects of importance to the biochemical system. CISS is revealing a new world of biological device complexity.
And these are not subtle effects that are buried in the noise of a much grander biological framework. Not at all. CISS is the source of the high chemical- and enantio-selectivities in biological reactions. CISS points the way to a solution of the mystery behind an insect’s ability to synthesize organic molecules with far more efficiency and much higher yields and optical purities than the world’s top synthetic organic chemists can achieve. In the insect, chemical reactions are selected with matched electron spin, traveling down matched chiralities in molecules, to afford energy profiles that strongly favor the desired product. The insect’s reaction chemistry controls electron spin while the synthetic chemist does not. Other effects, such as steric hand-and-glove models or dipolar interactions within the enzymatic clefts, are often addressed, but an electron’s spin and its coupling with the host molecule has never before been considered. CISS is likely the dominant property in biochemistry to which kinetics and thermodynamics must pay homage.
Biology is exquisite in its precision, capitalizing upon electron-spin dominated information that is read by the homochiral molecules. Thus DNA and RNA are not the only information storage systems in a cell. The chiral molecules are the readers and conduits of that information, while electrons of specific spin are the information carriers.
The authors write:
Charge polarization in chiral molecules, the experiment indicated, is accompanied by spin polarization. The spin polarization imposes a symmetry constraint from the Pauli exclusion principle that affects the electron cloud overlap. As two chiral molecules interact, they induce a charge redistribution and a spin polarization in their electronic clouds which change the interaction energy.
Who knew? Almost no one. And all of these electron cloud permutations, which are virtual photon interactions, are occurring at near the speed of light.
Naaman and Waldeck gently peel back the layers of complexity, revealing aspects that were formerly obscured. The neglect of CISS can and has caused data misinterpretation and collective cluelessness regarding the manner by which biological systems respire, synthesize, process, and transfer information. Herein lies the problem. For one to accurately appreciate the complexity of biochemistry, it is essential to be well-versed in quantum mechanics. While Naaman and Waldeck suggest that CISS is often ignored, my only criticism of their article is that the authors are not sufficiently explicit. A higher banner must be raised. The educators of biological phenomena are themselves hamstrung in their interpretations due to their lack of knowledge of quantum mechanics.
Biology is far more sophisticated than we had imagined. Such is the state of most modern science. We appreciate little. We know even less.
James Tour is a synthetic organic chemist at Rice University.
- Karen Michaeli et al., “The Electron’s Spin and Molecular Chirality—How Are They Related and How Do They Affect Life Processes?” Chemical Society Reviews (2016), doi:10.1039/C6CS00369A. ↩
- James Tour, “Chiral Induced Spin Selectivity,” Inference: International Review of Science 2, no. 4 (2016). ↩