Biological systems are a treasure chest. Jewels are there for the asking. Chiral induced spin selectivity (CISS) provides an invigorating example. In 1815, Jean-Baptiste Biot observed that certain chemical compounds could cause the rotation of plane polarized light;1 and in 1848, Louis Pasteur deduced that this phenomenon must have a molecular basis.2 Lord Kelvin suggested the term “chirality” to cover this class of compounds.3 Any object that cannot be superimposed on its mirror image displays chiral properties. From a symmetry standpoint, chiral objects do not possess a mirror plane or point of symmetry.
Chirality is ubiquitous in biological molecules. Aside from water, glycine, and acetic acid (among others), the majority of such molecules are chiral. The polymers of chiral molecules, such as the polysaccharides, polypeptides, and polynucleotides, are composed of chiral molecules. Such structures take on new shapes, including helices and spiral clefts, that are themselves chiral.
Chiral systems are lower in entropy than systems in which molecules are true mirror images of one another. If this is so, why has nature taken such pains to preserve chirality? Work by Ron Naaman, David Waldeck, and coworkers reveals a reason for nature’s selection which, until recently, had never been considered.4
Quantum mechanics demonstrates that for two electrons sharing a region of space their electrostatic repulsion energy is contingent upon whether their spins are parallel or anti-parallel. Both electrons can be spin-up, spin-down, or one can be spin-up and the other spin-down. The CISS effect exploits the spin-properties of electrons. CISS was first reported in a 1999 paper by Ron Namaan et al. demonstrating that electron transmission through a chiral molecule depends on the electron’s spin.5 Electrons in the spin-up state preferentially traverse a chiral molecule in one direction, while electrons in the spin-down state traverse the same chiral molecule more easily in the reverse direction.
Chemists now know that chiral molecules act as electron spin filters, permitting the one-way passage of electrons of one spin in preference to electrons of the other spin. Selective transmission probabilities can be a hundred times larger in a chiral molecule than in a non-chiral molecule. For an electron of the proper spin, chiral molecules show far less backscattering of the electron; this in turn greatly reduces the heat released from the molecule during the electron’s passage. Lower heat affords any biological system an advantage.
Scientists have often wondered why living creatures do not overheat while undertaking normal biochemical functions. The existence of exceedingly efficient biochemical routes is something like an a priori deduction. Kwabena Boahen estimated that a microelectronics processor functioning with the capacity of a human brain would need at least ten megawatts to operate. This is equivalent to the output of a small hydroelectric power plant. The human brain needs only about ten watts.6
Might CISS help to explain biology’s secret to efficiency?
CISS confers other benefits. Enantioselection designates the process of selecting one chiral isomer over another—the right, as opposed to the left, glove for the right hand. Enantioselective reactions are difficult in a laboratory, but easy in nature.
The hand-in-glove model is most often invoked when explaining nature’s exquisite ability to carry out enantioselective syntheses—a right hand only properly fits in a right-handed glove, for example. A preferential molecular fitting causes a molecule to enter into the chiral cleft with one preferred orientation. Van der Waals interactions favor one molecular alignment in the chiral cleft over another.
Little else is generally considered.
CISS reveals that more is going on, and it is not subtle in its influence. The attractive interactions between molecules of matched chirality can be higher by as much as one electron volt over their mirror images. This strongly favors enantioselection. Preferred chiral interaction is due to neighboring spin-spin interactions; the electrostatic potentials upon which they depend can interact at near the speed of light.
Is this how nature accomplishes its ultra-high degree of precision in enantioselective synthesis? Namaan et al. suggest that this is so.
The overall yield of biochemical processes in which CISS figures is often in excess of 99.99%. Astounding! In the laboratory, if the chemist achieves an eighty percent chemical yield after thorough optimization, he is usually satisfied. By controlling the spin on reactive chemical intermediates, CISS processes influence the course of chemical reactions so that they shovel their yields to one product instead of another. Reversing the spin interactions generates a different product.
There are other mysteries in nature. Consider the electronic transfers that routinely occur in photosynthesis and respiration. In the laboratory, electron transfer distances are often on the order of 0.5 nanometers or less. Biological systems routinely transfer electrons over distances of the tens of nanometers. Chiral molecules may act as nanoscale conduits for the 100-fold enhanced transmission of these electrons, provided that the spin on the electron is matched to the particular chiral molecule through which it passes.
There are still other mysteries. Consider how a repair-enzyme senses that there is damage to the DNA chain from a distance of tens of nanometers. How? It may well be by the transmission of electrons through the chiral DNA conduit. Is the plethora of molecule-molecular recognition events impacted by CISS?
Probably.
