Theoretical origin of life; Chirality needed by nature (Introduction)

by David Turell , Saturday, August 05, 2017, 22:46 (2667 days ago) @ David Turell

Chiral molecules guide electrons' spin and transfer under tight control. These quantum effects are required for efficiency. The latest studies show why left and right handed molecules are needed and used exclusively by living organisms:

http://inference-review.com/article/spin-in-quantum-biology#footnote-6

"Electrons have two intrinsic properties, charge and spin. The first is widely known, the second, less appreciated. The spin of an electron is a purely quantum mechanical property; it specifies the electron’s intrinsic angular momentum. ... While classical electrostatics does not allow two electrons to occupy the same region of space in a stable system, it is possible in quantum mechanics as long as their spins are different. This principle guides the organization of electrons in atoms, as well as in the chemical bonds between them.

***

"Here we discuss recent discoveries pointing to the importance of spin in biological systems, such as its role in long-range electron transfer, in reactions that involve multiple electrons, and in biorecognition. All these phenomena arise from chiral-induced spin selectivity (CISS), which links electron transfer through chiral molecules to a specific spin state of the electron. When electrons are transferred through chiral molecules, one spin state of the electron is preferred.

"Chirality is the name for the particular kind of symmetry that arises when one object is the mirror image of the other. An example is the human hand. Mirror images require no internal symmetries. Our two hands are called enantiomers of one another. Chiral molecules are referred to as being a right-handed enantiomer or a left-handed enantiomer.

***

"The components from which proteins are made—the amino acids—are enantiomerically pure in all organisms, and this purity is preserved with extremely high fidelity. Such enantiomeric selectivity is very difficult to achieve artificially in the laboratory. Moreover, the enantiopurity of drug medications is known to be essential to their efficacy. And without it, side effects are common and sometimes deadly.

***

"The CISS effect provides a possible answer. When electrons move elastically through chiral molecules, their spin and linear momentum are coupled; they cannot be reflected back without also flipping their spin, an improbable event in organic molecules. An electron should be able to propagate farther through a chiral molecule than through an achiral analog. This feature enhances the efficiency of electron transfer through chiral proteins.

"It is important to appreciate that molecules in a biological environment undergo significant thermal fluctuations. Nuclear and electronic motions vary and electron transfer thus occurs under the influence of an electronic potential that fluctuates. Under such conditions, backscattering could be significant if the CISS effect did not constrain the process.The use of chiral molecules as bridges for electron transfer could provide symmetry constraints that enhance electron transfer efficiency. Indeed, recent experiments reveal that spin plays a role in electron transfer through peptides, proteins, and DNA.5 For example, the electrons transferred through Photosystem I are spin-polarized.

"Many biochemical processes, such as oxygen formation in photosynthesis and respiration, require the transfer of multiple electrons.

***

"Spin polarization, which in chiral molecules accompanies charge polarization, is another way quantum mechanics affects biology. The coupling of the spin of an electron to its motion in chiral molecules and the resulting CISS have significant effects, even at physiological temperatures. For biomolecules, the effects are wide ranging, from the efficient transfer of electrons over relatively long distances, to enhanced selectivity of oxidation reactions, to the efficiency of enantioselective biorecognition."

Comment: These new finding explain why chirality is necessary. It is to control electron transfer in life's processes. As James Tour expresses in previous entries about synthesizing handedness in the laboratory, maintaining pure handedness of one type is very difficult. Nature does it very easily. Not by chance.