CISS theory

The Chirality-Induced Spin Selectivity ( CISS ) effect was first discovered over two decades ago in the field of optical polarization. (ref.)

It was found that a fundamental quantum property of electrons - known as 'spin' - is strongly influenced by the 'chirality' i.e. 'handedness' of molecules.

The 'spin' of an electron is crucial in determining whether or not it can occupy a particular electron 'shell' - or 'orbit' - around the atomic nucleus. The presence or absence of electrons in the shells very dramatically alters the chemical properties of an atom.

And, in laboratory settings, electron spin can now be manipulated with weak magnetic fields - leading to a a new area of study called 'Spin Chemistry'. (ref.)

Since most complex biological molecules are all 'handed' in the same way (see Homochiralityplugin-autotooltip__plain plugin-autotooltip_bigHomochirality

"Symmetry is a fundamental aspect of nature. For example, certain molecules exist in two forms which are symmetrical mirror images of each other. They are called chiral molecules. Common chemical synthesis generally produces equal amounts of the two forms of chiral molecules. In living systems, however, this symmetry is broken. Naturally occurring proteins are composed of L-amino acids but not D-forms, whereas DNA and RNA contain only D-sugars.) then, by implication, CISS effects might be highly important factors for bio-chemical reactions, Including vital processes such as Enzyme catalysisplugin-autotooltip__plain plugin-autotooltip_bigEnzyme catalysis

Almost all metabolic processes in living cells need enzyme catalysis in order to proceed at rates fast enough to sustain life. Although enzyme processes have been investigated for many decades, and more than 5000 have been identified, the extreme complexity of the biochemistry involved has meant that the way in which a particular enzyme operates can be far from clear. and Protein structuringplugin-autotooltip__plain plugin-autotooltip_bigProtein structuring

Genes set the order that amino acids (the chemical building blocks of proteins) appear in the proteins which they code for. But, working from the gene, the form which the protein's 3-D structure will take cannot as yet be predicted. The extremely complex shapes in which the protein 'folds' has a profound effect on the properties it has within an organism.. It's one of the research areas where it has recently become obvious that quantum effects can be highly important for biological systems.

Nonetheless, to date, the implications of CISS and the possible influence of magnetic fields for life systems have only been sketchily investigated.

In addition, CISS theory itself is poorly understood. Theoretical calculations for the strength of the effect differ very substantially from the results shown in practical experiments.

Early theoretical efforts have indeed confirmed that SOC ^{[ spin-orbit coupling ]} may provide a qualitative explanation for some aspects of the experimental findings. Quantitatively, however, such calculations have consistently predicted effects that were smaller by up to several orders of magnitude than those observed experimentally. While additional theoretical research efforts, described in more detail below, have shed more light on CISS, a complete quantitative theory of the effect remains elusive and its microscopic origins are insufficiently understood.
[…]
At present, a unifying scheme that would allow one to interpret all experiments in terms of only a single microscopic effect – the “CISS effect” - has not yet been identified. While such a framework cannot be ruled out, chirality-induced spin selectivity may perhaps be thought of as a set of phenomena that have a unifying scheme only in the sense that they all derive from the interplay of spin-orbit interaction and chirality.

Source : Theory of Chirality Induced Spin Selectivity: Progress and Challenges Advanced Materials, Volume 34, Issue13 ^{[ paywalled ]}
A full copy of which may be accessed here, at arXiv

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