It’s the spin that makes the difference

The so-called homophily of life—the fact that all biological molecules in living organisms exist in only one of two mirror-image forms—has puzzled many scientific luminaries, from Louis Pasteur, the discoverer of molecular chirality, to William Is. Thomson (Lord Calvin) and Nobel Laureate Pierre Curie. A definitive explanation is still lacking, as both forms, for example, have the same chemical stability and do not differ from each other in their physico-chemical properties. However, the hypothesis that interactions between electric and magnetic fields could explain a molecule’s preference for one or the other mirror image — so-called enantiomers — emerged early.

It was only a few years ago, though, that the first indirect evidence emerged that different combinations of these force fields could actually “distinguish” between two mirror images of a molecule. This was achieved by studying the interaction of chiral molecules with metal surfaces that exhibit a strong electric field over short distances. Surfaces of magnetic metals such as iron, cobalt or nickel thus allow electric and magnetic fields to couple in different ways — changing the direction of magnetization from “north up — south down” to “south up — north.” is changed. below.” If the interaction between magnetism and electric fields actually drives “antiselective” effects, then the strength of the interaction between chiral molecules and magnetic surfaces should also vary, for example — depending on whether a right-handed or left-handed molecule “lives” on the surface.

Mirror images prefer opposite magnetic fields.

And indeed this is the case, as a team of researchers led by Karl-Heinz Ernst of Empa’s Surface Science and Coating Technologies Lab and colleagues at the Peter Grunberg Institute of Forschungszentrum Julich in Germany recently reported in the journal Science. Advanced content. The team coated a (non-magnetic) copper surface with tiny, ultra-thin “islands” of magnetic cobalt and determined the direction of the magnetic field in them using spin-polarized scanning tunneling microscopy. As mentioned earlier, it can run in two different directions perpendicular to the metal surface: north up or south up. They then assembled spiral-shaped spiral molecules — a 1:1 mixture of left- and right-handed heptahelical molecules — on cobalt islands in high vacuum.

They then counted the number of right- and left-handed helicine molecules on “only” differently magnetic cobalt islands, about 800 molecules in total, again using scanning tunneling microscopy. And lo and behold: depending on the direction of the magnetic field, one or the other form of the Helician spiral was preferentially populated (see right side of graphic).

Furthermore, the experiments show that the selectivity — the preference for one or the other enantiomer — occurs not only during binding to the cobalt islands, but already. Before the molecules assume their final (preferred) position on one of the cobalt islands, they move long distances on the copper surface in a significantly weakly bound precursor state in “search” for an ideal position. They are bound to the surface only by so-called van der Waals forces. They are simply caused by fluctuations in the electronic shells of atoms and molecules and are therefore relatively weak. The fact that these are also affected by magnetism, i.e. the direction of spin of the electrons, was not known until now.

Electrons with the “wrong” spin are filtered out.

Using scanning tunneling microscopy, the researchers were also able to solve another puzzle, as they reported last November in the journal Small. Electron transport — that is, electric current — also depends on a combination of molecular handedness and surface magnetism. Depending on the craft of the bound molecule, electrons with one direction of spin preferentially flow — or “tunnel” — through the molecule, meaning that electrons with the “wrong” spin are filtered out. This chirality-induced spin selectivity (the CISS effect, see left side of graphic) had already been observed in previous studies, but it remained unclear whether pairs of molecules were necessary or whether individual molecules also exhibited this effect. are Ernst and his colleagues have now been able to show that individual helicine molecules also exhibit the CISS effect. “But the physics behind it is not yet understood,” admits Ernst.

The MPA researcher also believes that his findings may not ultimately provide a complete answer to the question of the meaning of life. In other words, the question that Nobel laureate in chemistry and ETH chemist Vladimir Prelog described in his Nobel Prize lecture in 1975 as “one of the first problems of molecular theology”. — as might have occurred in a chemical “primary soup” on early Earth — a particular combination of electric and magnetic fields could have led to a permanent accumulation of some form of various biomolecules — and that Kind of ultimately life quotes.