Molecule 'handedness' determines the strength of a coupling between nuclear spins

This finding, published in Nature Communications, is significant because for many decades, it was believed that such couplings were unaffected by chirality.

This knowledge could be used to investigate the handedness of molecules as they interact with other molecules, potentially revealing whether specific chiralities lead to different outcomes. Such interactions might also provide insights into the role of electron spin in chemistry and biology, as nuclear spins can serve as indirect indicators of electron spin.

The nucleus of an atom contains protons and neutrons bound together, each possessing a quantum property called "spin." This spin generates a magnetic field, similar to that of a bar magnet or a circulating electrical current. When magnetic nuclei are in close proximity, each nucleus influences the spin of the other. This is called spin-spin coupling and is analogous to two magnets tugging on each other.

These coupled spin states are used in various applications, such as chemistry, to determine molecular structures, and in biomedical research, in a technique called magnetic resonance spectroscopic imaging, or MRSI. MRSI can be a valuable tool in medical diagnostics and research by measuring the concentration of certain chemicals in tissues.

The magnetic field created by a nuclear spin has a direction, similar to a pointing arrow. However, unlike a compass needle that consistently points north, the direction of nuclear spin -- known as the spin state -- can point up, down, or in other directions. This orientation can vary between different molecules in a sample and can be influenced or controlled by external magnetic fields, neighboring atoms and molecules, as well as externally applied radiofrequency fields.

The direction of the spin state is important because it affects how nuclear spins can be used in applications. Understanding the factors that influence spin states, such as spin-spin couplings, and how to control them has been a significant area of study for scientists.

Since 1999, scientists have known that chirality, one of the most fundamental properties of certain molecules, has a powerful effect on spin state, but it was thought to have no effect on coupling. Chirality refers to a geometric property of molecules where the same set of atoms can be arranged in two distinct forms that are non-superimposable mirror images of each other, much like left and right hands.

Just as left and right hands cannot be perfectly aligned through any combination of translations and rotations, these mirror-image forms, known as enantiomers, are identical in composition but differ in their interactions with other chiral molecules and environments.

The new research shows that handedness affects how magnetic nuclear spins are coupled. While the effect is subtle and small, it is still large enough to be detected in experiments. It is the first demonstration that purely magnetic effects within a molecule can contribute to nuclear spin-spin couplings.

"We discovered that the coupling between nuclear spins can vary depending on whether the molecule is left-handed or right-handed," said corresponding author and UCLA chemistry professor Louis Bouchard. "The strength of the coupling differs between the two chiral forms. We were surprised to find that chirality actually alters these couplings. Our finding could potentially be used to selectively probe molecules based on their chirality."

Since chirality can be detected and chemical reactions can be manipulated, Bouchard suggests that techniques sensitive to nuclear spins could be utilized as sensors that do not disturb chemical reactions involving chiral groups. This would permit observation and analysis of the reaction as it occurs. One potential application is in the development of nonperturbative spectroscopic sensors for biological systems.

"We need better methods to probe the state of electrons and spin in chemical and biological systems," Bouchard said. "This discovery adds a new tool to the chemistry and biochemistry toolboxes, enabling us to design studies that probe the state of spins during chemical reactions."