Chirality is a geometric characteristic that allows molecules to exist in two distinct forms that, while chemically identical, are three-dimensional mirror images of one another—akin to right and left hands. Previously, it was believed that chirality had no influence on the coupling between nuclear spins, but a recent study indicates otherwise. The research shows that chirality, or handedness, actually affects how strongly nuclear spins are coupled. This discovery could enhance techniques for examining electrons and spins in both chemical and biological contexts.
According to new research conducted by scientists from UCLA, Arizona State University, Penn State, MIT, and Technische Universität Dresden, the strength of coupling between nuclear spins is influenced by the chirality, or handedness, of a molecule. The study also found that in chiral molecules of a specific handedness—either left or right—the nuclear spin tends to align in a particular direction. Conversely, in molecules with the opposite chirality, like right-handed ones, the spin aligns in the opposing direction.
This important finding, published in Nature Communications, challenges the long-held belief that chirality does not affect these couplings.
This newfound knowledge could aid in exploring how the handedness of molecules interacts with other molecules, potentially highlighting whether different chiralities yield different reactions. Such interactions may also shed light on how electron spin plays a role in chemistry and biology, since nuclear spins can indirectly reflect electron spin.
The nucleus of an atom comprises protons and neutrons that are bound together, each exhibiting a quantum property known as “spin.” This spin produces a magnetic field, similar to that of a bar magnet or flowing electrical current. When magnetic nuclei are close to one another, each one influences the spin of the other, a phenomenon referred to as spin-spin coupling, analogous to magnets pulling at each other.
These coupled spin states find utility in various applications, such as determining molecular structures in chemistry and in biomedical research through a method known as magnetic resonance spectroscopic imaging (MRSI). MRSI can be an invaluable method in medical diagnostics and research by analyzing the concentration of specific chemicals in tissues.
The magnetic field generated by a nuclear spin has direction, akin to an arrow pointing. However, unlike a compass needle that points north, the direction of a nuclear spin—termed the spin state—can vary, pointing upwards, downwards, or in different orientations. This orientation can differ among various molecules and can be affected or controlled by external magnetic fields, neighboring atoms and molecules, and applied radiofrequency fields.
The orientation of the spin state is crucial, as it determines how nuclear spins can be implemented in various applications. Researchers have extensively studied the elements that influence spin states, including spin-spin couplings, and how they can be controlled.
Since 1999, researchers have recognized that chirality, a basic property of specific molecules, significantly affects spin state, yet it was previously assumed to have no impact on coupling. Chirality is a geometric characteristic of molecules where identical atoms can be organized into two distinct forms that cannot be superimposed on each other, much like left and right hands.
Just as left and right hands cannot align perfectly through any method of translation or rotation, these mirror-image forms, known as enantiomers, share the same composition but interact differently with other chiral molecules and environments.
The latest research demonstrates that handedness influences the coupling of magnetic nuclear spins. While the effect is subtle and not very strong, it is still sufficiently noticeable in experiments. This work is the first to establish that purely magnetic effects within a molecule can impact nuclear spin-spin couplings.
“We found that the coupling between nuclear spins can change based on whether a molecule is left-handed or right-handed,” stated Louis Bouchard, a chemistry professor at UCLA and the study’s corresponding author. “The strength of this coupling varies between the two chiral forms. We were surprised to discover that chirality truly modifies these couplings. Our finding paves the way for selectively probing molecules based on their chirality.”
Given that chirality can be identified and chemical reactions can be manipulated, Bouchard suggests that techniques sensitive to nuclear spins could serve as sensors without interfering with chemical reactions involving chiral groups. This would allow detailed observation and analysis of reactions in real-time. One promising application could be in the development of non-invasive spectroscopic sensors for biological systems.
“We require improved methods to investigate the state of electrons and spins in both chemical and biological systems,” Bouchard noted. “This discovery provides an additional tool for chemists and biochemists, enabling the design of studies that examine spin states during chemical reactions.”
