Some bacteria produce tiny magnetite crystals inside their cells. These crystals act as nanomagnets, enabling the bacteria to orient themselves in the Earth’s magnetic field.
A Franco-Japanese research team has shown that these nanocrystals acquire an unexpected optical property, chiroptical activity, under the influence of their biological environment. But only while they remain inside the bacterium! Once removed from the cell, they lose this property. It can be restored by attaching a chiral amino acid to their surface.
Published in Nano Letters, this study provides the first direct evidence of biologically induced chiroptical activity in magnetite nanoparticles.
What is chirality?
An object is chiral when it exists in two mirror-image forms that cannot be superimposed, like a snail shell coiled to the right or to the left. Sugars and amino acids can also exist in two mirror-image forms, designated D and L. When a material interacts differently with right- and left-circularly polarized light, it is said to exhibit chiroptical activity.
Magnetism + chirality: a rare combination
Combining magnetism and chirality in a single material opens up possibilities in spintronics and selective catalysis (preferentially producing one of the two mirror-image forms of a molecule, an important challenge in pharmaceuticals).
Chirality is well documented in gold nanoparticles, but much less so in oxides. Magnetic oxide nanoparticles that display both magnetic and chiroptical properties remain rare and have been poorly studied.
Living systems have made their choice
Our proteins are made up almost exclusively of L-amino acids, while DNA contains almost exclusively D-sugars. This preference, known as biological homochirality, is a fundamental feature of life. But can this chirality also be transferred to the minerals that organisms produce?
What if we let a bacterium do the work?
This led to the idea of entrusting the production of nanomagnets to a bacterium, rather than attempting to make them solely through chemical synthesis. Magnetospirillum gryphiswaldense strain MSR-1, a magnetotactic bacterium, produces magnetite (Fe₃O₄) crystals approximately 37 nanometres across in its cytoplasm, each enclosed in a membrane: a magnetosome. Arranged in a chain, they form a magnetic rod that orients the cell in the Earth’s magnetic field.
This bacterium is, by its very nature, a chiral environment: can it transfer its chirality to the iron crystals it shapes?
How can we tell whether the signal comes from the iron, or from its environment?
Three samples were analyzed by natural circular dichroism (NCD), which is sensitive to chirality, and magnetic circular dichroism (MCD), which is sensitive to magnetism:
– the intact wild-type bacterium, with its magnetosome chain;
– a mutant (ΔmamAB) unable to produce magnetosomes, used as a biological control;
– magnetosomes extracted after the cells were disrupted.
The key finding: chiroptical activity disappears outside the cell
The mutant lacking magnetosomes shows a negative NCD signal at 233 nm, characteristic of transitions generally associated with proteins, while its MCD signal remains very weak. Extracted magnetosomes remain magnetic but no longer show detectable chiroptical activity.
The intact bacterium also exhibits a positive NCD peak at 362 nm, absent from the mutant, that coincides with an iron transition identified by MCD in the isolated nanoparticles. This weak but reproducible signal (asymmetry factor of 3.4 × 10⁻⁵) shows that the intact biological environment induces chiroptical activity associated with magnetite.
Can the nanoparticles’ chiroptical activity be restored?
To reproduce this effect outside the bacterium, the researchers attached pure aspartic acid, first in its D form and then in its L form, to the surface of extracted nanoparticles. The nanoparticles become optically active again, with mirror-image spectra: the signature of an effect governed by the ligand’s chirality. The signal from the D form coincides with that of the living bacterium, suggesting that small chiral amino acids present around magnetosomes may adsorb onto them and induce the observed asymmetry.
Towards biofactories for chiral nanomagnets
This discovery opens up a route to designing nanomaterials that combine magnetism and chirality: for the first time, the researchers show that a biological environment can induce chiroptical activity in magnetite nanoparticles. They also demonstrate that this effect can be reproduced outside the cell by modifying the nanoparticles’ surface with a chiral amino acid.
In the longer term, magnetotactic bacteria could become biofactories for magneto-chiral nanomaterials with applications in spintronics, enantioselective catalysis and sensing.
The biological factor responsible for this chiroptical activity has yet to be identified. But an important first step has been taken: the biological environment can confer chiroptical activity on minerals produced by living organisms.
Reference : Mendes, Matheus S. L.; Lefevre, Christopher T.; Benzerara, Karim; Nlate, Sylvain; Oda, Reiko; Hillard, Elizabeth A.; Pouget, Emilie « Chiral by Nature: Chiroptical Activity in Fe3O4 Magnetic Nanoparticles from Bacterial Biomineralization », Nano Letters, 2026. https://doi.org/10.1021/acs.nanolett.6c02286.
Collaboration: CBMN, ICMCB (Bordeaux), BIAM, IMPMC (Sorbonne University), WPI-AIMR (Tohoku University).
Contact : Christopher.LEFEVRE@cea.fr