Quantum Sensing Proteins and Engineering Chassis

Bottom line: a few biological sensors may depend on quantum effects, and
the best-supported case is radical-pair magnetoreception, where blue light
creates spin-correlated flavin-tryptophan radical pairs in cryptochromes
whose chemistry is sensitive to weak magnetic fields. From one deep-research
report (January 2026) we summarized the three proposed magnetoreception
mechanisms (radical pair, magnetite, induction), shortlisted five candidate
proteins (robin CRY4, monarch DpCRY1, human CRY2, fly CRY and the engineered
MagLOV proteins) and five engineering chassis, and listed contested areas such
as fly behavioural assays and vibrational olfaction. We did this to scope
whether an AI Gene Review project could usefully cover quantum-sensitive
proteins, or support engineering them. Scoped, not yet started as a dedicated
native-robin/monarch/MagLOV review set:
#3971 tracks
accession triage and the absent projects/quantum-sensing-bioinformatics/
workflow. Existing reviews made for other projects already carry
magnetoreception rows: fly DROME/CRY has four magnetoreception /
response to magnetism rows plus GO:0050980 detection of light stimulus involved in magnetoreception; Arabidopsis ARATH/CRY1 has two response to magnetism rows; all are KEEP_AS_NON_CORE. Human CRY2 is reviewed as
a mammalian circadian repressor and has no magnetoreception row.
The page also holds an unrelated side note on open light-source datasets
(LIGHT_SOURCE_OPEN_DATASETS.md).

Scope

Working definition: biological systems in which quantum effects (spin chemistry, quantum coherence, or tunneling) materially affect a sensing or signal transduction step. The strongest evidence base today is for radical-pair magnetoreception in cryptochromes; other areas are more speculative or are about energy transfer rather than sensing.

Deep research synthesis (Jan 31, 2026)

Based on projects/QUANTUM_SENSING/QUANTUM_SENSING-deep-research-openai.md, the field splits into three main biological mechanisms:
- Radical-pair magnetoreception (cryptochromes): strongest mechanistic evidence; light-dependent, spin-chemistry based sensing in birds/insects; still debated in some models (notably Drosophila behavior).
- Magnetite-based sensing: magnetite crystals act as mechanical transducers; strong evidence in magnetotactic bacteria and growing evidence in some fish/vertebrate tissues; molecular players remain unclear.
- Induction-based sensing: possible in aquatic animals moving through fields; less protein-specific.

Practical constraints from the literature:
- Earth-field sensitivity is weak and noisy; most lab demonstrations use stronger fields.
- Orientation and immobilization of the sensor protein matter for directional sensing.
- Signal readout and controls are a persistent challenge (risk of false positives).

Application themes that are emerging:
- Magneto-sensitive reporters (e.g., engineered flavoproteins with ODMR).
- Magnetogenetics (mixed evidence and reproducibility; careful controls needed).
- Conservation/behavioral applications (e.g., manipulating magnetic cues for navigation studies).

Proteins with evidence for quantum-level sensing

Cryptochromes (radical-pair magnetoreception)

Mechanism hypothesis: blue-light excitation of FAD in cryptochrome triggers electron transfer along a tryptophan chain, forming spin-correlated radical pairs. The spin dynamics are sensitive to weak magnetic fields, modulating the yield or lifetime of signaling states.

Key evidence and notes:
- Avian CRY4: European robin CRY4 shows magnetic field effects on radical pair yields in vitro; site-specific mutations implicate multiple FAD-Trp radical pairs in the magnetic response. This is one of the most direct biochemical demonstrations of magnetically sensitive cryptochrome photochemistry. (Nature 2021)
- Broad review: The radical-pair mechanism of magnetoreception is comprehensively reviewed with spin-chemistry and biological context. (Annual Review of Biophysics 2016)
- Monarch butterfly DpCRY1: light-dependent inclination sensing requires CRY1, not CRY2, with antennae and eyes implicated as magnetosensory organs; the accession and local evidence still need triage under #3971 before a review is seeded. (Nature Communications 2021)
- Human CRY2: Human cryptochrome 2 can function as a light-dependent magnetosensor in a Drosophila transgenic system, suggesting the photochemistry is conserved even if a human magnetosense is not established. (Nature Communications 2011)
- Drosophila CRY: GOA still has pre-replication magnetism rows on fly CRY; this project treats them as non-core and contested pending the replication-dispute review tracked in #3971.

Implication: cryptochromes are the leading protein family for quantum-sensitive sensing via spin chemistry.

Engineered LOV-domain proteins (MagLOV; ODMR in living cells)

• A 2026 Nature paper reports optically detected magnetic resonance (ODMR) in living bacterial cells using engineered LOV2-domain magneto-sensitive fluorescent proteins (MagLOV variants). The ODMR and magnetic-field effects are explained by a radical-pair mechanism involving the protein backbone and a bound flavin cofactor, with signal-to-noise sufficient for single-cell detection at room temperature.
• The study uses directed evolution to tune magnetic and radio-frequency responses and demonstrates applications including spatial localization with gradient fields (fluorescence MRI-like), microenvironment sensing, multiplexing/lock-in detection, and mitigation of scattering/autofluorescence. (Nature 2026)

Magnetite-based systems (classical magnetic sensing, not quantum)

• Magnetotactic bacteria use magnetosome gene clusters to build magnetite particles that align cells in fields.
• Vertebrate evidence is suggestive but mixed; recent work points to magnetite-bearing cells in fish olfactory tissues and candidate biomineralization gene signatures.

Shortlist: candidate proteins and constructs to pursue

Priority protein candidates (mechanistic evidence strongest)

• ErCRY4 (European robin cryptochrome 4): in vitro magnetic sensitivity with radical-pair signatures and mutational mapping of Trp radical pairs; strong biochemical anchor for magnetoreception. (Nature 2021)
• DpCRY1 (monarch butterfly cryptochrome 1): reported in light-dependent inclination-sensing work, but the native accession and local evidence still need triage.
• hCRY2 (human cryptochrome 2): supports light-dependent magnetosensing when expressed in Drosophila; good for readily available human sequence and protein tools. (Nature Communications 2011)
• DmCRY (Drosophila cryptochrome): historically used in magnetosensing assays, but behavioral evidence is contested by large-scale replication; still useful as a biochemically tractable CRY. (Nature 2023; Nature 2024)
• MagLOV (engineered LOV2 magneto-sensitive fluorescent proteins): ODMR and magnetic-field effects in living cells at room temperature; directed-evolution variants enable tunable responses and practical imaging/sensing. (Nature 2026)

Engineering-ready constructs (signal coupling or structural leverage)

• AtCRY2-CIB1 optogenetic module: blue-light inducible dimerization with rapid kinetics; useful to wire cryptochrome photochemistry into readouts. (Nature Methods 2010)
• AtCRY1 PHR domain-based constructs: structural templates with bound FAD enable rational tuning of electron-transfer paths; useful starting point for designing radical-pair lifetimes. (Swiss-Model Repository, PDB 1u3d/1u3c)
• AsLOV2-derived MagLOV variants: LOV2 backbone with flavin cofactor; demonstrated ODMR and strong magnetic-field effects in vivo with single-cell readouts. (Nature 2026)

UniProt snapshot (cursory)

Examples of cryptochrome entries and accessions linked from UniProt-connected resources:
- Homo sapiens CRY1: UniProt Q16526 (CRY1_HUMAN). (iPTMnet)
- Homo sapiens CRY2: UniProt Q49AN0. (Human Protein Atlas)
- Arabidopsis thaliana CRY1: UniProt Q43125 (CRY1_ARATH). (iPTMnet; Swiss-Model)
- Arabidopsis thaliana CRY2: UniProt Q96524. (InParanoid)
- Drosophila melanogaster CRY: UniProt O77059. (RCSB PDB/AlphaFold entry)
UniProt-linked annotations for CRY2 include FAD as a ligand/cofactor, consistent with flavoprotein photochemistry. (Human Protein Atlas)

Reproducible pipeline (draft)

I sketched a minimal, reproducible pipeline for candidate triage in projects/quantum-sensing-bioinformatics/:
- Fetch UniProt JSON + FASTA by accession list.
- Pull RCSB PDB mappings/metadata and ligands (flag FAD).
- Produce a per-accession summary table for quick screening.

This is intentionally conservative and not a functional claim; it’s meant to surface evidence and prioritize targets.

Photosynthetic energy-transfer complexes (quantum coherence; not sensing per se)

• FMO complex in green sulfur bacteria showed signatures interpreted as wavelike (coherent) energy transfer in ultrafast spectroscopy. (Nature 2007)
• Follow-up theoretical/experimental analyses argue that coherence signatures can be affected by ensemble dephasing and that the functional role in vivo is still debated. (J Phys Chem B 2011)

Implication: strong evidence of quantum coherence in pigment-protein complexes, but these systems are energy-transfer devices rather than sensors.

Olfaction (vibrational theory; speculative and contested)

• Drosophila can discriminate isotopomers in conditioning assays, interpreted as evidence for a vibration-sensing component of olfaction. (PNAS 2011)
• A later study found no isotopomer discrimination in several human and mouse olfactory receptors and argued against the vibrational theory for those receptors. (PNAS 2015)

Implication: potential quantum-tunneling or inelastic electron transfer mechanisms in odorant recognition remain controversial and receptor-specific at best.

Engineering chassis: candidates and rationale

1) Purified protein / in vitro reconstitution

• Best for mechanistic benchmarking of magnetic field effects (EPR, transient absorption, opto-chemistry) and rapid mutational screening.
• Straightforward to express cryptochromes in E. coli or insect cells and reconstitute with FAD.
• Enables decoupling photochemistry from cellular signaling, useful for engineering stronger magnetic field effect sizes.

2) Yeast and mammalian cell culture (HEK, U2OS, etc.)

• Good for eukaryotic folding, post-translational modifications, and targeting to specific subcellular locations.
• Compatible with reporter assays (luciferase, transcriptional switches) to couple cryptochrome conformational changes to measurable outputs.
• Suitable if the goal is to engineer a quantum-sensitive signal transduction module.

3) Insect systems (Drosophila S2 cells, insect cell lines, monarch)

• Useful for expressing insect CRY1s and leveraging existing genetic tools for sensory assays.
• Caution: behavioral magnetosensitivity in Drosophila is contested; monarch DpCRY1 has stronger evidence but is a more specialized chassis.

4) Plant systems (Arabidopsis, Nicotiana)

• Plants already use cryptochromes as blue-light photoreceptors; robust expression and optical readouts.
• Could be a chassis for engineering magnetically sensitive photoreceptor signaling pathways.

5) Magnetotactic bacteria (magnetosome proteins; classical magnetic sensing)

• Not quantum sensing, but a practical chassis for magnetic field coupling via magnetite crystals and magnetosome-associated proteins.
• Useful as a parallel track if the project goal is magnetic sensing rather than specifically quantum spin effects.

Engineering targets and design levers

• Radical-pair lifetime and yield: mutate residues in the electron-transfer chain (Trp triad/tetrad) or tune FAD redox chemistry.
• Signal coupling: fuse cryptochrome C-terminus to transcriptional regulators, ion channels, or dimerization modules to convert spin effects to outputs.
• Orientation and localization: membrane anchoring or anisotropic scaffolds may be required to translate magnetic anisotropy into directional signals.
• Photophysics: optimize absorption spectrum to desired illumination regimes (e.g., 380-500 nm for many CRYs).

Open questions for project scoping

• Do you want a biological sensor (in vivo behavior or cell signaling) or an in vitro quantum-chemistry module?
• Is the goal magnetic-field sensing (geomagnetic range) or broader quantum coherence/tunneling effects?
• What constraints on chassis (biosafety level, genetic tools, throughput) are acceptable?

References (selected)

• Hore PJ, Mouritsen H. The radical-pair mechanism of magnetoreception. Annu Rev Biophys. 2016. doi:10.1146/annurev-biophys-032116-094545.
• Xu J et al. Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature. 2021. doi:10.1038/s41586-021-03618-9.
• Wan G et al. Cryptochrome 1 mediates light-dependent inclination magnetosensing in monarch butterflies. Nat Commun. 2021. doi:10.1038/s41467-021-21002-z.
• Foley LE et al. Human cryptochrome exhibits light-dependent magnetosensitivity. Nat Commun. 2011. doi:10.1038/ncomms1364.
• Gegear RJ et al. Cryptochrome mediates light-dependent magnetosensitivity in Drosophila. Nature. 2008. doi:10.1038/nature07183.
• Bassetto M et al. No evidence for magnetic field effects on the behaviour of Drosophila. Nature. 2023. doi:10.1038/s41586-023-06397-7.
• Reppert SM. Magnetic field effects on behaviour in Drosophila. Nature. 2024. doi:10.1038/s41586-024-07319-x.
• Kyriacou CP. Magnetic field responses in Drosophila. Nature. 2024. doi:10.1038/s41586-024-07320-4.
• Bassetto M et al. Bassetto et al. reply. Nature. 2024. doi:10.1038/s41586-024-07321-3.
• Engel GS et al. Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature. 2007. doi:10.1038/nature05678.
• Ishizaki A, Fleming GR. On the interpretation of quantum coherent beats observed in two-dimensional electronic spectra of photosynthetic light harvesting complexes. J Phys Chem B. 2011. doi:10.1021/jp112406h.
• Franco MI et al. Molecular vibration-sensing component in Drosophila melanogaster olfaction. PNAS. 2011. doi:10.1073/pnas.1012293108.
• Block E et al. Implausibility of the vibrational theory of olfaction. PNAS. 2015. doi:10.1073/pnas.1503054112.
• Peigneux A et al. Learning from magnetotactic bacteria: synthesis of biomimetic nanoparticles mediated by magnetosome-associated proteins. J Struct Biol. 2016. doi:10.1016/j.jsb.2016.06.026.
• Abrahams G et al. Quantum spin resonance in engineered proteins for multimodal sensing. Nature. 2026. doi:10.1038/s41586-025-09971-3.
• Kennedy MJ et al. Rapid blue-light-mediated induction of protein interactions in living cells. Nat Methods. 2010. doi:10.1038/nmeth.1524.
• iPTMnet Report for Q16526 (CRY1_HUMAN). University of Delaware.
• iPTMnet Report for Q43125 (CRY1_ARATH). University of Delaware.
• Human Protein Atlas: CRY2 (UniProt Q49AN0).
• Swiss-Model Repository: Q43125 (CRY1_ARATH) structures (PDB 1u3d/1u3c).
• InParanoid: CRY1_ARATH ortholog group lists Q96524 (ARATH/CRY2).
• RCSB PDB/AlphaFold: UniProt O77059 (Drosophila cryptochrome).
• Deep research report: file:projects/QUANTUM_SENSING/QUANTUM_SENSING-deep-research-openai.md