SABRE: Signal Amplification by Reversible Exchange

• SABRE is a hyperpolarization method that uses reversible catalyst exchange with parahydrogen to massively enhance nuclear magnetization in NMR and MRI.
• It relies on finely tuned exchange kinetics, pulse sequence engineering, and field modulation to optimize polarization transfer without chemical modification.
• Recent advances in catalyst design and multiaxial pulse schemes have enabled high-efficiency hyperpolarization for low-γ nuclei and applications like metabolic imaging.

Signal Amplification by Reversible Exchange (SABRE) is a parahydrogen-based hyperpolarization technique that enables massive enhancement—often by four to five orders of magnitude—of nuclear magnetization in solution-phase nuclear magnetic resonance (NMR) without chemical modification of the substrate. SABRE achieves this by transferring spin order from parahydrogen (pH₂), in its nuclear singlet state, to target molecules via reversible, catalyst-mediated exchange. Since its introduction, SABRE has had profound impact on sensitivity-limited NMR and MRI applications, particularly for low-γ nuclei (e.g., ¹³C, ¹⁵N), small-volume analytics, and metabolic imaging. The method’s theoretical underpinnings, kinetic principles, pulse sequence engineering, field-dependence, and molecular scope have been extensively developed and refined, enabling optimization far beyond initial implementations.

1. SABRE Mechanistic Principles

The foundational step in SABRE is the formation of a transient organometallic complex, typically based on an Ir(I)–N-heterocyclic carbene (Ir–NHC) catalyst, which reversibly binds both pH₂ and a target substrate. Parahydrogen (the J=0 singlet isomer, ρ = |S₀⟩⟨S₀|) supplies a reservoir of zero-magnetization two-spin order, which—via strong J-couplings within the complex—is coherently transferred to one-spin or two-spin order on the targeted substrate nuclei.

The “classic” SABRE process can be summarized as:

1. Complex Formation:

$$\text{Ir(catalyst) + pH}_2 + S \rightleftharpoons \text{Ir}(H)_2(S)_n$$

1. Spin-Order Transfer: Hyperfine (scalar) couplings—such as $J_{HH}$ (hydride–hydride, ~−8–10 Hz) and $J_{HL}$ (hydride–ligand, ~1–25 Hz)—enable mixing between the hydride singlet and the Zeeman states of the ligand.
2. Substrate Release and Repetition: The hyperpolarized substrate is released into solution; continued bubbling with fresh pH₂ repopulates the singlet order and sustains signal amplification via chemical exchange (Arunkumar et al., 2020, Smith et al., 6 Nov 2025, Eriksson et al., 2021).

This mechanism is distinct in that the chemical identity of the substrate remains unchanged—contrasting with PHIP, in which hydrogenation occurs.

2. Kinetics, Exchange Dynamics, and Spin Physics

SABRE polarization build-up is governed by coupled chemical kinetics and spin evolution. The prototypical kinetic model considers association/dissociation rates ($k_\mathrm{on}$, $k_\mathrm{off}$), the polarization transfer rate ($k_\mathrm{ex}$), and relaxation time ($T_1$) of the substrate in free solution (Arunkumar et al., 2020, Xu et al., 2023). The build-up equation:

$$\frac{dP}{dt} = k_\mathrm{ex} (P_{H_2} - P) - \frac{P}{T_1}$$

integrates in the fast-exchange regime to

$$P(t) = P_\mathrm{max} (1 - e^{-k_\mathrm{ex} t}) \quad,\quad P_\mathrm{max} = P_{H_2} \frac{k_\mathrm{ex}}{k_\mathrm{ex} + 1/T_1}$$

Where $P_{H_2}$ is the effective singlet order drawn from pH₂ bubbling (ideally ~0.5).

A critical facet is that, in actual SABRE complexes, binding of pH₂ breaks symmetry, promoting singlet–triplet ($S$–$T_0$) mixing and the rapid formation of anti-phase ($I_{1z}I_{2z}$) hydride order, not pure singlet (Knecht et al., 2018, Smith et al., 6 Nov 2025, Knecht et al., 2018). This directly impacts polarization transfer efficiency and necessitates tailored pulse sequences for high-field experiments, as conventional assumptions (pure-singlet initial state) often fail.

Exchange and relaxation rates set an optimal window—too rapid exchange prevents complete transfer; too slow exchange leads to relaxation-dominated losses (Xu et al., 2023). Field-dependent kinetic models, incorporating the full Liouville superoperator formalism, have been developed for reliability and optimization (Eriksson et al., 2021, Xu et al., 2023).

3. Hyperpolarization Pulse Schemes: Continuous, Pulsed, and Multiaxial Strategies

Efficient SABRE hyperpolarization requires precise control over both magnetic field and pulse sequence. The original method employed static low field matching ("level anti-crossing," LAC) conditions, with the optimum field determined by a resonance between hydride Zeeman and J-coupling interactions, e.g., $B_\mathrm{LAC} \approx J_{HH}/\gamma_H$ for ¹H (6.6 mT) (Arunkumar et al., 2020). For X-SABRE (heteronuclei), the LAC field is typically sub-microtesla.

However, LAC-based resonance conditions have severe limitations:

• Exchange and relaxation lead to nonoptimal and shifted fields (empirically, best polarization for X-SABRE is at $B \sim 0.6\,\mu$T, not the classical LAC value) (Eriksson et al., 2021, Smith et al., 6 Nov 2025).
• More generally, SABRE efficiency can be dramatically improved by using non-intuitive, far-from-LAC field modulations: two-state, compensated, oscillating, or multiaxial pulse sequences (Eriksson et al., 2021, Lindale et al., 2023, Li et al., 2022).
• These optimized schemes exploit average Hamiltonian theory (AHT), engineering effective couplings and controlled phase relationships to maximize polarization transfer pathways otherwise hidden or suppressed by symmetry (Li et al., 2022).

Key advancements include:

• Oscillating and Asymmetric Pulses: Asymmetric (e.g., ramp or chirped-square) pulses remove symmetry-imposed polarization "blind spots" and stabilize performance against field and timing errors, providing 3–6× higher polarization than conventional approaches (Li et al., 2022).
• Multiaxial Field Control (MACHETE-SABRE): Algorithmic optimization in three spatial field axes achieves 10-fold enhancement over static-field SABRE-SHEATH by preserving hydride singlet order during catalytic exchange (Lindale et al., 2023, Eriksson et al., 2022).
• High-Field Phase-Coherent Excitation: Phase-cycled broadband RF schemes now permit simultaneous, high-field hyperpolarization of multiple NMR targets for multiplexing and broad nuclei access (Lindale et al., 2021).

4. Molecular Scope, Substrate Binding, and SABRE-Relay

SABRE hyperpolarization can target hundreds of molecules given suitable binding affinities to the Ir–NHC catalyst and compatible scalar J-couplings. Efficient polarization is achievable for a diverse range of small molecules, including pyridines, heterocycles, carboxylates, and metabolic substrates like pyruvate (Assaf et al., 7 Jan 2026, Boele et al., 12 Sep 2025). The mechanism’s generality is further enhanced by:

• Ligand and Catalyst Engineering: Co-ligands such as DMSO, ligand deuteration, and counterion tuning modulate binding equilibria and spin exchange routes, significantly impacting rates and observable multi-species polarization (e.g., stabilization of resting vs. active pyruvate–Ir complexes by Na⁺) (Assaf et al., 7 Jan 2026).
• SABRE-Relay: Hyperpolarization can be relayed between substrates via either direct chemical exchange or through secondary coordination complexes, provided by explicit SABRE-relay kinetic models and confirmed experimentally (Knecht et al., 2020).
• Singlet-State Storage: Dual-nucleus systems (e.g., azobenzene (Sheberstov et al., 2021)) allow for long-lived singlet-state preservation, enabling sustained hyperpolarization far beyond conventional T₁ limits.

5. Experimental Realizations: Sensitivity, Miniaturization, and Applications

SABRE has fundamentally transformed the sensitivity limits of NMR—enabling, for example, the detection of millimolar concentrations in tens of picoliters (e.g., 10^−14 mol in a ∼10 pL diamond NV–NMR volume) with sensitivity enhancements exceeding $10^5$ over thermal polarization (Arunkumar et al., 2020). Applied advances include:

• Microfluidic and Diamond NV–NMR: Integration of SABRE with optically detected NV-center magnetometry provides unprecedented small-volume analytics, down to single-cell-scale molecular analysis (Arunkumar et al., 2020).
• Metabolic MRI: SLIC-SABRE and related variants enable hyperpolarization of ¹³C-pyruvate for in situ MRI at ultra-low field (e.g., 6.5 mT), with million-fold enhancement and direct MRI readout (Boele et al., 12 Sep 2025).
• Compact Field Cycling and On-Spectrometer Integration: Devices enabling rapid sample shuttling and field cycling over 9.4 T→nT span facilitate robust SABRE-SHEATH operation, exchange-rate measurement, and minimal polarization loss during transfer (Peters et al., 10 Jun 2025).

A summary table of SABRE operational regimes:

SABRE Variant	Field Regime	Target Nuclei	Max Enhancement	Notable Features
SABRE-HF (¹H)	~6–7 mT	¹H	10³–10⁵	Zeeman–J matching; single-bond couplings
SABRE-SHEATH (X-SABRE)	~0.1–1 µT	¹⁵N, ¹³C, etc.	10³–10⁴	Ultra-low field, LAC or optimized field-cycling
Nonresonant SABRE	1–10³ µT	¹⁵N, ¹³C	10³–10⁴	Field-invariant, two-spin order hyperpolarization
MACHETE-SABRE (multiaxial)	µT, any direction	¹⁵N, ¹³C, etc.	up to 10× SHEATH	Evolutionary-optimized, robust phase-engineering
High-field phase-coherent	≥1 T (spectrometer)	all	∼100–200×	Broadband, multi-target, in situ operation

6. Theoretical Developments and Future Trajectories

Recent theoretical work has revealed that the commonly used LAC approach is often suboptimal, and that optimal field and time parameters are dictated by detailed balance among coherent mixing, exchange kinetics, relaxation, and actuator-induced symmetry breaking (Eriksson et al., 2021, Smith et al., 6 Nov 2025, Li et al., 2022, Lindale et al., 2023).

Key insights and future vectors:

• Nonresonant Pathways: Field-independent, direction-independent two-spin hyperpolarization, robust to hardware constraints and extending generality to bench-top setups (Smith et al., 6 Nov 2025).
• Pulse and Field-Sequence Engineering: Use of average Hamiltonian theory and evolutionary algorithms to design robust, scalable pulse shapes that maximize steady-state polarization and mitigate imperfections (Li et al., 2022, Lindale et al., 2023).
• Catalyst and Exchange Control: Understanding the role of co-ligands, counterions, and temperature in controlling active and resting states of catalyst complexes to optimize hyperpolarization throughput (Assaf et al., 7 Jan 2026).
• Long-Lived State Storage and Signal Retention: Integration of post-polarization field steps to preserve spin order during shuttling and isomerization-based methods for lifetime extension (Sheberstov et al., 2021, Peters et al., 10 Jun 2025).
• Mutinuclear and Multiplexed Applications: High-field, phase-coherent multi-target approaches open routine SABRE hyperpolarization for metabolomics, in vivo molecular imaging, and quantum sensing (Lindale et al., 2021).

7. Limitations and Optimization Guidelines

While SABRE offers dramatic enhancements, limitations arise due to finite relaxation, exchange rates, and incomplete singlet preservation. Key limiting factors and guidelines include:

• Exchange Rate Tuning: Maximize $k_\mathrm{ex}$ to balance singlet build-up and nuclear relaxation; avoid rapid dissociation that precludes mixing, or sluggish exchange leading to relaxation losses (Knecht et al., 2018, Xu et al., 2023).
• Singlet–Triplet Mixing: Minimize S–T₀ mixing via fast pH₂ replenishment, low-temperature stabilization, or transverse decoupling pulses to preserve usable spin order (Knecht et al., 2018, Eriksson et al., 2022).
• Field Precision vs. Modulation Robustness: Time-asymmetric fields (chirp, ramp, multi-axis) confer insensitivity to imperfections; careful calibration of B₀ is still required for static or symmetric schemes (Li et al., 2022).
• RF and Catalyst Limitations: Higher-power phase-coherent pulse sequences impose increased RF heating; catalyst and ligand concentrations must be set to maximize transfer while minimizing unproductive resting complexes (Lindale et al., 2021, Assaf et al., 7 Jan 2026).
• Relay and Multi-step Architectures: For extensions such as SABRE-relay, exchange rates and binding affinities for both "main" and "relay" substrates must be systematically optimized to enable efficient secondary polarization transfer (Knecht et al., 2020).

In summary, SABRE and its rapidly expanding suite of variants constitute a highly versatile, theoretically mature, and practically generalizable platform for hyperpolarization in solution NMR and MRI. Advances in pulse-sequence design, multi-axis field modulation, and catalyst engineering continue to expand its reach across chemistry, analytical science, and molecular imaging (Arunkumar et al., 2020, Eriksson et al., 2021, Lindale et al., 2023, Smith et al., 6 Nov 2025, Li et al., 2022, Lindale et al., 2021, Assaf et al., 7 Jan 2026).