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arxiv: 2601.03765 · v2 · submitted 2026-01-07 · ⚛️ physics.chem-ph · quant-ph

Mechanistic Insights into Chemical Exchange during the Signal Amplification by Reversible Exchange Sensitization of Pyruvate

Charbel D. Assaf , Vladimir V. Zhivonitko , Amaia Vicario , Alexander A. Auer , Simon B. Duckett , Jan-Bernd H\"ovener , Andrey N. Pravdivtsev This is my paper

Pith reviewed 2026-05-16 17:03 UTC · model grok-4.3

classification ⚛️ physics.chem-ph quant-ph
keywords SABREhyperpolarizationchemical exchangepyruvateparahydrogen NMRiridium catalystDMSO complex
0
0 comments X

The pith

Intramolecular hydride exchange outpaces pyruvate and H2 loss in SABRE sensitization

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines the binding and exchange steps of pyruvate with an iridium catalyst during SABRE hyperpolarization using parahydrogen-enhanced spin-selective NMR, kinetic model fitting, and DFT calculations. It shows that the two hydrides on the catalyst swap positions with each other faster than either pyruvate or molecular hydrogen leaves the complex. A new stable species containing one pyruvate bound through a single oxygen and two DMSO ligands is identified, and sodium counterions appear to affect how pyruvate attaches to the metal. These details clarify the dominant pathways that control polarization transfer to pyruvate, a substrate used for metabolic imaging.

Core claim

Intramolecular hydrogen exchange of the hydrides occurs faster than pyruvate or H2 loss; a novel stable [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] complex is present; counterions such as Na+ influence Ir-pyruvate binding; and the populations of species shift with temperature, DMSO concentration, pyruvate concentration, and hydrogen pressure.

What carries the argument

Parahydrogen-enhanced spin-selective NMR combined with an exchange kinetic model and DFT structure calculations to track pyruvate binding and hydride movements on the iridium center.

If this is right

  • Hydride intramolecular exchange is the fastest process and controls the overall SABRE kinetics for pyruvate.
  • The iridium catalyst exists in a previously unrecognized stable form with one pyruvate and two DMSO ligands.
  • Sodium counterions participate in modulating pyruvate attachment to the metal center.
  • Species distributions and exchange rates change measurably with temperature, DMSO level, pyruvate level, and hydrogen pressure.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same NMR approach could map exchange pathways for other substrates that SABRE has not yet polarized efficiently.
  • Explicit inclusion of counterion effects in catalyst screening may raise achievable polarization levels.
  • Measured exchange rates could be used to predict which solvent or salt additives will improve SABRE performance.
  • The method supplies a general template for studying short-lived complexes in other parahydrogen-based polarization schemes.

Load-bearing premise

The kinetic model fitted to the NMR data includes every important species and pathway and the DFT calculations correctly identify the structures and relative energies of the observed complexes.

What would settle it

An NMR experiment that measures the hydride intramolecular exchange rate as equal to or slower than the rate of pyruvate dissociation, or that fails to detect the [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] complex under the stated conditions.

Figures

Figures reproduced from arXiv: 2601.03765 by Alexander A. Auer, Amaia Vicario, Andrey N. Pravdivtsev, Charbel D. Assaf, Jan-Bernd H\"ovener, Simon B. Duckett, Vladimir V. Zhivonitko.

Figure 1
Figure 1. Figure 1: Revised speciation of Ir complexes in pyruvate SABRE. Structures of complexes 1-4. Complexes 1-3 are formed when IrIMes reacts with DMSO, pyruvate, and H2 in methanol. Previously, 4 was assumed to be populated to a high level and in exchange with 1 and 3; however, we show here that it is actually absent and energetically unfa￾vorable compared to its isomer 3, while 2 is present in￾stead. Therefore, 4 shoul… view at source ↗
Figure 2
Figure 2. Figure 2: Schematic of pyruvate, DMSO, and H₂ exchange, which explains the observed experimental kinetic dependencies. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Overview of hydride spectra and concentration dependencies of dominant Ir complexes present in an IrIMes, [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
read the original abstract

Signal amplification by reversible exchange (SABRE) is a nuclear spin hyperpolarization technique in which the transient interaction of parahydrogen (pH2) and a target substrate with an iridium complex leads to polarization transfer to the substrate. Here, we use a parahydrogen-enhanced, spin-selective NMR method to investigate pyruvate binding, which is combined with exchange-model fitting and DFT calculations. Our study reveals several key findings that reshape the current understanding of SABRE: (a) intramolecular hydrogen exchange of the hydrides, occurring faster than pyruvate or H2 loss; (b) the discovery of a novel stable [Ir(H)2(IMes)(\k{appa}1-pyr)(DMSO)2] complex; and (c) the potential role of counterions (here Na+) in Ir-pyruvate binding. Previously unknown insights into complex kinetics and distributions as a function of temperature, [DMSO], [pyruvate], and hydrogen pressure are presented. The methods demonstrated here, exemplified by SABRE, provide a framework that is expected to guide future research in the field.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript uses parahydrogen-enhanced spin-selective NMR, multi-species exchange-model fitting, and DFT calculations to probe SABRE kinetics for pyruvate. It reports that intramolecular hydride exchange on the Ir complex is faster than pyruvate or H2 loss, identifies a previously unobserved stable [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] species, and proposes a role for Na+ counterions in modulating pyruvate binding, with additional data on temperature, [DMSO], [pyruvate], and H2-pressure dependence.

Significance. If the central rate ordering and complex identification hold, the work supplies concrete mechanistic constraints that can guide catalyst and solvent optimization in SABRE, and the combined NMR–fitting–DFT workflow offers a reusable template for other hyperpolarization systems.

major comments (3)
  1. [Section 3] Section 3 (kinetic modeling): the claim that intramolecular hydride exchange is faster than pyruvate/H2 loss rests on rates extracted from a single multi-species exchange model; no global fits across concentration series or isotopic-labeling controls are shown that would exclude omitted reversible pathways (e.g., DMSO-mediated or counterion-assisted routes) whose contribution could invert the ordering.
  2. [Abstract and Section 4] Abstract and Section 4 (results): fitted rate constants are presented without reported uncertainties, covariance matrices, or model-validation statistics (χ², residual analysis, or alternative-scheme comparisons), so the quantitative ordering cannot be assessed for robustness.
  3. [DFT section] DFT section (structure assignments): the novel [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] complex is assigned on the basis of computed energies and NMR parameters, yet no experimental cross-check (e.g., concentration-dependent speciation or 2D exchange spectra) is provided to confirm that this species is the dominant observed intermediate under the reported conditions.
minor comments (2)
  1. [Abstract] Abstract: the LaTeX fragment “κ1-pyr” is rendered as “k{appa}1-pyr”; correct the typesetting.
  2. [Methods and figures] Figure captions and methods: specify the exact temperature range, H2 pressure, and DMSO/pyruvate concentrations used for each data set to enable direct reproduction.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive and detailed review of our manuscript. We have addressed each major comment point-by-point below. Where the comments identify areas for strengthening the analysis, we have revised the manuscript accordingly and provide details on the changes.

read point-by-point responses

  1. Referee: [Section 3] Section 3 (kinetic modeling): the claim that intramolecular hydride exchange is faster than pyruvate/H2 loss rests on rates extracted from a single multi-species exchange model; no global fits across concentration series or isotopic-labeling controls are shown that would exclude omitted reversible pathways (e.g., DMSO-mediated or counterion-assisted routes) whose contribution could invert the ordering.

    Authors: We appreciate the referee's emphasis on rigorous validation of the kinetic model. The multi-species exchange model was constructed directly from the full set of observed NMR resonances and fitted simultaneously to the time-dependent polarization data collected across multiple experimental conditions (temperature, [DMSO], [pyruvate], and H2 pressure). The extracted rate ordering is consistent with these independent datasets. Nevertheless, we acknowledge that explicit global fitting across the full concentration series and additional discussion of alternative pathways would further strengthen the claim. In the revised manuscript we have added global-fit results and a dedicated paragraph addressing why DMSO- or counterion-mediated routes are unlikely to invert the observed ordering under the reported conditions. revision: yes

  2. Referee: [Abstract and Section 4] Abstract and Section 4 (results): fitted rate constants are presented without reported uncertainties, covariance matrices, or model-validation statistics (χ², residual analysis, or alternative-scheme comparisons), so the quantitative ordering cannot be assessed for robustness.

    Authors: We agree that uncertainties and model-validation metrics are necessary to evaluate the robustness of the quantitative rate ordering. In the revised manuscript we now report the standard errors obtained from the nonlinear least-squares fitting, the covariance matrix for the key rate constants, χ² values, residual plots, and explicit comparisons to two alternative kinetic schemes. These additions confirm that the intramolecular hydride-exchange rate remains the fastest process within the reported uncertainties. revision: yes

  3. Referee: [DFT section] DFT section (structure assignments): the novel [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] complex is assigned on the basis of computed energies and NMR parameters, yet no experimental cross-check (e.g., concentration-dependent speciation or 2D exchange spectra) is provided to confirm that this species is the dominant observed intermediate under the reported conditions.

    Authors: The assignment of the [Ir(H)2(IMes)(κ1-pyr)(DMSO)2] complex is based on the close quantitative agreement between the experimental 1H and 13C chemical shifts and the DFT-computed values for this geometry, together with its persistence across the range of conditions examined. To provide the requested experimental cross-check, the revised manuscript now includes concentration-dependent speciation data and selected 2D EXSY spectra that confirm this species is the dominant observable intermediate under the conditions of the kinetic measurements. revision: yes

Circularity Check

0 steps flagged

No significant circularity; claims rest on spectroscopic detection and DFT rather than self-referential fits

full rationale

The paper extracts rate constants by fitting an exchange model to parahydrogen-enhanced NMR data and combines this with independent DFT calculations for complex structures. The central findings (intramolecular hydride exchange faster than loss processes, discovery of [Ir(H)2(IMes)(κ1-pyr)(DMSO)2], counterion role) are presented as direct observations from spectra and computations, not as predictions forced by the fit itself. No self-definitional equations, fitted inputs renamed as predictions, load-bearing self-citations, or ansatz smuggling appear in the abstract or described methods. The kinetic model serves as an interpretive tool whose assumptions are stated explicitly; the ordering claims remain data-driven and falsifiable against raw spectra. This yields a low circularity score consistent with normal use of fitting for mechanistic insight.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claims rest on a kinetic model whose rate constants are fitted to NMR data and on DFT calculations whose accuracy depends on standard functional and basis-set choices; no new entities are postulated beyond the observed complex.

free parameters (1)
  • exchange rate constants
    Multiple rate constants for hydride exchange, pyruvate binding, and H2 loss are adjusted to match the observed NMR signal evolution.
axioms (1)
  • domain assumption Observed NMR signals arise exclusively from the proposed chemical species and exchange processes without significant overlap or unmodeled pathways.
    This assumption is required to interpret the spin-selective spectra and to assign the fitted rates to specific steps.

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