Fluorine Spin States That Outlast Relaxation Open New Window on Drug Binding

Nuclear magnetic resonance has long lived with an uncomfortable truth: the spin order that spectroscopists so carefully prepare begins to decay the moment it is created. Longitudinal relaxation times, the familiar T1 values that govern how quickly nuclear magnetization returns to equilibrium, typically set a hard ceiling on how long any NMR experiment can watch a molecule before its signal fades. Now a team at the École Normale Supérieure in Paris has shown that in a family of environmentally notorious chemicals, that ceiling can be pushed dramatically higher. In three per- and polyfluoroalkyl substances, the researchers created nuclear spin states whose lifetimes exceed the ordinary relaxation limit by roughly a factor of two to more than three, and they demonstrated that these extended lifetimes are exquisitely sensitive to whether the fluorinated molecules are bound to a protein.

The work, published in the journal Magnetic Resonance by Coline Wiame, Sebastiaan Van Dyck, Kirill Sheberstov, Aiky Razanahoera, and Geoffrey Bodenhausen, extends a concept that has fascinated magnetic resonance scientists for more than a decade: long-lived states, or LLSs. The idea traces back to the spin isomers of dihydrogen, where the population imbalance between para- and ortho-hydrogen can persist for days. In solution NMR, the canonical long-lived state is the singlet state of a pair of nuclei, in which the two spins form a combination with zero total spin. The population difference between this singlet and the average of the three triplet states, known as a triplet–singlet population imbalance, is immune to the dominant relaxation mechanism, the dipole–dipole interaction between the two spins of the pair itself. Because that intrapair pathway is switched off, the imbalance decays far more slowly than ordinary magnetization.

What makes the new study distinctive is the jump from pairs to manifolds of spins. The team worked with achiral molecules containing chains of two or three consecutive CF2 groups, which means four or six fluorine-19 nuclei arranged in symmetric pairs. In Pople’s notation such a six-spin system is labeled AA′MM′XX′, and the theoretical menu of long-lived spin order is correspondingly richer. Beyond the two-spin order terms that describe a singlet imbalance within a single CF2 group, the chains can host four-spin order terms such as the product of two singlet imbalances on different groups, and in principle even a six-spin term spanning the entire chain. Simulations on analogous protonated chains suggest that six-spin terms are produced only in small yields, but the experiments show that admixtures of two-, four-, and minor six-spin order relax together with an effective mono-exponential decay, yielding a single measurable lifetime.

Exciting such states in a symmetric, achiral molecule is far from trivial. The two fluorine atoms attached to the same carbon are chemically equivalent, so their geminal coupling does not split the spectrum, and the standard pulse sequences that exploit chemical shift differences fail. The key requirement is magnetic inequivalence: a pair of fluorines on one carbon becomes magnetically inequivalent only if its vicinal couplings to neighbors on adjacent groups are not degenerate. That degeneracy is lifted when rotation about the carbon–carbon bonds produces rotamers of unequal energy, creating small differences between nominally identical coupling constants. For fluorine these differences can reach tens of hertz, considerably larger than the few-hertz differences typical of protons, which makes fluorinated chains particularly favorable terrain for the method.

The Paris group used spin-lock induced crossing, or SLIC, a technique in which a radio-frequency field applied at the right amplitude drives a level anti-crossing that converts ordinary magnetization into singlet order. Applying this to fluorine required confronting a striking difference in coupling magnitudes: geminal fluorine–fluorine couplings in CF2 groups run between roughly 250 and 290 hertz, an order of magnitude larger than the geminal proton couplings of about −15 hertz in CH2 groups. The team employed mono- and polychromatic SLIC, applying one, two, or three radio-frequency fields simultaneously to the multiplets of one, two, or three CF2 groups, and distinguished single-quantum from double-quantum conditions, which demand radio-frequency amplitudes equal to the geminal coupling or twice that value, respectively.

A crucial preparatory step involved the so-called outer singlet–triplet transitions, weakly allowed combination lines that appear on the flanks of the fluorine multiplets with intensities about ten thousand times weaker than the allowed transitions. These forbidden lines encode the geminal couplings that SLIC needs, but they are essentially invisible in a conventional spectrum. By irradiating a multiplet with a carefully tuned radio-frequency field, the researchers amplified the outer singlet–triplet transitions by up to two orders of magnitude, allowing them to measure geminal couplings in the range of 280 to 295 hertz and to optimize the SLIC parameters. In one demonstration on perfluorobutanoic acid, a spectrum that would have required 1024 scans to reveal the forbidden lines was captured with just four scans once the amplification was in place.

With the pulse sequence optimized, the team measured long-lived state lifetimes in three achiral molecules: perfluorobutanoic acid, perfluorobutane sulfonic acid, and perfluoropentanoic acid, all dissolved in deuterated DMSO at 500 millimolar concentration. At a static field of 11.7 tesla, the fluorine-19 long-lived state lifetimes exceeded the corresponding T1 values by factors between 2.1 and 3.4. The researchers also repeated the measurements at 7 tesla and found that both lifetimes lengthened, a consequence of chemical shift anisotropy, which relaxes fluorine nuclei more efficiently at higher fields. Notably, the ratio of long-lived state lifetime to T1 remained near two at both fields, indicating that chemical shift anisotropy erodes both types of spin order in a similar fashion. Selective decoupling experiments confirmed that the CF3 methyl groups at the ends of the chains contribute nothing to the long-lived states.

The most consequential result may be the demonstration that these lifetimes report on molecular binding. The team titrated perfluorobutane sulfonic acid with bovine serum albumin, a protein known to interact with fluorinated compounds, using 50 millimolar ligand and protein concentrations ranging up to 100 micromolar. The long-lived state relaxation rates shifted measurably as the ligand bound, and the contrast between free and bound ligand, defined through the change in relaxation rate, was far stronger when measured through the long-lived state lifetime than through ordinary T1. Strikingly, good contrast persisted even when the protein was ten thousand times more dilute than the ligand, a regime relevant to fragment-based drug screening where weak binders must be detected against a large excess of small molecule.

The connection to medicine is direct. Roughly a third of small-molecule drugs contain fluorine, because carbon–fluorine bonds resist enzymatic degradation and thereby extend drug lifetimes in the body. Fluorine-19 is nearly as sensitive as the proton in NMR terms, its chemical shifts span a wide range, and biological tissues contribute essentially no background fluorine signal, making fluorinated ligands ideal probes. A long-lived state spanning multiple fluorines in a ligand chain amplifies the spectroscopic signature of binding through changes in chemical shifts, a reduction of symmetry when an achiral ligand meets a chiral protein target, and slower rotational diffusion of the bound complex. The Paris results suggest that attaching fluorinated aliphatic chains to existing drug candidates could turn their spin order into a sensitive reporter of protein engagement.

There is also a broader scientific payoff. Per- and polyfluoroalkyl substances, the PFAS often called forever chemicals, are under intense scrutiny for their persistence and toxicity, and previous work has shown that they bind to a variety of proteins. A technique that can detect such binding with high contrast, using the fluorine nuclei already present in these molecules, offers a new tool for probing how PFAS interact with living systems at the molecular level. The authors note that contrast should improve further at lower magnetic fields, where chemical shift anisotropy relaxes fluorine less aggressively, though that prediction remains to be verified experimentally. From the spin isomers of hydrogen to the fluorinated chains of industrial chemistry, the lesson of long-lived states continues to hold: some of the most useful information in NMR belongs to spin order that refuses to fade on schedule.

Subject of Research: Long-lived fluorine-19 nuclear spin states in fluorinated aliphatic chains and their use for detecting ligand–protein binding

Article Title: Long-lived states involving a manifold of fluorine-19 spins in fluorinated aliphatic chains

Article References: Long-lived states involving a manifold of fluorine-19 spins in fluorinated aliphatic chains. (n.d.). https://doi.org/10.5194/mr-6-273-2025

Image Credits: AI Generated

DOI: 10.5194/mr-6-273-2025

Keywords: long-lived states, fluorine-19 NMR, PFAS, spin-lock induced crossing, singlet order, drug screening, relaxation, chemical shift anisotropy, protein binding, magnetic resonance, perfluoroalkyl substances, NMR spectroscopy