Fluorine Probes Reveal Hidden Side-Chain Rotamers in a Model Protein

Fluorine is a tiny atom with an outsized talent for revealing what proteins are really doing in solution. In a study published in the journal Magnetic Resonance, researchers at the Australian National University, working with colleagues in Latvia and the United Kingdom, replaced every valine residue in the small protein GB1 with fluorinated analogues and used the resulting fluorine-19 nuclear magnetic resonance (NMR) spectra to work out, with unprecedented detail, which rotational conformations the fluorinated side chains preferentially adopt. The work demonstrates a practical toolkit for reading side-chain conformations directly from NMR data, and it uncovers a surprising preference for geometries that place fluorine atoms perilously close to backbone carbonyl groups.

The team, led by Elwy Abdelkader and Gottfried Otting, used cell-free protein synthesis to produce three versions of GB1, a compact 56-residue immunoglobulin-binding domain that has long served as a workhorse of protein biophysics. In one version, all four valines carried a single fluorine in one stereochemical arrangement, designated (2S,3S)-4-fluorovaline; in a second, the fluorine was installed in the alternative stereochemistry, (2S,3R)-4-fluorovaline; and in a third, both methyl hydrogens of one valine methyl group were replaced, yielding 4,4′-difluorovaline. Mass spectrometry confirmed that the dominant product in each case was the protein in which every valine site had been substituted, although a fraction of molecules retained one canonical valine, particularly in the difluorinated sample where the bacterial valyl-tRNA synthetase struggles to recognise the heavily fluorinated amino acid.

Crucially, the substitutions barely disturbed the protein. Melting temperatures measured by circular dichroism fell only about 10 degrees Celsius, from the wild-type value to a range of 67 to 70 degrees, and the characteristic proton chemical shifts of the native fold were conserved. That structural conservation proved to be more than a reassurance: because the relative ordering of proton chemical shifts was preserved between wild-type and fluorinated samples, the researchers could transfer resonance assignments from conventional two-dimensional NMR spectra to the fluorine dimension, opening a convenient route to assign the fluorine-19 signals without resorting to collections of point mutants. Short-delay proton-fluorine correlation experiments and fluorine-detected TOCSY spectra then linked each fluorine resonance to its parent residue, including stereospecific assignments distinguishing the two fluorines of each difluorovaline side chain.

The central analytical trick relies on two complementary NMR observables that report on the rotation of the CH2F group about its carbon-carbon bond. The first is the three-bond coupling between the fluorine nucleus and the beta proton of the same residue. According to the Karplus relationship, this coupling is large, roughly 30 to 44 hertz, when the fluorine sits trans to the beta proton, and small, around 8 to 9 hertz, in the gauche geometries. A coupling of 38 hertz measured for the fluorine at position 54 therefore unambiguously identified the trans rotamer for that group. The second observable is the so-called gamma effect on carbon-13 chemical shifts: a fluorine atom three bonds away from a carbon-13 nucleus shifts that resonance upfield by several parts per million, and the size of the shift depends on the dihedral angle between the two nuclei. Because the remaining methyl group of each fluorovaline provides a built-in carbon-13 reporter, the gamma effect distinguishes the two gauche rotamers that the proton-fluorine coupling alone cannot separate.

Density functional theory calculations underpinned the interpretation. Computations on the fluorovaline residues of a related protein, for which a 1.3 angstrom crystal structure exists, showed that the gamma effect on the methyl carbon reaches about minus 6.3 parts per million when the fluorine and the methyl carbon are nearly anti-periplanar, but only about minus 5.1 parts per million in the gauche geometries. Calculations on the model compound 1-fluoro-2-methylpropane, sweeping the torsion angle systematically, confirmed the same dihedral dependence whether or not the remaining molecular coordinates were relaxed. Experiment matched theory: the measured gamma effects ranged from about 5.8 to 7.4 parts per million, and they correlated with three-bond fluorine-carbon coupling constants measured by a constant-time HSQC difference experiment, exactly as expected if rotamer populations govern both quantities.

The analysis showed that CH2F groups almost never lock into a single rotamer, even when buried in the hydrophobic core. Only the most deeply buried valine, residue 54, showed a strong preference for one staggered conformation. Solvent-exposed groups sampled multiple rotamers, yet preferential populations could still be identified from the combination of coupling constants and chemical-shift effects. The energy barrier separating the three staggered rotamers of a CH2F group is roughly 4 to 5 kilocalories per mole, high enough to make eclipsed conformations unfavourable but low enough to allow interconversion at room temperature, so the observed spectra reflect a population-weighted average rather than a frozen geometry.

One of the most striking findings concerns where the fluorine atoms end up. In several residues, the preferred rotamer positions the fluorine atom as close as 2.85 angstroms to the carbonyl carbon of the same residue’s backbone, a distance that nominal van der Waals radii would suggest is sterically forbidden. The authors note that standard van der Waals radii may not apply to sp2 carbons bearing electron-withdrawing substituents, and they suggest that a favourable electrostatic interaction between the negatively polarised fluorine and the positively polarised carbonyl carbon may stabilise these geometries. Similar arrangements had been spotted before in a high-resolution crystal structure of a different fluorinated protein, but the new solution-state data show that the effect is systematic rather than a crystallographic artefact. Notably, the interaction is not strong enough to trap the CH2F group in a single rotamer, and it did not produce unusually large changes in the carbonyl carbon-13 chemical shifts.

The study also documented transient contacts between fluorine atoms on neighbouring residues. In GB1, the gamma carbons of valines 39 and 54 lie within about 4 angstroms of each other, and through-space scalar fluorine-fluorine couplings of roughly 1.5 hertz in the monofluorinated sample and 2.7 hertz in the difluorinated sample were detected in fluorine-fluorine TOCSY spectra. These couplings arise from short-range orbital overlap and therefore report direct, if fleeting, fluorine-fluorine contacts. Intriguingly, the difluorinated protein switched which pair of fluorines made contact, from the two gamma-1 fluorines to the gamma-1 fluorine of residue 39 paired with the gamma-2 fluorine of residue 54, suggesting that the extra fluorination nudges the local structure toward the conformation seen in crystal structures of the wild-type protein. The polarity of the carbon-fluorine bonds appears to discourage sustained fluorine-fluorine proximity, which also explains why the two CH2F groups within a single difluorovaline avoid conformations that would bring their fluorines together.

Beyond its structural insights, the work points toward practical applications. Fluorinated amino acids are now commercially available, and proteins labelled with them yield fluorine-19 spectra of exceptional width and resolution while remaining only modestly destabilised. Because the gamma effect translates rotamer populations into measurable carbon-13 shifts, a fluorinated isopropyl group could serve as a sensor of ligand binding that can be read out even on spectrometers not equipped for fluorine detection. The conservation of relative proton chemical shifts between wild-type and fluorinated proteins likewise offers a general shortcut for assigning fluorine spectra in larger systems, where broad lines and chemical-shift anisotropy usually force researchers into laborious mutagenesis. As the authors conclude, the ease with which proteins accommodate fluorine atoms, and the sensitivity of CH2F groups to their chemical environment, make these labels excellent non-invasive probes of protein structure, dynamics and molecular recognition.

Subject of Research: Rotamer preferences of fluorinated valine side chains in the protein GB1 determined by fluorine-19 NMR spectroscopy

Article Title: γ effects identify preferentially populated rotamers of CH2F groups: side-chain conformations of fluorinated valine analogues in a protein

Article References: γ effects identify preferentially populated rotamers of CH2F groups: side-chain conformations of fluorinated valine analogues in a protein. (n.d.). https://doi.org/10.5194/mr-6-257-2025

Image Credits: AI Generated

DOI: 10.5194/mr-6-257-2025

Keywords: fluorine-19 NMR, fluorinated amino acids, GB1 protein, rotamers, CH2F groups, gamma effect, Karplus relationship, DFT calculations, cell-free protein synthesis, through-space coupling, side-chain conformations, protein stability