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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

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The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.

Cooperative Allosteric Transitions

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Cooperative Allosteric Transitions

Cooperative allosteric transitions can occur in multimeric proteins, where each subunit of the protein has its own ligand-binding site. When a ligand binds to any of these subunits, it triggers a conformational change that affects the binding sites in the other subunits; this can change the affinity of the other sites for their respective ligands. The ability of the protein to change the shape of its binding site is attributed to the presence of a mix of flexible and stable segments in the...

Spin–Spin Coupling: One-Bond Coupling

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...

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Examining the Conformational Dynamics of Membrane Proteins in situ with Site-directed Fluorescence Labeling

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Conformational Switching Controls Biradical Spin Dynamics in Flavin-Tryptophan Dyads.

Guzel Musabirova¹, Olga B Morozova², Alexey S Kiryutin²

¹Department of Analytical Chemistry, Leipzig University, Linnéstr. 3, 04103 Leipzig, Germany.

Journal of the American Chemical Society

|March 23, 2026

Summary

Proline isomerization in flavin-tryptophan dyads acts as a switch, controlling radical recombination pathways. Field-cycling NMR reveals how proline conformation impacts spin dynamics and biradical formation.

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Area of Science:

Photochemistry and photophysics
Biophysical chemistry
NMR spectroscopy

Background:

Flavin-tryptophan dyads are crucial for studying photoinduced electron transfer.
Proline isomerization (cis-trans) significantly impacts molecular conformation and dynamics.
Photo-chemically induced dynamic nuclear polarization (photo-CIDNP) is sensitive to radical pair spin dynamics.

Purpose of the Study:

To investigate how proline cis-trans isomerization influences spin dynamics in flavin-tryptophan dyads.
To elucidate the role of proline conformation in controlling intramolecular versus intermolecular radical recombination.
To assess the utility of field-cycling NMR and time-resolved photo-CIDNP in studying these systems.

Main Methods:

Synthesis of flavin-tryptophan dyads with varying oligoproline linkers.
Field-cycling NMR spectroscopy under photo-CIDNP conditions.
Time-resolved photo-CIDNP measurements.

Main Results:

Minor cis-proline conformers favor compact structures enabling intramolecular biradical recombination, observed via J-resonance photo-CIDNP.
Dominant trans-proline conformers and longer linkers lead to intermolecular radical encounters, indicated by Δg-dominated patterns.
Time-resolved studies confirmed the prevalence of intermolecular processes, with some dyads showing intramolecular contributions.

Conclusions:

Proline isomerization acts as a structural switch, modulating donor-acceptor distances and the balance of intra- vs. intermolecular spin-selective pathways.
Field-cycling photo-CIDNP is effective for identifying transient biradical states in biomolecular models.
Understanding these dynamics is key for designing systems with controlled radical pair behavior.