Light-Induced Triplet-Triplet Electron Resonance Spectroscopy.

Arnau Bertran,Marina Gobbo,Marilena Di Valentin,Kevin B Henbest,Marta De Zotti,Christiane R Timmel,Alice M Bowen

doi:10.1021/acs.jpclett.0c02884

Abstract

We present a new technique, light-induced triplet–triplet electron resonance spectroscopy (LITTER), which measures the dipolar interaction between two photoexcited triplet states, enabling both the distance and angular distributions between the two triplet moieties to be determined on a nanometer scale. This is demonstrated for a model bis-porphyrin peptide that renders dipolar traces with strong orientation selection effects. Using simulations and density functional theory calculations, we extract distance distributions and relative orientations of the porphyrin moieties, allowing the dominant conformation of the peptide in a frozen solution to be identified. LITTER removes the requirement of current light-induced electron spin resonance pulse dipolar spectroscopy techniques to have a permanent paramagnetic moiety, becoming more suitable for in-cell applications and facilitating access to distance determination in unmodified macromolecular systems containing photoexcitable moieties. LITTER also has the potential to enable direct comparison with Förster resonance energy transfer and combination with microscopy inside cells.

Highlights

We present a new technique, light-induced triplet−triplet electron resonance spectroscopy (LITTER), which measures the dipolar interaction between two photoexcited triplet states, enabling both the distance and angular distributions between the two triplet moieties to be determined on a nanometer scale
By measuring the electron−electron dipolar interaction between two paramagnetic species, Electron spin resonance (ESR) pulse dipolar spectroscopy (PDS) techniques allow for the determination of the distance distributions and in some cases the relative orientations of the paramagnetic centers, giving direct insight into conformational dynamics
This technique was successfully applied to a synthetic model peptide ruler with porphyrin−nitroxide distances ranging from 1.8 to 8.1 nm, rendering spin−spin distance distributions in good agreement with the values predicted by density functional theory (DFT) calculations

Summary

■ METHODS

The experimental and computational methods are described in the Supporting Information. Authors Arnau Bertran − Centre for Advanced Electron Spin Resonance and Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom Kevin B. Henbest − Centre for Advanced Electron Spin Resonance and Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom Marta De Zotti − Department of Chemical Sciences, University of Padova, 35131 Padova, Italy; orcid.org/0000-00023302-6499 Marina Gobbo − Department of Chemical Sciences, University of Padova, 35131 Padova, Italy; orcid.org/0000-00026316-0525 Christiane R. Timmel − Centre for Advanced Electron Spin Resonance and Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom; orcid.org/0000-0003-1828-7700.

■ ACKNOWLEDGMENTS

■ REFERENCES