A ¹³C‐Labeled Triarylmethyl Radical as an EPR Spin Probe Highly Sensitive to Molecular Tumbling

Abstract

A stable triarylmethyl spin probe whose electron paramagnetic resonance (EPR) spectrum is highly sensitive to molecular tumbling is reported. The strong anisotropy of the hyperfine coupling tensor with the central carbon of a 13C1‐labeled triarylmethyl radical enables the measurement of the probe rotational correlation time with applications to measure microviscosity and molecular dynamics.

In Situ Labeling and Distance Measurements of Membrane Proteins in E. coli Using Finland and OX063 Trityl Labels

“…Also, several membrane proteins form homo‐ or heterooligomeric complexes [14] and orthogonal labeling strategies would greatly facilitate their investigations. The carbon‐centered trityl radicals have attained much attention for ESR and also as an orthogonal tag with other spin labels [8, 15–22] . Like the Gd 3+ ‐based labels, they are stable in the reducing cellular environment and their narrow linewidth provides high sensitivity.…”

Biological Applications of Electron Paramagnetic Resonance Viscometry Using a 13C-Labeled Trityl Spin Probe

“…The EPR spectrum exhibits a doublet pattern due to the splitting of the 13 C nucleus (I = 1 2 ) with a hyperfine coupling constant of 23.35 G [21]. A weak single peak corresponding to the 1% 12 C 1 -dFT in the center of the EPR spectrum possesses a very narrow linewidth (about 32 mG measured at low modulation amplitude [21]) and was virtually insensitive to viscosity due to the low anisotropy of the g-factor and the absence of hyperfine splitting. The widths of the lines of the doublet of the 13 C-dFT spectrum were much larger due to the incomplete averaging of the 13 C anisotropic hyperfine coupling even in the case of fast tumbling of the probe in a relatively nonviscous aqueous buffer solution (∆B pp = 550 mG, η = 0.96 cP).…”

“…For viscosity measurements in the presence of oxygen, samples were prepared in Eppendorf tubes and placed inside the loop of the surface coil resonator. Viscosity values of glycerol solutions were calculated from glycerol content using an online tool (http://www.met.reading.ac.uk/~sws04cdw/viscosity_calc.html, accessed May 2021; see also [21]). For calibration viscosity measurements in the absence of oxygen, samples were prepared in glass vials and sealed using rubber septum.…”

“…Halpern et al applied 250 MHz EPR with penetration depths of approximately 7 cm and a specially designed partially deuterated nitroxide probe [ 19 , 20 ] for microviscosity studies in a mouse model of cancer. Recently we synthesized 13 C 1 -substituted trityl radical ( 13 C-dFT, Scheme 1 ) [ 21 ] with viscosity-dependent EPR linewidth broadening of about two orders of magnitude higher than that for the nitroxide radical probes [ 17 , 18 , 20 ]. Another important advantage of the trityl radicals over the nitroxides is their extraordinary stability in living tissues [ 22 ], where the nitroxides are rapidly reduced to EPR silent hydroxylamines [ 23 ].…”

Improved, Odorless Access to Benzo[1,2-d;4,5-d′]- bis[1,3]dithioles and Tert-butyl Arylsulfides via C-S Cross Coupling

“…Another widespread application lies in the synthesis of triarylmethyl radicals [ 4 ] (“TAM-radicals” 4 ), a class of highly stable radicals employed in site-directed spin labeling of biomolecules for electron paramagnetic resonance (EPR)-based structure determination in vitro [ 5 , 6 , 7 ], at room temperature [ 5 , 6 ] and within cells [ 8 , 9 , 10 ]. Furthermore, such triaryl methyl (TAM)-radicals are used as sensors for local oxygen concentrations within EPR-imaging [ 11 , 12 ], as pH-probes [ 13 ], viscosimetry [ 14 ], and as polarizing agents for dynamic nuclear polarization (DNP) [ 15 , 16 ]. Additionally, TAM radicals incorporating benzo[1,2-d;4,5-d′]bis[1,3]dithiol moieties were recently taken into account for the design of magnetic materials and information transfer [ 17 , 18 ].…”