13C isotope enrichment of the central trityl carbon decreases fluid solution electron spin relaxation times

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

“…Both nuclear magnetic resonance (NMR) and electron para magnetic resonance (EPR) techniques have been used to measure solvent viscosity in op tically nontransparent biological samples [12][13][14][15]. While NMR directly interrogates wate protons, EPR relies on the application of paramagnetic probes to report solvent viscosity A higher viscosity slows down the tumbling of the probe, increasing its rotational auto correlation time and affecting the spectral linewidth, which is inversely proportional to transverse relaxation time, T2 [16][17][18]. EPR probe relaxation times are up to six orders o magnitude shorter than water proton NMR relaxation times; therefore, EPR measure ments report more specific localized viscosity of the probe microenvironment at the sub micrometer scale, termed further as microviscosity [15,17].…”

“…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 ].…”

Perchlorinated Triarylmethyl Radical 99% Enriched 13C at the Central Carbon as EPR Spin Probe Highly Sensitive to Molecular Tumbling

“…The calculated hyperfine anisotropy A ∥ − A ⊥ = 195.5 MHz (69 G) is higher than the A anisotropy for the 13 C 1 -dFT (A ∥ − A ⊥ = 144 MHz, 52 G). 11 To be used as a spin probe that is sensitive to molecular tumbling a 13 C 1 −PTMTC with higher than 50% 13 C enrichment is desirable for the maximum signal-to-noise ratio. Therefore, we synthesized 99% enriched 13 C 1 −PTMTC in a four-step sequence as depicted in Scheme 1.…”

“…29,30 The experimental τ R = 0.22 ns leads to a C slip = 0.78, which is somewhat larger than the C slip = 0.66 for 13 C 1 -dFT. 11 Below 67.5% of glycerol (26.7 cP), the effect of viscosity on the X-band spectrum is a line broadening (Figure 4). This broadening is asymmetric between the two peaks because the anisotropy of the g tensor leads to a more substantial effect on the low-field peak (Table 2 and Figure 5B).…”

Perchlorinated Triarylmethyl Radical 99% Enriched 13C at the Central Carbon as EPR Spin Probe Highly Sensitive to Molecular Tumbling

“…9 We recently devised a tetrathiatriarylmethyl radical labeled 99% at the central carbon ( 13 C 1 ), namely, 13 C 1 -deuterated Finland trityl or 13 C 1 -dFT (Figure 1), whose EPR spectrum is highly sensitive to molecular tumbling and, therefore, to the media microviscosity. [10][11] This sensitivity arose from the strong anisotropy of the hyperfine coupling to the 13 C 1 (A x =A y =17, A z =162 MHz) that is not entirely averaged out by molecular tumbling, even in low viscosity medium such as water at room temperature. Below a microviscosity of 6 cP, the spectrum of 13 C 1 -dFT is a doublet, and the effect of viscosity is a line broadening (820 mG/cP at X-Band, 9.5 GHz) affecting both lines equally because of the minimal g anisotropy of the probe (g x =2.0033, g y =2.0032, g z =2.00275).…”