Abstract
Electron spin relaxation times for perdeuterated Finland trityl 99% enriched in 13C at the central carbon (13C1-dFT) were measured in phosphate buffered saline (pH=7.2) (PBS) solution at X-band. The anisotropic 13C1 hyperfine (Ax=Ay=18±2, Az=162±1MHz) and g values (2.0033, 2.0032, 2.00275) in a 9:1 trehalose:sucrose glass at 293K and in 1:1 PBS:glycerol at 160K were determined by simulation of spectra at X-band and Q-band. In PBS at room temperature the tumbling correlation time, τR, is 0.29±0.02ns. The linewidths are broadened by incomplete motional averaging of the hyperfine anisotropy and T2 is 0.13±0.02µs, which is shorter than the T2~3.8µs for natural abundance dFT at low concentration in PBS. T1 for 13C1-dFT in deoxygenated PBS is 5.9±0.5µs, which is shorter than for natural abundance dFT in PBS (16µs) but much longer than in air-saturated solution (0.48±0.04µs). The tumbling dependence of T1 in PBS, 3:1 PBS:glycerol (τR=0.80±0.05ns, T1=9.7±0.7µs) and 1:1 PBS:glycerol (τR=3.4±0.3ns, T1=12.0±1.0µs) was modeled with contributions to the relaxation predominantly from modulation of hyperfine anisotropy and a local mode. The 1/T1 rate for the 1% 12C1-dFT in the predominantly 13C labeled sample is about a factor of 6 more strongly concentration dependent than for natural abundance 12C1-trityl, which reflects the importance of Heisenberg exchange with molecules with different resonance frequencies and faster relaxation rates. In glassy matrices at 160K, T1 and Tm for 13C1-dFT are in good agreement with previously reported values for 12C1-dFT consistent with the expectation that modulation of nuclear hyperfine does not contribute to electron spin relaxation in a rigid lattice.
Accepted Version
Published Version
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