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

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