Room-Temperature Electron Spin Relaxation of Triarylmethyl Radicals at the X- and Q-Bands

Abstract

Triarylmethyl radicals (trityls, TAMs) represent a relatively new class of spin labels. The long relaxation of trityls at room temperature in liquid solutions makes them a promising alternative for traditional nitroxides. In this work we have synthesized a series of TAMs including perdeuterated Finland trityl (D36 form) , mono-, di-, and tri-ester derivatives of Finland-D36 trityl, deuterated form of OX63, dodeca-n-butyl homologue of Finland trityl, and triamide derivatives of Finland trityl with primary and secondary amines attached. We have studied room-temperature relaxation properties of these TAMs in liquids using pulsed Electron Paramagnetic Resonance (EPR) at two microwave frequency bands. We have found the clear dependence of phase memory time (Tm~T2) on magnetic field: room-temperature Tm values are ~1.5-2.5 times smaller at Q-band (34 GHz, 1.2 T) compared to X-band (9 GHz, 0.3 T). This trend is ascribed to the contribution from g-anisotropy that is negligible at lower magnetic fields but comes into play at Q-band. In agreement with this, while T1~Tm at X-band, we observe T1>Tm at Q-band due to increased contributions from incomplete motional averaging of g-anisotropy. In addition, the viscosity dependence shows that (1/Tm-1/T1) is proportional to the tumbling correlation time of trityls. Based on the analysis of previous data and results of the present work, we conclude that in general situation where spin label is at least partly mobile, X-band is most suitable for application of trityls for room-temperature pulsed EPR distance measurements.

Triarylmethyl Radicals: An EPR Study of 13C Hyperfine Coupling Constants

“…The synthesis of FD , FH , FDAM1 , FDAM2 , FDAM3 , FDME3 , OX63D , DBT , FBA3 , FP3 was described in detail previously [11]. The synthesis of TAM-labeled DNA duplex and the procedure of its immobilization in trehalose matrix were also described [28, 39].…”

“…In particular, TAMs are widely used as in vivo spin probes for EPR oxymetry [5–9]. Compared to nitroxides, TAMs have several important advantages, namely: (i) extremely narrow EPR line, especially for deuterated analogues (5 μT); (ii) very long electron spin relaxation times even at room temperatures: T 1 up to 20 μs and T m up to 3 μs [10, 11]; (iii) very high stability against reduction in biological systems (tissues and blood). These properties make TAMs very perspective materials with numerous applications in biology [12–18], medicine [19–21], analytical chemistry [8, 22, 23], materials science[24], DNP applications [25, 26].…”

“…Recent studies of room-temperature relaxation in TAMs revealed that T m values are systematically longer at X-band compared to Q-band due to the g-anisotropy; therefore room-temperature pulsed EPR distance measurements using TAMs are more practical at X-band [11]. As was already mentioned above, the linewidth of TAM at X-band is too narrow for application of PELDOR, because it is impossible to accommodate both observer and pump frequencies within this line and provide small enough overlap of the corresponding excitation profiles.…”

Triarylmethyl Radical: EPR Signal to Noise at Frequencies between 250 MHz and 1.5 GHz and Dependence of Relaxation on Radical and Salt Concentration and on Frequency

“…Slower relaxation rates increase the sensitivity to oxygen concentration [3, 10]. Most pulsed EPR measurements suffer from a “dead time” after the end of the pulse during which data cannot be acquired because of the need to protect the detection system from high power pulses, resonator ring-down, switching times of devices, and reflections from impedance mismatches in the RF/microwave system.…”

A triarylmethyl spin label for long-range distance measurement at physiological temperatures using T 1 relaxation enhancement

“…Disadvantages of TAM1 compared to a nitroxide include the much larger steric bulk and the instability of the ester linked TAM due to hydrolysis. The half-life of hydrolysis for the ester linkage is ~50 hours and does not severely limit is use [30]. In future studies, other more stable linkages will be explored, and the degree to which the TAM1 side chain destabilizes the protein will be investigated.…”