Spin-label EPR T1 values using saturation recovery from 2 to 35 GHz

EPR saturation-recovery (SR) measurements of the electron spin-lattice relaxation time, T-1, of nitroxide-radical spin probes have been made from 2 to 35 GHz. T-1 values of small water-soluble spin probes increase linearly with microwave frequency throughout the full range of available frequencies. T-1 values of four commonly used hydrophobic probes in lipid bilayers also increase with frequency, but the dependence is weaker and complex. Contributions of dissolved molecular oxygen to relaxation rates were independent of microwave frequency. T-1 values of N-15-containing labels are always somewhat longer than for N-14 labels. Details of the Q-band SR spectrometer, which is based on frequency translation technology, are provided. A new way to suppress free induction decay signals in SR experiments has been found: pump and observing frequencies time-locked and separated by about 1 kHz in frequency. A novel three-loop-two-gap resonator with a sample volume of 30 nl was used for the Q-band measurements. It is concluded that Q-band is a favorable frequency for SR spin-label oximetry studies.