ESR spectra of samples that were incubated and then frozen and analyzed by ESR at 6.3 K. "Cells" (top): BEAS-2B cells were incubated with 400 µM Na2CrO4 for 5 min at 37°C prior to freezing. "His" (middle): 10 mM histidine plus 1.25 mM Cr(III) (as CrCl3) were incubated in 0.5 M HEPES buffer (pH 7.4) at room temperature for 96 h prior to freezing. "GSH" (bottom): 100 mM GSH plus 1.25 mM Cr(III) were incubated in 0.5 M HEPES buffer (pH 7.4) at room temperature for 72 h prior to freezing. Instrument settings were: temperature = 6.3 K, 5 G modulation amplitude, 0.63 mW microwave power, 60 dB receiver gain, 82 msec time constant, 9.633 GHz microwave frequency, scan time = 83 sec; number of scans, 9.