A spin labeling study of calf thymus DNA.

2,2,6,6-Tetramethyl-4-(bis-ethyleneimino) triazinamino-piperidine-1-oxyl was used as the spin label. In ESR spectra of the product obtained (SL-DNA), the strongly immobilized peak greatly prevailed over the weakly immobilized component and a significant difference existed between their microwave saturation characteristics. The spin labeling reaction was investigated in some details. With the aid of heat denaturation (100 degrees C, 15 min) of SL-DNA in conjunction with acid degradation (pH 1.5, 37 degrees C, 23 h), the quantity of labels bound to DNA was determined. Namely, the number of labels bound to 1000 nucleotides was found to be 4.0-12.6. The structural transition temperature of SL-DNA attained from variable temperature measurements was about 63 degrees C and a rise in this temperature was observed with increasing salt concentration.