NMR studies of the structure and stability of the 1 : 2 actinomycin D-d-pG-C complex in aqueous solution

We describe NMR studies at superconducting fields which characterize aspects of the structure and stability of the 1 : 2 actinomycin-d-pG-C complex in solution as monitored at the Watson-Crick base pairs and backbone phosphate groups. Two guanine N1H resonances (12.17 and 11.66 ppm) are observed in the 360 MHz proton NMR spectra of the complex in water at -4 degrees C. These slowly exchangeable resonances, which demonstrate the presence of two Watson-Crick G + C base pairs in the complex, broaden in a sequential manner with increasing temperature. The terminal and internucleotide phosphates of both d-pG-C molecules are observable in the 145.7 MHz 31P spectra of the 1 : 2 actinomycin-d-pG-C complex at 0 degrees C. The internucleotide phosphate resonance at 1.905 ppm broadens prior to that at 2.385 ppm with increasing temperature, consistent with a sequential breakage of the G + C base pairs in the complex. The lifetime of the complex (4.5 +/- 0.5 X 10(-4) s, 33 degrees C) was deduced from the variation of the d-pG-D internucleotide 31P resonance line width on gradual addition of the antibiotic in solution.