Comparison of the phosphorus magnetic resonance and circular dichroism properties of calf thymus DNA and chromatin

Dual measurements of the 31P magnetic resonance spectra and the circular dichroism spectra have been made on calf thymus DNA and purified chromatin. The 31P magnetic resonance signals for all samples fell at 1.2 +/- 0.1 ppm relative to 85% orthophosphoric acid. The full width at half-height of the signal of samples in which the molecular weight of the native DNA component was in the 2-9 X 10(6) range was ca. 50 Hz. This bandwidth was reduced dramatically to ca. 20 Hz by reducing the molecular weight to 140 000 (by sonication) or by heat denaturation of the high-molecular-weight DNA. The position of the signal and the bandwidth of the chromatin samples did not differ significantly from that of the DNA samples of comparable molecular weight and state of nativity. The intensities of the chromatin signals, however, were all less than those of signals of DNA in companion runs conducted under comparable experimental conditions. The reduction of the intensity of the magnetic resonance signal paralleled the lowering of the intensity of the positive band above 260 nm in the circular dichroism spectrum of the given sample of chromatin relative to the spectrum of protein-free DNA. In fact, the percent reduction of the magnetic resonance signal of chromatin relative to protein-free DNA was, within experimental error, equal to the percentage of nucleotide residues in the Watson-Crick B secondary structure. Since the latter fraction of residues can be correlated with those in the interbead regions of the superstructure of chromatin, we have concluded that the signal of the nucleotide residues in the beads, or v bodies, has been broadened to the point of extinction by a packing arrangement which maximizes phosphate-protein interactions and structural rigidity.