Carbon 13 spin-lattice relaxation, linewidth, and nuclear Overhauser enhancement measurements of nucleosome length DNA.

Carbon 13 NMR data were obtained at four magnetic fields for double-stranded DNA samples 120 and 160 nucleotide pairs long. Spectral linewidths are several-fold smaller than predicted for overall rigid rod rotation and increase with increasing field, indicating significant chemical shift dispersion contributions to high field linewidths. Spin-lattice relaxation times are short (e.g. T1 values for CH carbon atoms are approximately 0.5 s at 67.9 MHz), and increase with increasing field. Nuclear Overhauser effects of 0.6 to 0.8 (theoretical maximum = 2.0) were measured for the same protonated carbon atoms at 100.6 MHz. These short T1 values nd relatively large nuclear Overhauser effects show that double-stranded DNA undergoes rapid internal motions with effective correlation times of a few nanoseconds. Preliminary data indicate that DNA base carbon motions occur on essentially the same time scale as sugar motions, thus arguing against independent flexibility of the DNA backbone. Linewidths decreased 2- to 5-fold and nuclear Overhauser effects doubled, upon heat denaturation of DNA, as expected for increased motion. On the other hand, NT1 values of native and denatured DNA were nearly identical, suggesting insensitivity of spin-lattice relaxation times to motions in the nanosecond range in these relatively stiff chains.