Overall and internal dynamics of DNA as monitored by five-atom-tethered spin labels.

Electron paramagnetic resonance (EPR) spectra of the two-atom-tethered six-membered ring thymidylate spin label (DUMTA) incorporated into duplexes of different sizes were found to display a helix length dependence and a local-order parameter S = 0.32 +/- 0.01 for B-DNA based on the dynamic cylinder model (Keyes, R. S., and A. M. Bobst. 1995. Detection of internal and overall dynamics of a two-atom-tethered spin-labeled DNA. Biochemistry. 34:9265-9276). This sensitivity to size, which reflects global tumbling, is now reported for the more flexible five-atom-tethered five-membered ring thymidylate spin label (DUAP) that can be readily incorporated enzymatically and sequence specifically into nucleic acids of different sizes. The DUAPs containing B-DNA systems were simulated with the same dynamic cylinder model, giving S = 0.20 +/- 0.01 for the more flexibly tethered spin label. This shows that S is dependent on tether length but not on global motion. An analysis with the same motional model of the B-Z transition in a (dG-dC)n polymer containing the five-atom-tethered six-membered ring cytidylate spin label (DCAT) (Strobel, O. K., R. S. Keyes, and A. M. Bobst. 1990b. Base dynamics of local Z-DNA conformations as detected by electron paramagnetic resonance with spin-labeled deoxycytidine analogues. Biochemistry. 29:8522-8528) revealed an increase in S from 0.15 +/- 0.01 to 0.26 +/- 0.01 in response to the B- to Z-DNA transition. This indicates that S is not only sensitive to tether length, but also to conformational changes in DNA. Both the DUAP- and the DCAT-labeled systems were also simulated with a base disk model. From the DUAP spectral series, the perpendicular component of the correlation time tau perpendicular describing the spin-labeled base diffusion was found to be sensitive to global tumbling, confirming earlier results obtained with DUMTA. The DCAT polymer results demonstrated that tau perpendicular monitors a conformational change from B- to Z-DNA, indicating that tau perpendicular is also sensitive to local base dynamics. These results confirm that the dynamics of five-atom-tethered nitroxides are coupled to the nucleic acid dynamics and, as with two-atom-tethered spin labels, can be characterized by S and tau perpendicular. The analyses of both spin-labeled systems provide good evidence for spin-labeled base motions within double-stranded DNA occurring on the nanosecond time scale, and establish that both labels can be used to monitor changes in global tumbling and local order parameter due to variations in DNA conformation and protein-DNA interactions.