Theory for Spin-Lattice Relaxation of Spin Probes on Weakly Deformable DNA

The weakly bending rod (WBR) model of double-stranded DNA (dsDNA) is adapted to analyze the internal dynamics of dsDNA as observed in electron paramagnetic resonance (EPR) measurements of the spin−lattice relaxation rate, R1e, for spin probes rigidly attached to nucleic acid−bases. The WBR theory developed in this work models dsDNA base-pairs as diffusing rigid cylindrical discs connected by bending and twisting springs whose elastic force constants are κ and α, respectively. Angular correlation functions for both rotational displacement and velocity are developed in detail so as to compute values for R1e due to four relaxation mechanisms: the chemical shift anisotropy (CSA), the electron−nuclear dipolar (END), the spin rotation (SR), and the generalized spin diffusion (GSD) relaxation processes. Measured spin−lattice relaxation rates in dsDNA under 50 bp in length are much faster than those calculated for the same DNAs modeled as rigid rods. The simplest way to account for this difference is by allowing ...