Theory for Spin−Lattice Relaxation of Spin Probes on Weakly Deformable DNA
详细信息 查看全文
- 作者：Alyssa L. Smith ; Pavol Cekan ; David P. Rangel ; Snorri Th. Sigurdsson ; Colin Mailer ; Bruce H. Robinson
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：July 31, 2008
- 年：2008
- 卷：112
- 期：30
- 页码：9219-9236
- 全文大小：353K
- 年卷期：v.112,no.30(July 31, 2008)
- ISSN：1520-5207
文摘
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 for internal flexibility in models of DNA. Because of this discrepancy, we derive expressions for the spectral densities due to CSA, END, and SR mechanisms directly from a weakly bending rod model for DNA. Special emphasis in this development is given to the SR mechanism because of the lack of such detail in previous treatments. The theory developed in this paper provides a framework for computing relaxation rates from the WBR model to compare with magnetic resonance relaxation data and to ascertain the twisting and bending force constants that characterize DNA.