Why Structured Water Causes Sharp Absorption by DNA at Microwave Frequencies

Aqueous solutions of oligopolymer DNA have been observed by G.S. Edwards, C.C. Davis, M.L. Swicord and J.D. Saffer, Phys. Rev. Lett. 53, 1284 (1984) to show structured absorption of microwave energy in the region of several gigahertz, characteristic of an ordered series of compressional normal mode vibrations propagating on the polymer chain. Although hydrodynamic coupling of such vibrations to the surrounding solvent would preclude the existence of sharp resonances, the molecular nature of the solvent in the near neighborhood of the polymer and- paradoxically- the strong water/polymer interactions provide a means for effectively decoupling the polymer motion from the dissipation of the liquid. Recent measurements of DNA/water relaxation times allow estimating numerical values in a parameterization of the decoupling effect. The resulting predicted frequency dependence explains many of the smaller features of Edwards' experiment as well as the overall anomaly. A simple model gives a surprisingly complete account of the features of the data using only values determined from other experiments.