Anomalous power law decay in solvation dynamics of DNA: a mode coupling theory analysis of ion contribution

Abstract

Several time dependent fluorescence Stokes shift (TDFSS) experiments have reported a slow power law decay in the hydration dynamics of a DNA molecule. Such a power law has neither been observed in computer simulations nor in some other TDFSS experiments. Here we observe that a slow decay may originate from collective ion contribution because in experiments DNA is immersed in a buffer solution, and also from groove bound water and lastly from DNA dynamics itself. In this work we first express the solvation time correlation function in terms of dynamic structure factors of the solution. We use mode coupling theory to calculate analytically the time dependence of collective ionic contribution. A power law decay in seen to originate from an interplay between long-range probe–ion direct correlation function and ion–ion dynamic structure factor. Although the power law decay is reminiscent of Debye–Falkenhagen effect, yet solvation dynamics is dominated by ion atmosphere relaxation times at longer length scales (small wave number) than in electrolyte friction. We further discuss why this power law may not originate from water motions which have been computed by molecular dynamics simulations. Finally, we propose several experiments to check the prediction of the present theoretical work.

Keywords:

• DNA hydration dynamics
• power law decay
• Debye–Falkenhagen effect
• wave number-dependent ion atmosphere relaxation

Acknowledgements

It is a pleasure to dedicate this paper to Prof. Pierre Turq on the occasion his 70th birthday. Pierre has been, for a long time, a valuable and helpful colleague and a trusted friend and great collaborator. We join his innumerable colleague in wishing him a Happy Birthday and a productive, healthy life. I thank Ms Susmita Roy for discussions and for much help in preparing the manuscript. I thank Prof. S. Yashonath, Prof. S. Sen and Prof. S.K. Pal for helpful discussions. Particular thanks to Prof. Yashonath, Dr. Praveen Kumar and Ms Susmita Roy for collaborations. We also thank an anonymous reviewer for constructive criticism and suggestions.

Additional information

Funding

This work was supported in parts from grant by Department of Science and Technology (DST) (India) and a Sir JC Bose Fellowship.