Vibrational dynamics of DNA.  I. Vibrational basis modes and couplings. 
Carrying out density functional theory calculations of four DNA bases, base derivatives, Watson-Crick (WC) base pairs, and multiple-layer base pair stacks, we studied vibrational dynamics of delocalized modes with frequency ranging from 1400 to 1800 cm(-1).  These modes have been found to be highly sensitive to structure fluctuation and base pair conformation of DNA.  By identifying eight fundamental basis modes, it is shown that the normal modes of base pairs and multilayer base pair stacks can be described by linear combinations of these vibrational basis modes.  By using the Hessian matrix reconstruction method, vibrational coupling constants between the basis modes are determined for WC base pairs and multilayer systems and are found to be most strongly affected by the hydrogen bonding interaction between bases.  It is also found that the propeller twist and buckle motions do not strongly affect vibrational couplings and basis mode frequencies.  Numerically simulated IR spectra of guanine-cytosine and adenine-thymine bases pairs as well as of multilayer base pair stacks are presented and described in terms of coupled basis modes.  It turns out that, due to the small interlayer base-base vibrational interactions, the IR absorption spectrum of multilayer base pair system does not strongly depend on the number of base pairs.