Helical ordering of hydrogen bonds between pairs of DNA bases

The interaction between hydrogen bonds and conformational elastic degrees o f freedom has been investigated using the simplest model of a double-strand DNA molecule. The hydrogen bonds are described in terms of two-level quant um systems. After excluding conformational degrees of freedom, one has effe ctive interaction among two-level systems. In the ground state of an ideal double helix, hydrogen bonds in a DNA molecule also have a helical order in duced by conformational degrees of freedom. The pitch of the hydrogen-bond helix (and even its sign under certain conditions) is different from that o f the basic helix pitch and, generally speaking, is incommensurate with the latter. This effect can, possibly, lead to an inversion of the sign of the circular dichroism in spectral bands, which was detected in some experimen ts. (C) 1999 American Institute of Physics. [S1063-7761(99)01103-8].