  By taking into account base-base stacking interactions we improve the
Generalized Model of Polypeptide Chain (GMPC). Based on a one-dimensional
Potts-like model with many-particle interactions, the GMPC describes the
helix-coil transition in both polypeptides and polynucleotides. In the
framework of the GMPC we show that correctly introduced nearest-neighbor
stacking interactions against the background of hydrogen bonding lead to
increased stability (melting temperature) and, unexpectedly, to decreased
cooperativity (maximal correlation length). The increase in stability is
explained as due to an additional stabilizing interaction (stacking) and the
surprising decrease in cooperativity is seen as a result of mixing of
contributions of hydrogen bonding and stacking.
