A base pairing model of duplex formation I: Watson-Crick pairing geometries

We present a base-pairing model of oligonuleotide duplex formation and show in detail its equivalence to the Nearest-Neighbour dimer methods from fits to free energy of duplex formation data for short DNA-DNA and DNA-RNA hybrids containing only Watson Crick pairs. In this approach the connection between rank-deficient polymer and rank-determinant oligonucleotide parameter, sets for DNA duplexes is transparent. The method is generalised to include RNA/DNA hybrids where the rank-deficient model with 11 dimer parameters in fact provides marginally improved predictions relative to the standard method with 16 independent dimer parameters ($\Delta G$ mean errors of 4.5 and 5.4 % respectively).