Swapna "mutates" nucleic acid residues by replacing their base moieties. Every residue with at least one atom in atom-spec is changed to a new_type residue. New_type is a one-letter code representing one of the standard nucleic acid bases: A, T, G, C, or U (case is unimportant). The geometries of the new residues may not be optimal.
The preserve option (default true) retains the existing torsion angle around the base-sugar (glycosidic) bond and the position of the base nitrogen involved in that bond.
The temperature factor for the new residue is set to the highest currently found in the model.
See also: swapaa, Adjust Torsions, Build Structure