Fractals for hybrid orbitals in protein models

The structure and shape of the polypeptide chain of proteins are determined by the hybridized states of atomic orbitals in the molecular chain. The calculated s ratio in the sp hybrid orbitals is computed from the fractal dimension D of 81 proteins. The tertiary structures of 8 proteins covering 4 structural classes are analyzed. The proteins in the β-structural class are distinguished quantitatively from other classes. They show a higher s ratio in sp hybrids, rather similar to planar sp. Comparison of the proteins with a Gaussian chain is interpreted in terms of steric repulsion. Iron proteins are compared with the SAW model and two classes are distinguished quantitatively: ferric hemeproteins and iron-sulphur proteins. A curious discovery that a protein takes sp orbitals is reported and discussed. An apparent dependence of fractal hybrid orbitals on ionic strength has been observed.