Localized and extended electronic eigenstates in proteins: the role of hydrogen bonds

We present a numerical study of the electronic structure of an azurin mutant of the bacterium Alcaligenes denitrificans. The electronic structure is described by a tight-binding Hamiltonian, which permits the computation of electronic properties in the presence and in the absence of hydrogen bonds. We have investigated the density of states, the population and the localization properties of eigenfunctions around the HOMO-LUMO gap. In the absence of hydrogen bonds, eigenstates are extended up to a length scale of an isolated secondary structure element of the biopolymer, typically a beta -sheet. The introduction of tight-binding matrix elements for hydrogen bonds induces a considerable spread of valence and conduction band eigenfunctions, and nonsequentially couples up to three beta -sheets. We discuss the implications of this delocalization effect for electron transfer reactions and visualize the contribution of an extended eigenfunction to a possible intramolecular electron pathway.