  We report first principles density functional calculations for hydroquinone
(HQ), indolequinone (IQ) and semiquinone (SQ). These molecules are believed to
be the basic building blocks of the eumelanins, a class of bio-macromolecules
with important biological functions (including photoprotection) and with
potential for certain bioengineering applications. We have used the DeltaSCF
(difference of self consistent fields) method to study the energy gap between
the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO), Delta_HL. We show that Delta_HL is similar in IQ and
SQ but approximately twice as large in HQ. This may have important implications
for our understanding of the observed broad band optical absorption of the
eumelanins. The possibility of using this difference in Delta_HL to molecularly
engineer the electronic properties of eumelanins is discussed. We calculate the
infrared and Raman spectra of the three redox forms from first principles. Each
of the molecules have significantly different infrared and Raman signatures,
and so these spectra could be used in situ to non-destructively identify the
monomeric content of macromolecules. It is hoped that this may be a helpful
analytical tool in determining the structure of eumelanin macromolecules and
hence in helping to determine the structure-property-function relationships
that control the behaviour of the eumelanins.
