Ab initio electronic spectra of peptides

We present ab-initio results on the electronic spectra of a peptide corresponding to the (83-92) fragment of HIV-1 protease, a molecule of biological interest. This study is part of a project aimed at the study of circular dichroism (CD), a useful technique able to discriminate between different secondary structures of biomolecules. Since circular dichroism is due to to the difference in molar extinction between left and right circularly polarized light by chiral media, it is related to absorption itself. Therefore, our study started from the calculation of optical spectra. The investigated system (a 175-atoms peptide) displays interesting features, such as charge transfer excitations, typical of "large" molecules, and resulting from specific properties and spatial localization of wavefunctions. This behaviour is therefore identifiable and understandable only through ab-initio electronic structure calculations, which we do on geometrical structures obtained by classical molecular dynamics simulations.