$Ab initio$ studies of the electronic structure and optical properties of amino acids and cyclo-dipeptides.

Besides their importance as monomer units of proteins, amino acids can be also used in the synthesis of nanomaterials. Cyclo-dipeptides (CDPs), $i.e.$, the smallest cyclic peptides, made of two amino acids, are interesting both due to their biological and pharmacological activities and as possible building blocks for nanodevices. We report on our $ab initio$ investigations of the electronic and optical properties of some biomolecules of the above-mentioned classes, focusing on the valence electronic structure of three aromatic cyclo-dipeptides, and on the electronic properties and absorption and circular dichroism (CD) spectra of alanine. For cyclo-dipeptides, our calculations have helped interpreting measured photoemission spectra, highlighting the role of the side chain in determining the photochemical properties of the molecules. Moreover, both for alanine and for CDPs we have investigated the effects of choosing different $ab initio$ schemes, such as plane wave $vs.$ Gaussian basis codes, different exchange-correlation functionals, and Independent Particle $vs.$ GW $vs.$ TDDFT methods. For circular dichroism we have used our recent implementation of CD calculations in the Yambo code.