  Amino acids are essential to all life. However, our understanding of some
aspects of their intrinsic structure, molecular chemistry, and electronic
structure is still limited. In particular the nature of amino acids in their
crystalline form, often essential to biological and medical processes, faces a
lack of knowledge both from experimental and theoretical approaches. An
important experimental technique that has provided a multitude of crucial
insights into the chemistry and electronic structure of materials is X-ray
photoelectron spectroscopy. Whilst the interpretation of spectra of simple bulk
inorganic materials is often routine, interpreting core level spectra of
complex molecular systems is complicated to impossible without the help of
theory. We have previously demonstrated the ability of density functional
theory to calculate binding energies of simple amino acids, using $\Delta$SCF
implemented in a systematic basis set for both gas phase (multiwavelets) and
solid state (plane waves) calculations. In this study, we use the same approach
to successfully predict and rationalise the experimental core level spectra of
phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His) and
gain an in-depth understanding of their chemistry and electronic structure
within the broader context of more than 20 related molecular systems. The
insights gained from this study provide significant information on the nature
of the aromatic amino acids and their conjugated side chains.
