QM/MM calculation of absorption spectra of complex systems: The case of human serum albumin

In this contribution we report the strategies to obtain reliable descriptions of excited states, and in particular of absorption spectra, in the case of complex systems composed of different chromophores embedded in macromolecular matrices. In particular the use of hybrid quantum mechanics/molecular mechanics technique and of multiscale simulation is evidenced. We also highlight the necessity to take into account on the same footing the mechanical, electrostatic and polarization effects induced by the environment in tuning the spectral response. In case of complex macromolecules, due to their inherent flexibility the necessity to perform a sounding sampling of the conformational space by using classical molecular dynamics technique is underlined. As a case study we report the total UV/Vis absorption spectrum of human serum albumin, that is basically due to the absorption of eighteen tyrosines and one tryptophane residues.