Phycocyanobilin was computationally investigated by means of DFT calculations in combination with implicit solvation starting from X-ray data. Different conformations and degrees of protonation were considered, and the acidity constants were estimated. The computed data suggest a syn-syn-syn conformation for the free molecule, with the two carboxylic groups deprotonated under physiological conditions and weak acidic behaviour of one of the terminal pyrrolone heterocycles. The absorption transitions in the visible range were studied by means of TD-DFT calculations, focusing on the molecular orbitals involved. The frontier orbitals have a dominant role in the lowest energy absorption.

The computational outcomes provided in this communication support the comprehension of the acid-base behaviour of PCB, which influences the absorption features of the molecule. Another factor that affects the absorption maximum in the red range is the conformation assumed by the tetrapyrrolic fragment, that resulted noticeably different on comparing the free molecule and PCB-containing proteins. All these parameters should be considered when PCB is embedded in a matrix and applied as bio-based pigment. Moreover, the use of PCB as luminescent sensor for heavy metal ions can be rationalized on considering the low energy values computed for the triplet excited states, which favour non-radiative decay routes.