Complexes of prodigiosin (a naturally occurring pyrrolylpyrromethene) with H+, Cl−, and CO2: A computational study

Prodigiosin (a naturally occurring pyrrolylpyrromethene) is a red pigment which can be isolated from the marine bacterium serratia and has potent immunosuppressive and anticancer activities. It has recently been shown to function as an anion exchanger for Cl− and HCO 3 - across lipid cell membranes. It possesses three N(H) groups arranged in an arc. These groups generate a convergent set of H-bonds (directed toward a common point) on a rigid scaffold which allow prodigiosin to bind strongly to anions like HCO 3 - . It may therefore also provide a means for sensing HCO 3 - or capturing HCO 3 - from solution. e carried out quantum mechanical calculations on the structures, energetics, UV–visible spectra and NMR spectra of prodigiosin and its complexes with H+, Cl−, HCO 3 - , ( HCO 3 - )2 and CO2. Geometry optimizations and energetic calculations have been performed at the 6-311G(d,p) B3LYP level with proper accounting for vibrational contributions to the free energy, corrections for basis set superposition error, and corrections for solvation effects using a polarizable continuum model. Comparison of the energetics for formation of complexes with HCO 3 - shows that the protonated prodigiosin forms the most stable complex. Our calculated geometry for the prodigiosinH+⋯Cl− complex is in good agreement with the experimentally determined structure for a close analog. The calculated stability of the complexes is strongly influenced by the polarity of the solvent. The UV–visible spectrum of neutral prodigiosin calculated at the TD DFT level, with the B3LYP potential, is also in good agreement with experiment. The lowest energy spectral peak is dominated by a HOMO to LUMO transition involving orbitals localized on the ring system of the molecule. Calculated changes in the UV–visible spectra produced by protonation and complex formation are small.