UV-visible absorption and circular dichroism spectra of the subunits of C-phycocyanin I: Quantitative assessment of the effect of chromophore—protein interaction in the α-subunit

Abstract

The influence of non-covalent interactions between the A84 chromophore and polar amino acid residues in the α-subunit of C-phycocyanin from Mastigocladus laminosus was investigated using quantum-mechanical model calculations based on the high resolution X-ray structures of chromophore and protein. The theoretical results are discussed in comparison with steady state absorption and circular dichroism spectra. It is shown that these interactions with the protein surrounding are required to bring the longest wavelength transition of the pigment into a wavelength region where the energy transfer in the photosynthetic antenna may take place. The presence of different tautomeric forms of propionic acid side-chains and arginine residues, which are bound with different distributions of partial charges near the central methine bridge, is able to tune the transition in the visible wavelength region between 550 and 620 nm. Since the differences in free enthalpy for these arrangements of the side-chains are of the order of RT at room temperature, an inhomogeneous line broadening of the visible absorption band takes place, which results in a short wavelength shoulder in the absorption and circular dichroism spectra of the α-subunit.