UV-visible absorption and circular dichroism spectra of the subunits of C-phycocyanin II: A quantitative discussion of the chromophore—protein and chromophore—chromophore interactions in the β subunit

Abstract

In analogy with our previous paper on the α subunit, we discuss the influence of non-covalent interactions between the covalently bound chromophores and the polar protein environment in the β subunit of C-phycocyanin from Mastigocladus laminosus. The steady state absorption and circular dichroism spectra were theoretically investigated using quantum-mechanical model calculations. The energetics of the chromophore—protein interaction were systematically investigated using a force field. It is shown that the interaction with the protein, e.g. the formation of a salt bridge, and the presence of different tautomeric forms of amino acids with carboxylate or amino and imino functions are essential for the tuning of the visible absorption band to a region where effective light harvesting in the antenna complex may occur. Compared with the α subunit, the surrounding of the chromophores in the β subunit causes a reduction in the degrees of freedom as concluded from the differences in the free enthalpy values. This reduction is mainly attributed to the tighter folding of the polypeptide backbone around the pigments. Thereby, the high energy absorbing chromophore is anchored firmly and exposed to a limited heterogeneity of the protein environment. In the β subunit the number of interactions is augmented by non-negligible chromophore—chromophore interactions. It is shown that even the small coupled oscillator interaction (approximately 10 cm −1) of the long wavelength π—π* transitions located on the two chromophores appears clearly in the circular dichroism spectrum. The induction of large rotatory strength of opposite sign in the two transitions in the visible spectral region gives rise to a straightforward explanation for the unusual shape of the circular dichroism spectrum compared with the absorption spectrum of the β subunit.