10.1021/bi802320q.s001 Tim Schulte Tim Schulte Frank P. Sharples Frank P. Sharples Roger G. Hiller Roger G. Hiller Eckhard Hofmann Eckhard Hofmann X-ray Structure of the High-Salt Form of the Peridinin-Chlorophyll <i>a</i>-Protein from the Dinoflagellate <i>Amphidinium carterae</i>: Modulation of the Spectral Properties of Pigments by the Protein Environment American Chemical Society 2009 pigment arrangement pigment composition chlorophyll phytol chains trimeric MFPCP lipid molecules monomeric HSPCP macrocycle geometry distortion Spectral Properties carotenoid molecules peridinin Qy bands form PCP 2.1 Å resolution binding site asymmetry Dinoflagellate Amphidinium carterae complex glutamate 202 sequence variations photosynthetic dinoflagellate Amphidinium carterae 2009-06-02 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/X_ray_Structure_of_the_High_Salt_Form_of_the_Peridinin_Chlorophyll_i_a_i_Protein_from_the_Dinoflagellate_i_Amphidinium_carterae_i_Modulation_of_the_Spectral_Properties_of_Pigments_by_the_Protein_Environment/2853277 Light-harvesting complexes have evolved into very different structures but fulfill the same function, efficient harvesting of solar energy. In these complexes, pigments are fine-tuned and properly arranged to gather incoming photons. In the photosynthetic dinoflagellate <i>Amphidinium carterae</i>, two variants of the soluble light-harvesting complex PCP have been found [main form PCP (MFPCP) and high-salt PCP (HSPCP)], which show small variations in their pigment arrangement and tuning mechanisms. This feature makes them ideal models for studying pigment−protein interactions. Here we present the X-ray structure of the monomeric HSPCP determined at 2.1 Å resolution and compare it to the structure of trimeric MFPCP. Despite the high degree of structural similarity (rmsd C<sub>α</sub>−C<sub>α</sub> of 1.89 Å), the sequence variations lead to a changed overall pigment composition which includes the loss of two carotenoid molecules and a dramatic rearrangement of the chlorophyll phytol chains and of internal lipid molecules. On the basis of a detailed structural comparison, we favor a macrocycle geometry distortion of the chlorophylls rather than an electrostatic effect to explain energetic splitting of the chlorophyll <i>a</i> Q<sub>y</sub> bands [Ilagan, R. P. (2006) <i>Biochemistry 45</i>, 14052−14063]. Our analysis supports their assignment of peridinin 611* as the single blue-shifted peridinin in HSPCP but also highlights another electrostatic feature due to glutamate 202 which could add to the observed binding site asymmetry of the 611*/621* peridinin pair.