pigmentbinding

Pigment binding refers to the reversible association of a pigment molecule with another molecule, most commonly a protein, to form a pigment-protein complex. Pigments are small, light-absorbing molecules such as chlorophylls, carotenoids, hemes, retinal, flavins, and other porphyrin or polyene structures. Binding is typically non-covalent and occurs in a defined binding site that shapes the pigment’s environment and spectral properties.

In biology, pigment-binding proteins play essential roles in light-based processes. In photosynthesis, plant and algal light-harvesting complexes bind chlorophylls and carotenoids to capture light and transfer excitation energy to reaction centers. In vision, opsin proteins bind retinal and convert light into a neural signal. Carotenoid-binding proteins regulate pigment positioning and protect against photodamage, while bacterial photoreceptors use bound pigments to detect light quality and intensity.

Mechanistically, pigment binding relies on hydrophobic interactions, hydrogen bonds, electrostatics, and π-π interactions between pigment rings and amino acid residues. Binding can induce conformational changes in the protein and often shifts the pigment’s absorption spectrum (bathochromic or hypsochromic shifts), adjust redox state, or influence photostability. Binding also serves to protect pigments from degradation and to facilitate transport or storage.

Common examples include chlorophyll-binding proteins in photosynthetic membranes, phycobiliproteins such as phycoerythrin and phycocyanin in cyanobacteria and red algae, heme proteins like hemoglobins and cytochromes, and retinal-binding opsins in vision. In non-biological contexts, pigment binding is explored in materials science and sensor design to stabilize or modulate pigment properties.

Binding affinity is typically quantified by dissociation constants (Kd) and is studied using spectroscopy, calorimetry, crystallography, and computational methods.