A chromophore is part (or moiety) of a molecule responsible for its color.
When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.
In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change of the molecule when hit by light.
Chromophores almost always arise in one of two forms: conjugated pi systems and metal complexes.
In the former, the energy levels that the electrons jump between are extended pi orbitals created by a series of alternating single and double bonds, often in aromatic systems. Common examples include retinal (used in the eye to detect light), various food colorings, fabric dyes (azo compounds), lycopene, β-carotene, and anthocyanins.
The metal complex chromophores arise from the splitting of d-orbitals by binding of a transition metal to ligands. Examples of such chromophores can be seen in chlorophyll (used by plants for photosynthesis), hemoglobin, hemocyanin, and colorful minerals such as malachite and amethyst.
A common motif in biochemistry is chromophores consisting of four pyrrole rings. These come in two types:
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