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The yellow color of urine is due due a compound known as urochrome, or urobilin. Why is this chemical yellow? There isn't any coordination complexes. What else could cause this color?

I have not learned organic chemistry yet so please make it simple.

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Many organic molecules, including food dyes, are colored because they contain chromophores. A chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Light that hits the molecule can thus excite the electrons in the chromophore, resulting in the emittance of a particular wavelength of light.

$\hspace{1cm}$urobilin

In urobilin (seen above) specifically, the tetrapyrrole moiety, as a result of its conjugated pi-bond system, is the chromophore that gives this compound its characteristic yellow color.

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  • $\begingroup$ What is the tetrapyyrole moiety? $\endgroup$
    – TanMath
    Oct 25, 2015 at 5:35
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    $\begingroup$ "Tetra" meaning four, "pyrrole" meaning an aromatic, 5-memered ring containing 4 carbons and 1 nitrogen, and "moiety" meaning a part of a molecule. These 4 pyrroles constitute the main body of urobilin. $\endgroup$
    – ringo
    Oct 25, 2015 at 5:43
  • $\begingroup$ The color is probably due to the fact that the middle two pyrroles are conjugated, further reducing the HOMO-LUMO gap. Regular pyrrole is a colorless liquid. $\endgroup$
    – Zhe
    Oct 7, 2016 at 21:15

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