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EmilyU1F984 t1_j9t8zcs wrote

It’s relevant but not the whole explanation, also all organic molecules get their colour from those conjugated double bonds, whether they are blue yellow or red doesn‘t matter.

It‘s just that yellow requires the lowest amount of bonds, so any break down products are likely have enough bonds to go from only absorbs UV to also absorbs blue.

In urine the natural healthy colour is made from the molecule that oxygen is transported around in your body.

It’s gets broken down to smaller bits, shorter conjugated electron systems hence yellowish colour.

Additionally: the way our body metabolizes random molecules, like pigments in plant food etc, is by oxidizing them into more water solubles derivatives.

This oxidation is usually pretty efficient at double bonds (which make conjugated systems) And if your body breaks the double bond in the middle of a conjugated system, you usually end up with something that doesn’t have a large enough conjugated system to even absorb blue. So it appears colorless.

Take the imine in your example: break that, and you end up with just a benzene ring, which is colorless.

That‘s why foods and other colorful substances rarely dye the urine.