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Looking at familiar bodily fluids for a learning session. At first glance they both look the same. Then I noticed the N=C in biliverdin and HN-C in bilirubin. How can a N=C (imine group) and HN-C (secondary amine) make such a difference in colour? They look like structural isomers of each other to me. Or is there any stereoisomeric differences as well to this colour functional group change?

(Jaundice is directly related to bilirubin, urine is a product of bilirubin, I reckon there must be a real good reason and help aid from the colour change representation.)

e.g.

image 1: bilirubin is a brown pigmented substance while biliverdin is green pigment

enter image description here

image2

enter image description here

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    $\begingroup$ That's actually two hydrogens. The compound on the right is an oxidized form of the compound on the left. $\endgroup$ – Zhe Aug 20 '17 at 3:39
  • $\begingroup$ @Zhe are you referring to first image? I don't see two hydrogens :@@ Bilirubin (brown) oxidised to create urobilinogen, then to urobilin for what we see in urine (yellow). $\endgroup$ – bonCodigo Aug 20 '17 at 3:41
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    $\begingroup$ There are indeed two hydrogen atoms. There's one on the nitrogen and one that's implicitly associated with the missing double bond. $\endgroup$ – Zhe Aug 20 '17 at 12:46
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    $\begingroup$ Actually there is little crucial on H yes or not. The entire pi system is changed in between. $\endgroup$ – Alchimista Aug 20 '17 at 15:03
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    $\begingroup$ Check how the conjugation is altered. $\endgroup$ – AS_1000 Aug 20 '17 at 16:25
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The difference in structure of these two compounds is perhaps the presence of a more conjugated structure in biliverdin compared to bilirubin. These products (degradation products of haeme1), just as most pigments, make use of the conjugated electron systems to absorb visible light, giving rise to strong colors. The long chain conjugated system in bilverdin leads to its strong green color, the reduced biliribin (methenyl bridge) has a less conjugated system with a different absorption consenquently different color.

This is a case of effects of conjugation in pigments than merely a difference in functional groups as I see it.

1Note The degradation of haeme occurs at the end of the red cells’ lifetime, where they are removed from the circulation and their components degraded, thus depends of many factors most importantly regulations carried out in the body, enzymes etc.

References

  • Voet and Voet Biochemistry Section 26-4. Amino Acids as Biosynthetic Precursors.

  • Conjugated system

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  • $\begingroup$ I am terrible at replying, I took a look at your reply then, but usually take a long time to digest it, when not sure. $\endgroup$ – bonCodigo Nov 19 '17 at 9:11

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