Hi. Teach me chemistry!

[Russ]

So, isomerization. Tricky stuff. I have a somewhat conceptual question.

This molecule (bilirubin) here has E/Z stereochemistry and is a photopigment


Under exposure to blue light, it specifically forms the E-Z isomer rather than the Z-Z isomer. Why? As in specifically what effect the light has upon the molecule to cause this isomerisation. Doesn't have to be for this specific example, but what effect does light energy of a specific wavelength have to cause bonds to break reform in this predictable way?

Furthermore, in this specific case, the E-Z isomer is more water soluble than the Z-Z isomer. Why is this? With no true structural changes, only orientation ones across the double bond, I'm at a loss as to how this can alter its polarity/solubility? Is it just steric hindrance?

Ty

e, I'm interested generally rather than specifically, since I probably don't need to know this for class but I'm curious and chemistry 101 was a loooong time ago

[Mao]

Oh man. This is a difficult question.

Firstly, the specific answer to to why E,Z is more soluble to Z,Z.
http://en.wikipedia.org/wiki/File:Bilirubin-from-xtal-1978-3D-balls.png <-- This picture is the Z,Z structure. If we were to rotate one of the ends, then we completely change the molecular structure. This increases solubility, some may say it is because more polar groups are exposed, the mechanism of dissolution is still under debate at the moment.

The more general effect you are talking about is 'photoisomerization'. I don't know how or why it happens. I remember going to a guest lecture a few weeks ago, where the guy was talking about excited state quantum calculations or something, and presented his results which he claims to predict photoisomerization. Essentially, in an atom, electrons absorb light, jump up energy levels, then release light and jump down energy levels. In more complex systems, electrons absorb light, but the system can relaxes via some non-radiative mechanism, which involves some structural changes, so the energy absorbed from the light is expended in the cis-trans conversion process. Bonds aren't broken in the process, the electron density simply shifts around a bit.


As for how to predict if this will always happen, what wavelength of light is needed, what isomer is more favourable, and so on, I don't think there is an easy theoretical answer, or worse, there might not be an answer at all.

[thushan]

Oh man. This is a difficult question.

Firstly, the specific answer to to why E,Z is more soluble to Z,Z.
http://en.wikipedia.org/wiki/File:Bilirubin-from-xtal-1978-3D-balls.png <-- This picture is the Z,Z structure. If we were to rotate one of the ends, then we completely change the molecular structure. This increases solubility, some may say it is because more polar groups are exposed, the mechanism of dissolution is still under debate at the moment.

The more general effect you are talking about is 'photoisomerization'. I don't know how or why it happens. I remember going to a guest lecture a few weeks ago, where the guy was talking about excited state quantum calculations or something, and presented his results which he claims to predict photoisomerization. Essentially, in an atom, electrons absorb light, jump up energy levels, then release light and jump down energy levels. In more complex systems, electrons absorb light, but the system can relaxes via some non-radiative mechanism, which involves some structural changes, so the energy absorbed from the light is expended in the cis-trans conversion process. Bonds aren't broken in the process, the electron density simply shifts around a bit.


As for how to predict if this will always happen, what wavelength of light is needed, what isomer is more favourable, and so on, I don't think there is an easy theoretical answer, or worse, there might not be an answer at all.

If Mao doesn't know it, then it hasn't been discovered yet.

[Russ]

Thanks for the outline Mao! Shame there's no well elucidated answer but I'll have to live with that

[Mao]

Thanks for the outline Mao! Shame there's no well elucidated answer but I'll have to live with that

Welcome to my life... It's full of things without any concrete answers and involves a whole bunch of very intelligent people doing very hand-wavy things..