Are disulfide bridges negative?

[laczlacylaci]

When placed in an electric field, which of the following compounds will migrate toward the cathode at pH=7.
HOOCCHNH2CH2-S-S-CH2CHNH2COOH
So I chose the correct answer based on that positive molecules will migrate toward the cathode (-)
In the answer key, it says this molecule has an overall negative charge.
2COOH=-2
2NH2=+2
Which is 0 charge.
Are the disulfide bridges making this molecule negative?

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[theonlytycrane]

Hmm... can you double-check the molecule? That should be neutral.

 

[laczlacylaci]

Hmm... can you double-check the molecule? That should be neutral.

*I completely forgot that cathodes are negative, thought they were positive.
But the bottom of the answer key: (D) is incorrect because these molecules will be - charged pH=7.
I understand that POE can be used to do this problem, but just wondering...

 

[theonlytycrane]

Yeah the key is incorrect. A and D are neutral at that pH, C is negatively charged, and B is positive. I use pKa = 2 for -COOH and pKa = 9 for :NH3.

It looks like you have this down, but have a quick way to remember that:

pH < pKa : (+)

pH = pKa : neutral

pH > pKa : (-)

Also, think of the gel electrophoresis setup as an electrolytic cell (battery driven), which means that the cathode is (-) and the anode is (+). This is different from a galvanic = voltaic cell (spontaneous cell) where the cathode is (+) and the anode is (-). If this last part was unfamiliar, it's from electrochemistry (g.chem).

 

[BerkReviewTeach]

When talking about migration in an electric field, electrophoresis, you need to consider pI and pH. While it is true that the vast majority of the species in solution will have a zero net charge (as determined in the posts above), there will be some that carry a net negative charge. The pI is around 6 [found from (2.5 + 9.5)/2 = 12/2 = 6)]. The pH is greater than the pI (7 > 6), so the species carries an average charge that is slightly negative. Anions migrate to the anode (the (+)-charged plate of the capacitor). I'm not sure what the exact question is, but that cys-cys dimer should slowly migrate to the anode if the gel is buffered at pH = 7.