Help with isoelectric points?

[Gauss44]

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Why do basic amino acids like Histidine migrate toward the negative electrode, like Mike is explaining at 20:21 in this video http://www.youtube.com/watch?v=iJQAeKluMws ?

I understand his math, but would someone explain this conceptually?

Thanks.

 

[dcamkl1]

Since the pH of the mixture (6) is less than the pI of histidine (7.59), overall the a.a. will be in the protonated form, thus having an overall positive charge that will move towards the negative electrode.

Likewise, if pH>pI, the a.a. will have a negative charge and if pH=pI it is neutral (zwitterion) and will not move in the gel.

Hope that helps?

 

[AureatEagle]

Why do basic amino acids like Histidine migrate toward the negative electrode, like Mike is explaining at 20:21 in this video http://www.youtube.com/watch?v=iJQAeKluMws ?

I understand his math, but would someone explain this conceptually?

Thanks.

The first important thing to understand is that the overall charge on any given amino acid typically varies from positive to negative as a result of the acidic/basic functional groups present in each one. Histidine, for example, could have a charge anywhere from +2 (at very low pH) to -1 (at very high pH).

The isoelectric point of a molecule is defined as the pH at which the molecule has a net charge of zero, making it neutral. As you might expect, the pI of each amino acid is different depending on whether or not the R group is acidic/basic/neutral. Histidine happens to have a pI around 7.6, which means that when it is placed into a solution of pH 7.6, it's overall neutral. This is the "balance point," where His is neither positive nor negative.

However, when placed into a solution with pH < pI, the solution tips the balance by being on the more acidic side of the balance point. As a consequence, the majority of Histidine at this range of pH's will have a net positive charge. Thus, His migrates towards the negative electrode. Conversely, if pH > pI, His would be overall negative, and would therefore migrate to the positive terminal.

 

[Gauss44]

This really helps. Thank you both.