Amino acid charge in different ph

[holdmystethoscope]

I have my MCAT this saturday and I am having a brain fart moment. A big one to be honest 🙁

So for Amino Acids, I believe pka of COOH portion is around 2 and pka of -NH2 portion is around 9. For acidic condition the charge of a aliphatic amino acid (ie valine, leucine or even phenylalanine) is - and then when the environment is neutral (ph=7), the charge is zero and when the environment is basic the charge is +.

Now my question is what's the environment pH range for having the aliphatic amino acid as net charge of zero? it seems like it's ph 3-8 No?

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[betterfuture]

For amino acids with neutral side chains, at a low pH (1) the amino acid would have an overall + charge because the alpha amine group is NH3+ and the carboxy group is COOH (pKa=2), giving an overall charge of +1.

At physiological pH (7.4), the carboxy group has deprotonated and carries a - charge while the amine has not yet reached its pKa value so it is still in its protonated form, giving off a + charge. The - and + give an overall charge of 0 (neutral). This is called a Zwitterion amino acid.

At high pH(10), both the carboxy and amine have deprotonated and the amine loses its positive charge after losing a proton in basic solution giving it a neutral charge while the carboxy group is in its deprotonated form with charge of -1. Amino acid here has overall charge of -1.

The general pattern is amino acids with neutral side chains have a pI at around 5.96, with an acidic amino acid having a pI below 5.96 and a basic amino acid above 5.96. Generally, that is.

 

[holdmystethoscope]

For amino acids with neutral side chains, at a low pH (1) the amino acid would have an overall + charge because the alpha amine group is NH3+ and the carboxy group is COOH (pKa=2), giving an overall charge of +1.

At physiological pH (7.4), the carboxy group has deprotonated and carries a - charge while the amine has not yet reached its pKa value so it is still in its protonated form, giving off a + charge. The - and + give an overall charge of 0 (neutral). This is called a Zwitterion amino acid.

At high pH(10), both the carboxy and amine have deprotonated and the amine loses its positive charge after losing a proton in basic solution giving it a neutral charge while the carboxy group is in its deprotonated form with charge of -1. Amino acid here has overall charge of -1.

The general pattern is amino acids with neutral side chains have a pI at around 5.96, with an acidic amino acid having a pI below 5.96 and a basic amino acid above 5.96. Generally, that is.

Totally understood. But my question was well for Glycine the PI is 5.96, meaning at ph of 5.96 the net charge is zero. Seems like in ph of 4 the net charge will be zero as well, NO? Since at pH of 4, COOH will be deprotonated (charge -1) and NH3 will stay protonated (charge +1), hence the net charge zero. So what's the significance of the isoelectric point of 5.96 then. I might be thinking wrong.lol.

 

[betterfuture]

In your original post, you had mixed up what charge the amino acids would have at certain pH that is why I wrote an explanation for why amino acids carry a charge as they do. I think you had just mixed it up but atleast you know why now.

You have to remember that every amino acid will behave slighly different due to their different R chains. With that said, their pI are not going to be the same, say for every nonpolar amino acid. To calculate the pI, you must use (pKa1+pKa2)/2.

I guess I do understand what you are trying to ask. But by calculating the pI, it is stating that all of the glycine molecules now exist in their zwitterion form. If you look at a titration graph of glycine, this would correspond to the equivalence point. Equal amounts of moles of base added have reacted with equal amounts of the moles of glycine, meaning no glycine exists in the positive form; all now exist as zwitterions.

 

[aldol16]

Totally understood. But my question was well for Glycine the PI is 5.96, meaning at ph of 5.96 the net charge is zero. Seems like in ph of 4 the net charge will be zero as well, NO? Since at pH of 4, COOH will be deprotonated (charge -1) and NH3 will stay protonated (charge +1), hence the net charge zero. So what's the significance of the isoelectric point of 5.96 then. I might be thinking wrong.lol.

At pH of 4, not all COOH on all molecules will be deprotonated. You can calculate how much exactly is deprotonated using the Henderson-Hasselbalch equation (which will only give you a rough estimate since you are no longer in the buffer region of the first pKa).