Isoelectric point

[oohpsjin]

1. For an amino acid with a carboxyl group (COOH) side chain ®, the isoelectric point is shifted towards an acidic pH. (compared to glycine with side chain H).

Glutamic acid (R = -CH2CH2CO2-): pI = 3.1

Glycine (R = -H): pI = 6.1

Lysine (R = -CH2CH2CH2CH2NH3+): pI = 10.0

 

However, I was expecting the isoelectric point to be shifted towards a basic pH, since more moles of base would be required to deprotonate the amino acid and bring it to a zwitterion state.

 

Note: I visited http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html for clarification, but I was only confused further: pI = (Pka1+Pka2)/2, however in calculating the pI for lysine, the pKa for the carboxyl group was omitted!?!

 

2. How many amino acids exist? There are 20 naturally occurring amino acids. Does that mean there are more amino acids that are not naturally occuring? please provide examples!

 

3. In amino acid classification of nonpolar, polar, acidic, and basic: tryptophan is considered a nonpolar amino acid, despite its amine in its R group. Shouldn't tryptophan be a 'basic' amino acid, which are those with R-group that contain amino groups?

 

Thanks in advance!

[srindogg@hotmail.com]

The isoelectric point should be shifted towards an acidic pH because:

 

At isoelectric point, it should exist as COO- NH3+ and the R group carboxylic should maintain a neutral charge. The R group carboxylic acid wants to donate the proton, so the isoelectric point should be shifted to more acidic pH to ensure that the R group carboxylic acid stays protonated (neutral).

[Pantaloons]

For question Two:

 

Correct me if I'm wrong, but I believe there can theoretically be any number of amino acids--it just depends whatever R group is bonded to the amino-acid backbone.

 

The important thing to note is that there are only 20 naturally occurring amino acids. That is to say, only 20 kinds of amino acids (20 different R groups) are coded for by the human genome.

 

I would think that non-natural/non-standard amino acids are unimportant for the MCAT; however, I Wikipedia has an entry for non-standard amino acids.

[Avinash]

There is almost a limitless amount of different amino acids, any slight modification to the R group makes a different "amino acid".

[gamma]

avinash, is there a general rule about basicity of amines? that primary > secondary > tertiary? if so, what's the rationale behind it?

[oohpsjin]

thank you everyone!!

[Avinash]

Sorry, it seems like I was mistaken. Primary amines are not more basic than secondary. The reason why the Nitrogen in Tryptophan's side chain is not basic is because the lone pair electrons are required to be in the p orbital to contribute to the aromatic system (Huckel's rule), so it will not readily enter sp3 to form a bond with hydrogen (protonation).