Guidelines for determining charge on amino acid

[reising1]

What guidelines do we need to know for determining charge on an amino acid? I know it depends on the specific amino acid and the solution it is in, but what are some general rules we can use to help eliminate answers if a question ever arises? For instance, what pH values should we use as a rule of thumb for the amine group and the carboxylic group?

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[loltopsy]

A running theme with you. The answer is not binary (yes or no). As pH drops, more and more of the AA becomes protonated and therefore positively charged, and vice versa for high pH.

For a generic AA with just a carboxylic acid and an amine group, pH<3 = +1, 3<pH<9 = neutral, pH>9 = -1. That's a ballpark for when 50% of the total AA is in that charge state.

 

[reising1]

A running theme with you. The answer is not binary (yes or no). As pH drops, more and more of the AA becomes protonated and therefore positively charged, and vice versa for high pH.

For a generic AA with just a carboxylic acid and an amine group, pH<3 = +1, 3<pH<9 = neutral, pH>9 = -1. That's a ballpark for when 50% of the total AA is in that charge state.

Okay, so you think that I should use the general rule that the amino group is ph=9 and the carboxylic ph=3?

 

[mehc012]

Okay, so you think that I should use the general rule that the amino group is ph=6 and the carboxylic ph=3?

That's...those aren't even the numbers he used

 

[reising1]

That's...those aren't even the numbers he used

Sorry, I meant "9" not "6"

 

[mehc012]

Sorry, I meant "9" not "6"

lol, fair enough. Anyway, Wiki's actually pretty good on this topic:
http://en.wikipedia.org/wiki/Amino_acid#Zwitterions