PH, pKa, pI, and net charge of amino acids

[momolemur]

okay well basically this is what i know or think i know:

i have the pKa's of the carboxyl, amino, and R group of the amino acid and the pH of the solution it is in. I'm supposed to figure out the net charge for it at different pHs. i taught myself one way to do it (by looking **** up) and i was told by a friend how to do it his way. it was working fine for me until i found an amino acid (cysteine) that the two methods gave me different answers for. so my question is how am i actually supposed to do this?

what i thought was that i am supposed to look at each pK value and see if it is > or < pH of the solution. if pKa<pH that means that the acid gets deprotonized and it becomes negatively charged (or if was positively charged it would lose its charge) and if pKa > pH it would be protonized. and i have to add up all the charges at the end to get the net charge of the amino acid.

what my friend told me was that I have to look at the pI value and see where the amino acid has an overall charge of 0. then I'm supposed to see where the pKa values were and where the pH is. if the pH <pI value I have to see how many pka values are inbetween the pI and the pH and this would be a negative net charge because it's to the left and more acidic. when pH>pI, it's the same thing but to the right and positive

can anyone explain this to me? it worked for arginine and histidine but when i got to cysteine, each approach gave me a different answer

 

[jbsteele]

.The best way to figure out the net charge of an amino acid is to use a Henderson-Hasselbalch equation. This equation takes into account all of the relevant pKa values and the pH of the solution in order to calculate the net charge of the amino acid. It is important to note that some amino acids, such as cysteine, have multiple ionizable groups which can give different results depending on the pH of the solution. In these cases, it is best to use the Henderson-Hasselbalch equation to ensure accurate results.