[ii2]

Hi,

*guess i may have posted on the wrong forum... sorrie folks!!

I've encounter some problems in assigning charges to the amino acids..any advice on how to to go about it?

Am i right to say:
For basic amino acids: pH>pKa, net charge is -1; pH<pKa, net charge is +1; pH=pKa, net charge is -0.5
For acidic amino acids: pH>pKa, net charge is +1; pH<pKa, net charge is -1; pH=pKa, net charge is +0.5

1) So when does an amino acid has a charge of 0? (when pH=pI??)
2) What categories do Cys, Gly and Pro belongs to?

Thanks for any guidance on the topic. ^0^

[renge ishyo]

My advice is to keep it simple and consider only one functional group location at a time. If the pH is less than the pKa, make sure that this group is in the protonated form. If the pH is greater than the pKa, remove the proton so that the group is in the deprotonated form. If the pH=pKa, draw both the protonated form and the deprotonated form as each exists in equal concentrations at this point.

For example, the side chain carboxyl group on glutamic acid has a pKa of 4.07. This group will have a neutral net charge with the proton attached to the carboxylic oxygen at pH values under 4.07, and it will have a net charge of -1 on the oxygen with the proton removed above pH=4.07. For the side chain for lysine with a pKa of 10.53, the ammonium nitrogen will be protonated and carry a net charge of +1 below 10.53, and it will be neutral with the proton removed above pH=10.53.