posted by Poonam on .
The amino acid R has how many ionizable protons?
The amino acid arginine (R) has how many ionizable protons?
Looks like one(1) COOH group to me so one proton that ionizes.
I think it would be three, depending on the environment. Basic environments would cause the CO2H group to become ionized because it would be deprotonated and would thus become negatively charged. Conversely, the amine group and the amino acid's R group would become ionized in an acidic environment by accepting protons to become positively charged.
Devron, you may be right; however, the way I interpret the problem is "ionizable proton" to me means the deprotonation of the COOH group. "Ionizable" to me does not mean the acceptance of a proton. Since this is a new term to me I googled "ionizable proton" and "ionizable proton for bases" and I received ONLY discussions that agreed with my thoughts. If you find different please let me know and please give me a source (link). As previously stated, this is a new term to me; therefore, I'm interested in learning all I can about it
The answer is three. Each of the twenty amino acids has a carboxyl terminus (CO2H), an amino-terminus (NH2), and a R-group. At least two of these groups of an amino acid will always be ionizable groups, and the R-group may or may not be depending on the amino acid. I am pretty sure that you would agree that the CO2H group is acidic because of its ability to donate protons. However, most people forget to look at the other two groups of an amino acid. The 20 amino acids can be broken down into four groups; non-polar amino acids, polar amino acids, acidic amino acids, and basic amino acids.The carboxyl-terminus of an amino acid has a pka value because it has the ability to donate its proton, and the amino-terminus also has the ability to donate a proton if it is ionized.Therefore, in biochemistry, the amino-terminus is assigned a pka value because it also can not donate a proton making the number of ionizable groups at least two.The specific amino acid in question in this post is a basic amino acid, and its R-group can also become ionized, so it s R-group is also assigned a pka value. This is why the answer is three and not two or one. But why isn't it four with this logic since their are two amines in Arginine's R-group? I am not sure, but I believe it has to do with resonance and the different structures that it can undergo because of resonance.You wouldn't have been able to see three ionizable groups looking at the image that you provided because the picture doesn't show you the structure of the amino acid in a very acidic environment, neutral environment (native), or basic environment. If you google isolelectric points, pka values, and/or titrations for amino acids, this will all make sense.