Amino Acids (e.g. Tyrosine) and their charge

[medium yus]

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I recently checked out a document from UCLA a which shows the pKa's of each amino acid. I know that each protein's environment can have a affect on pKa's, or so I was told, but that is besides the point.

My question is in regards to figuring out charge, specifically for tyrosine.

Assumptions:
1. Phenol = 10 pKa
2. Amine = 9 pKa
3. Carboxylic Acid = 4 pKa

The above compound is neutral, but I am assuming this is under strongly acidic conditions. My assumption is that its isoelectric point is 5.64 because the amine is protonated and the carboxylic acid is deprotonated while the phenol is still protonated. (Right? See my thought process below)

Phenol: (a) = 1 / ((10^(10−5.64)) + 1) = .004 % deprotonated (.004 * -1 = -.004)
Carboxylic Acid: (a) = 1 / ((10^(4−5.64)) + 1) = 97.7% deprotonated ( -.977)
Amine: (a) = 1 / ((10^(9−5.64)) +1) = .04 % deprotonated (.96 * +1 = ~+.96)
Total Charge Estimate = -0.021​

As pH increases to physiological pH the amine should lose its hydrogen only slightly (the phenol should lose it very, very slightly as well) while the carboxylic acid remains at a negative charge.

Phenol: (a) = 1 / ((10^(10−7.4)) + 1) = .25 % deprotonated (.25 * -1 = -.25)
Carboxylic Acid: (a) = 1 / ((10^(4−7.4)) + 1) = 99.96% deprotonated (~ -.9996)
Amine: (a) = 1 / ((10^(9−7.4)) +1) = 2.45 % deprotonated (.9755 * +1 = +.9755)
Total Charge Estimate = -0.2741 ​

Once we reach a pH of say 10, which I arbitrarily chose, tyrosine should be considered negative.

Phenol: (a) = 1 / ((10^(10−10)) + 1) = 50% deprotonated (.5 * -1 = -.5)
Carboxylic Acid: (a) = 1 / ((10^(4−10)) + 1) = 99.99% deprotonated (~ -.9999)
Amine: (a) = 1 / ((10^(9−10)) +1) = .90% deprotonated (.1 * +1 = +.10)
Total Charge Estimate = -1.3999
​

So here is my question--assuming everything above is correct. Do you think we should only be expected to know an amino acid's charge at physiological pH and if it is not quite negative or neutral, such as in the case of tyrosine, that we treat it as neutral?

 

[BerkReviewTeach]

You definitely need to know an amino acid's charge at a given pH, but not in the detail you have shown. If you say that tyrosine is about -0.5 at pKa1 (= 2.6), about +.5 at pKa2 (= 9.2), and +1.5 at pKa3 (= 10.1), then you will do just fine on MCAT questions involving charge.