Show the predominant form of the amino acid tyrosine at pH=13.9 and ph=7.8. Be sure to include the proper number of hydrogens on heteroatoms and the correct formal charges on atoms.
I calculated the pka values for COOH (pka=2.41) NH3 (pka=8.67), and OH (pka=11.01) but i'm not sure how to draw the structure for tyrosine and what the functional groups for each ph should look like.
It may be an easier way tho do this problem, but the way that I was taught is that you are going to have to use Henderson Hasselbalch's equation to determine the ratio of uncharged to charge for each one of the ionizable groups for tyrosine at the indicated pH's to determine if they are charged or not, and then draw the amino acid with either the groups deprotonated or not.
the R group= Ph-OH or pH-O^-
CO2H= CO2H or CO2^-
NH3= NH3 or NH4^+
After you determine the charges, you can google an image of the amino acid at the indicated states.
To determine the predominant form of tyrosine at different pH values, we need to refer to the pKa values of the relevant functional groups involved. Tyrosine contains three functional groups: a carboxylic acid group (COOH), an amino group (NH2), and a phenolic hydroxyl group (OH) attached to an aromatic ring.
Let's start with the structure of tyrosine and then examine how the different pH values affect its functional groups.
The structure of tyrosine is as follows:
HO
|
R
|
NH2
|
Cα
/ \
HC - N
| \
H3C HO - C - OH
|
HO - C - OH
|
OH
Now, let's analyze the impact of pH on tyrosine at pH 13.9 and pH 7.8:
1. pH 13.9:
At a high pH of 13.9, the environment is strongly basic. In this case, the amino group (NH2) will be deprotonated to form NH-. The carboxylic acid group (COOH) and the phenolic hydroxyl group (OH) will also be deprotonated. Therefore, the amino group will lose a hydrogen, and the carboxylic acid group and the phenolic hydroxyl group will both lose a proton. This results in the following structure:
HO
|
R
|
N-
|
Cα
/ \
HC O-
| /
H3C OH
|
HO - C - O-
|
O-
In this structure, both the carboxylic acid group and the phenolic hydroxyl group have negative charges.
2. pH 7.8:
At a pH of 7.8, the environment is slightly basic, but not strongly basic like at pH 13.9. In this case, the amino group will be protonated (NH3+), while the carboxylic acid group and the phenolic hydroxyl group will be deprotonated. This results in the following structure:
HO
|
R
|
NH3+
|
Cα
/ \
HC O-
| /
H3C OH
|
HO - C - O-
|
O-
In this structure, the amino group carries a positive charge, while both the carboxylic acid group and the phenolic hydroxyl group are negatively charged.
By considering the pKa values and the pH of the environment, we can predict the protonation and deprotonation states of the functional groups in tyrosine, thus determining its predominant form at different pH levels.