posted by Sam .
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.