Hi, Im a college student and came across this question recently:
The pH value of Cysteine, Isoleucine, Histidine and Lysine are 5.0, 6.0, 7.6 and 9.7 respectively. Which of the amino acid mentioned exists as anion at pH 6.0?
The answer given states that Lysine (with the pI of 9.7) will exist as anion at pH 6.0. However, my concept is amino acid will only exist as anion when the surrounding pH is greater than pI. So if my concept is correct, I suppose that the correct answer will be Cysteine, as it exist as Zwitterion at pH 5.0 and the surrounding pH 6 is higher than the isoelectric point.
Anyone can point out the flaws in my concept? Thanks in advance.