It Was Not Until 1974 That $\gamma$ -Carboxyglutamic Acid (Shown Below)was Discovered (By Researchers at the University

It was not until 1974 that $\gamma$ -carboxyglutamic acid (shown below) was discovered (by researchers at the University of Colorado) in polypeptides.Based on your knowledge of organic chemistry,what is the most likely reason that this particular amino acid escaped identification so long?

A) It tended to spontaneously form a cyclic anhydride:

B) It tended to spontaneously lose carbon dioxide:

C) It easily underwent a reverse-Knovenagle reaction:

D) It tended to spontaneously form a cyclic amide (lactam) :

E) It was simply too polar to extract into non-polar organic solvents.