I had the same problem this morning. I have also chosen 17
first. After trying everything and getting out 0 I had a
second look at the two structures.
The question was NOT to chose the L- or the R-form. In this
case 17 would have been the right answer.
There is something else special with the two chosen
aminoacids:
This are the only ones with more !! than 1 asymmetric C atom.
And if you look at them again you should find out, ALL 4 are
L-amino acids, but 2 off them are still wrong.
JensJL