monoacetylated residue monoacetylated L-cysteine 145.019744873046875 145.17999267578125 42.01056671142578125 42.04000091552734375 C natural AcCys C 5 H 7 N 1 O 2 S 1 C 2 H 2 O 1 A protein modification that effectively converts an L-cysteine residue to either N-acetyl-L-cysteine or S-acetyl-L-cysteine. PSI-MOD MOD:00646 modified L-cysteine residue