TABLE 4.

Typical neutral loss fragments of β-propiolactone-modified amino acid residues

Residue	Product	ΔM	Neutral loss fragments
		Da
Cysteine	I	106	HS-CO-CH2-CH2OH	C3H6O2S
	II	106	HS-CH2-CH2-COOH	C3H6O2S
	III	178	HOOC-CH2-CH2-S-CH2-CH2-COOH	C6H10O4S
	IV	72	H2CCH-COOH	C3H4O2
	IV	178	HS-CH2-CH2-(CO)-O-CH2-CH2-COOH	C6H10O4S
Aspartate	I	72	H2CCH-COOH	C3H4O2
	II	72	H2CCH-COOH	C3H4O2
	II	144	H2CCH-(CO)-O-CH2-CH2-COOH	C6H8O4
Glutamate	I	72	H2CCH-COOH	C3H4O2
	II	72	H2CCH-COOH	C3H4O2
	II	144	H2CCH-(CO)-O-CH2-CH2-COOH	C6H8O4
Histidine		–a
Lysine		–
Methionine	I	120	H3C-S-CH2-CH2-COOH	C4H8O2S
	II	192	H3C-S-CH2-CH2-(CO)-O-CH2-CH2-COOH	C7H12O4S
Serine	I	90	HO-CH2-CH2-COOH	C3H6O3
Threonine	I	90	HO-CH2-CH2-COOH	C3H6O3
Tyrosine		–

^a No neutral loss fragments were detected.