Table I. Observed amino acid modifications resulting from microwave-assisted acid hydrolysis sample preparation.

MAAH-modified residue	Modification	Mass shift	Equivalent mass residue
		Da
Asn	Deamidation	+0.98402	Asp
Gln	Deamidation	+0.98402	Glu
Arg	Citrullination	+0.98402
Cys	Carboxymethylationa	+58.00548

^a When IAA is used as the alkylating agent, the majority of cysteines are carboxymethylated and the rest are carbamidomethylated. When IAc is used, only carboxymethylation of cysteines occurs and can thus be considered as a fixed modification for de novo sequencing.