Table 1.

Amino Acids with Isobaric or Similar Molecular Weights Requiring a Binary Tag System^a

primary tag	MWb	secondary tag	MWb	PDc
Gln	128.06	Ac-Gln	170.08	+42
Glu	129.04
Lys	128.09	Ac-Dab	142.08	+14
Asn	114.04
Asp	115.03	Ac-Asp	157.05	+42
Ile	113.08	Me-Ile	127.08	+14
Leu	113.08	γ-Abu	85.05	−28
Pro	97.05
Val	99.01	Ile	113.08	+14
Thr	101.05

^aDetails of primary and secondary tag systems for these amino acids are given in Table S2.

^bResidual molecular weight.

^cMolecular weight difference from primary tag.