Table I. LC-MS results of modified residues in the activation loop.

Peptide	Sequence	MS2 ID'd Modification*	Theoretical Mass	Mass Centroid	Error (ppm)	Net Mass Change	Peak Retention Time (min)
842–848	LLEGDEK	E847	859.429	859.429	−0.1	57.0215	16.51
842–848	LLEGDEK	E847	887.46	887.459	−0.8	85.0528	21.56
842–856	LLEGDEKEYNADGGK	E847	1693.78	1693.78	−1.1	57.0215	20.94
842–856	LLEGDEKEYNADGGK	E844	1721.81	1721.81	−2.6	85.0528	23.69
849–856	EYNADGGK	E849	909.383	909.381	−1.5	57.0214	10.4
849–856	EYNADGGK	E849	937.414	937.412	−2.2	85.0527	17.58
849–860	EYNADGGKMPIK	M857	1337.63	1337.63	−0.7	15.9949	16.59
849–860	EYNADGGKMPIK	D853, M857	1394.65	1394.65	−0.4	73.0164	16.6
849–860	EYNADGGKMPIK	D853	1378.66	1378.65	−1.5	57.0215	21.06

* Note: +15.9949 Da, methionine oxidation; +57.0215 Da, ester hydrolysis product of carboxyl group footprinting reaction; +73.0164 Da, methionine oxidation and carboxyl group modification, +85.0527 Da, ethyl ester product of carboxyl group footprinting reaction. The unmodified peptide peak usually has similar retention time with the +57.0215 Da modified peptide.