Table 2. Identification of major reactive Gln residues in OPN by MALDI-MS and MS/MS.

Reactive Gln	Peptide	Measured	Expected	ppm
34	YPDAVATWLNPDPSQK	2113.02	2113.04	11
42	QNLLAPQNAVSSEETNDFK	2416.19	2416.17	−4
193	AIPVAQDLNAPSDWDSR	2166.05	2166.06	7
248	KANDESNEHSDVIDSQELSK	2556.19	2556.18	−1

Monoisotopic molecular masses (MH⁺) were measured by MALDI-MS. The expected masses (MH⁺) include the 5-(Biotinamido)pentylamine modification and were calculated using the GPMAW software. The ppm differences between the measured and expected masses are listed. 5-(Biotinamido)pentylamine labeling, peptide separation and MS analyses were repeated four times.