Table 2.

Sequence-based identity confirmation of the TetR protein

	E	T	P	T	T	D	S	M	P	P	L	L	R
			y11	y10	y9	y8	y7	y6	y5	y4	y3	y2	y1
Measured y fragment masses			1243.59	n.d.	1045.48	944.44	829.43	742.39	595.36	498.32	401.26	288.19	175.11
Calculated y fragment masses*			1243.64	1146.58	1045.54	944.49	829.46	742.43	595.39	498.34	401.29	288.2	175.12

*Sulfoxide taken into account in the calculation of fragment masses. n.d. : not determined. Database interrogations were performed using the ‘Mascot’ software (41); detailed mass spectral analyses and interpretations were performed using the ‘GNU polyxmass’ mass spectrometric software authored by Filippo Rusconi. ‘Mascot’ : software available at http://www.matrixscience.com. ‘ GNU polyxmass’ : software freely available at http://www.polyxmass.org.

MALDI-TOF mass spectrometric analysis of the peptide mixture yielded one molecular species of m/z 1472.74 ([M⁺H⁺]⁺) which was identified as the ETPTTDSMPPLLR tryptic TetR peptide with a putatively oxidized methionyl residue. Subsequent mass spectrometric analysis of the mixture on an ESI-qQTOF (Q-Star; Applied Biosystems) mass spectrometer afforded an ion at m/z 737.34 ([M+2H⁺]²⁺) corresponding to that same peptide. Fragmentation of this ion afforded fragment ions in the y-series (y1 through y9 and y11). Noteworthy, fragments y6 through y9 and y11 showed masses with an increment of 16 amu due to the sulfoxide formation on the methionyl residue.