TABLE 4.

ESI-MS data of His₆-Tpx after reaction with compounds 1–6

Compound	Molecular mass	Observed massesa	Mass increase	Mass of the leaving group
	Da	Da	Da	Da
		17921.5, 18100.2
1	294.7	18181.4, 18359.4	259.9, 259.2	34.8, 35.5
2	248.7	18135.0, 18313.3	213.5, 213.1	35.2, 35.6
3	412.4	18158.2, 18336.5	236.7, 236.3	175.7, 176.1
4	342.0	18229.8, 18408.0	308.3, 307.8	33.7, 34.2
5	263.7	18149.7, 18328.0	228.2, 227.8	35.5, 35.9
6	345.8	18232.5, 18410.5	311.0, 310.3	34.8, 35.5

^a The calculated mass of reduced His₆-Tpx without and with the N-terminal Met is 17,923.2 and 18,054.3 Da, respectively. The analysis revealed two masses, one for the protein lacking the N-terminal Met and another one that corresponded to the full-length protein with an additional mass of 48 ± 1 Da. The latter mass did not appear when tag-free Tpx was used (see Fig. 3) and probably represents an oxidation product. Both peaks showed an identical mass increase when Tpx was reacted with the respective compound. The spectra are depicted in the supplemental Fig. S1.