Fig 11. Release of the 648 Da linker by proteinase K cleavage.

Modified CTD proteins were semi-purified, deglycosylated with TFMS, digested with proteinase K and analysed by LC-MS/MS in positive ion mode using the ion trap MS. A singly charged peak of m/z 649.3 was obtained and MS/MS analysis showed a major loss of 346 Da to produce the m/z 303.1 ion. An additional fragment ion at m/z 199 corresponded to a further loss of 104 Da.