Why would mass spec (LCMS and MALDI-TOF) show peptide mass at M+108 after routine heterologous expression?

I have expressed my peptide (2.9 KDa) as a fusion protein to a 22 KDa protein in e coli (bl21 de3), and then cleave it off in vitro after Ni-NTA purification.  I have then purified further, and...

03 April 2016 439 6 View