Several E.coli proteins are capable of binding immobilized metal affinity matrices with moderate to high affinity. The most common contaminants include SlyD (28 KDa), GlmS (67 KDa), ArnA (74 KDa) and carbonic anhydrase (25 KDa). More information can be found in the following reference: Bolanos-Garcia et al. (2006) BBA 1760, 1304-1313. The named contaminants may be eliminated from the Ni-NTA elution fraction by employing NiCo21(DE3) Competent E. coli (NEB# C2529H) as the expression host and then performing a second binding step with chitin magnetic beads (NEB# E8036).