The surface densities of the immobilized proteins, as determined from surface plasmon resonance (SPR) measurements, were 2.9, 1.2, and 0.72 mg/m 2 for the 7-, 6-, and 5-repeat forms, respectively. In the absence of Ni 2þ there was no detectable nonspecific protein adsorption, indicating that all proteins bound selectively by their histidine tags. The molecular weights of the monomers of the length variants are 35 344 Da (7-repeat), 32 718 Da (6-repeat), and 30 119 Da (5-repeat), so that the calculated monomer surface densities were Γ 7 = 4.9 Â 10 16 , Γ 6 = 2.1 Â 10 16 , and Γ 5 = 1.4 Â 10 16 monomers/m 2 .