Studies by electrochemical impedance spectroscopy demonstrate that the wild-type glucose-galactose receptor (GGR) protein does not interact significantly with a Au surface, even through nonspecific interactions. Only the GGR A1C mutant, where alanine is replaced by a cysteine residue at the N-terminus, can form a protein film on Au suitable for creating an impedance biosensor. Impedance measurements of glucose binding at equilibrium allow estimation of the dissociation constant (K-d) as approximately 6.6 (1.5) mu M, about 32 times higher than that for the native protein in solution. This change likely arises from surface immobilization rather than from cysteine introduction. (c) 2006 The Electrochemical Society.