OBSERVATION OF CYTOCHROME B-562 ADSORPTION ON GOLD-PARTICLE SURFACE BY OPTICAL-ABSORPTION MEASUREMENT

The plasmon absorption peak of 31-nm gold particles in aqueous solutio n was redshifted and broadened by the adsorption of protein. This effe ct is more striking in the acidic solution than in the alkaline soluti on. The adsorption is irreversible. When a small amount of protein was added to the colloidal solution, this effect was weak, but a small sh oulder emerged at about 680 nm. With increasing quantity of the protei n, the effect of the interaction became very strong. The small shoulde r became larger and shifted to about 720 nm. But further addition of p rotein reduced the effect and finally the absorption spectrum became i dentical with the original colloidal solution. A model is presented wh ich considers two adsorption manners of the protein on gold. Cytochrom e b-562 is of cylindrical shape (height is 5.0 nm and diameter is 2.5 nm.) and has the chromophore at the position of about 3.5 nm from the bottom. Consequently, the ''side-on'' adsorption manner of the protein on gold which dominates at low concentrations and the ''tail-on'' ads orption fashion which dominates at high concentrations cause differenc e in the interaction distance between the surface plasmon and chromoph ore. The interaction between gold particles and the protein depends la rgely on the distance between the gold-particle surface and protoheme IX in the protein. The redshift and broadening of absorption peaks occ urred at the side-on adsorption manner. We disclose an initial effort for developing a novel method to determine the configuration of protei n adsorbed on colloidal particles.