Proteins immobilized at an interface are expected to behave differently from their counterparts in bulk solutions, and understanding the interactions of the proteins on the interface surface is crucial to designing a bio-composite device. Extensive studies of gold colloidal nanoparticles have been applied to biomaterial- nanoparticle conjugations. Our particular interest is in conformational changes in the conjugation of various sequences of the Amyloid Beta (A-Beta) protein on the surface of gold colloidal nanoparticles. A-Beta is involved in the reversible process of fibrillogenesis, a key hallmark of Alzheimer's disease. Absorption spectroscopy was utilized to identify changes in the optical properties of the gold nanoparticles coated with various A-Beta sequences (1-11, 12-28, 31-35, 1-40, and 1-42) for a pH range of 2 to 10. We have discovered that the surface net charge of the gold colloid became less negative due to the conjugation of A-Beta protein. Quite interestingly, a reversible color change was found only in A-Beta1-40 conjugated on 20 nm gold colloid between pH 4 and pH 10. This reversible process may involve a key intermediate of fibrillogenesis. Further details of pH dependence and the reversibility of A-Beta will be discussed.