The conjugation of amyloid beta protein on the gold colloidal nanoparticles’ surfaces

The conjugation of various sequences of amyloid β protein solution (Aβ); Aβ 1−11 , Aβ 12−28 , Aβ 31−35 , Aβ 1−40 , and Aβ 1−42 with gold colloidal suspension of 20 nm size was examined. Absorption spectroscopy was utilized to identify changes in the optical properties of gold colloid for pHs, ranging from pH 2 to pH 10. Colour changes were seen for all tested proteins in this study at a higher pH than where bare gold colloid exhibits its colour change at pH = 3.09 ± 0.02. All tested Aβ sequences except for Aβ 1−42 exhibited colour changes around pI values of Aβ 1−40 , about pH 5.2. The Aβ 1−42 exhibited precipitants in all pH lower than pH 7 and showed the colour change around pH 3.96 ± 0.05. The colour change observed at a pH lower than 5 is attributed to the unfolded Aβ monomer units around the gold colloidal surface. Interestingly, only Aβ 1−40 -coated gold colloidal nanoparticles exhibited a reversible colour change as the pH was externally altered between pH 4 and 10. This reversibility is an important implication of the observation of a reversible step reported for the fibrillogenesis. It was interpreted that the reversible process takes place when hydrophilic Aβ possesses a three-dimensional network containing both β-sheet and α-helices.