The biotin−avidin reaction is well studied and is often used as a prototypical interaction in the development
of immunoassays. In this paper, this reaction is studied on the surface of colloidal silver and gold particles
as a first step in the development of a sol-based assay. More specifically, silver and gold colloidal particles
were biotinylated by self-assembly of a biotin disulfide molecule, and the reaction of the surface-modified
colloidal particles with avidin molecules was followed using optical absorption spectroscopy. The specific
interaction of avidin, a tetrameric protein, with biotin leads to cross-linking of the colloidal particles
(“flocculation”) and a consequent growth of a long wavelength absorption peak. The degree of flocculation
was quantified using a semiempirical flocculation parameter, and the dependence of this parameter on
the extent of biotinylation of the colloidal particle surface as well as the concentration of avidin in solution
was studied to determine the optimum working conditions of the sol. The silver sol required electrostatic
stabilization of the biotin-capped particles through the simultaneous incorporation of a charged bifunctional
molecule, 4-carboxythiophenol, in the capping monolayer while the gold sol was stable with biotin capping.
Both biotinylated silver and gold sols showed a visible color change on addition of avidin. However,
changes in the optical absorption spectra were more marked for the silver sol.
alpha-Diazoketones possess high electric dipole moments, as a consequence of the dipolar nature of the diazocarbonyl functional group. The vectorial analysis, theoretical calculations (PM3 and ab initio), and literature reports based on experimental and theoretical calculations reveal a higher dipole moment for the Z-configuration of the diazo functional group. Microwave irradiation of alpha-diazoketone (1a-m) (Figure 1) promotes Wolff rearrangement specifically via the Z-configuration in excellent yields. The dielectric properties of the solvent govern the course of the microwave rearrangement. 3-Diazocamphor (1m) on microwave irradiation in benzylamine exhibits nonthermal effects to furnish exclusively the Wolff rearrangement product (4m), equivalent to its photochemical behavior. In the presence of an aqueous medium, through solvent heating predominates, leading to the formation of a tricyclic ketone (5) as the principal product, arising from an intramolecular C-H insertion. This behavior is similar to its known thermal and transition metal catalyzed reactivity pattern.
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