Morphology and protein adsorption of aluminum phosphate and aluminum hydroxide and their potential catalytic function in the synthesis of polymeric emulsifiers

Aluminum-containing salts are commonly used as antacids and vaccine adjuvants; however, key features of functional activities remain unclear. Here, we characterized vaccine formulations based on aluminum phosphate and aluminum hydroxide and investigated the respective modes of action linking physicochemical properties and catalytic ability. TEM microscopy indicated that aluminum phosphate gel solutions are amorphous, whereas aluminum hydroxide gel solutions have a crystalline structure consistent with boehmite. At very low BSA concentrations, 100 % adsorption of the protein on aluminum hydroxide could be achieved. As the protein concentration increased, the amount of adsorbed BSA decreased as fewer vacant sites were available on the surface of the adjuvants. Notably, less than 20 % adsorption was observed in aluminum phosphate. The protein adsorption profiles should confront the requirements for vaccine immunoavailability. In terms of catalytic ability, the prepared aluminum salts were tested for their ability to drive the amphiphilic engineering of oligo(lactic acid) (OLA) onto methoxy poly(ethylene glycol). It was concluded that aluminum hydroxide, rather than aluminum phosphate, is suitable to be a vaccine adjuvant according to the morphology and antigen adsorption efficiency results; on the other hand, aluminum phosphate may be a more efficient catalyst for the synthesis of polymeric emulsifiers than aluminum hydroxide. The results provide critical mechanistic insight into aluminum-containing salts in vaccine formulations.