Investigation of the reaction between a soy-based protein model compound and formaldehyde

To guide the preparation of soy protein-based adhesives, the reaction between protein and formaldehyde as a possible cross-linker was studied in this paper under different pHs. To simplify the investigations, dipeptide N-(2)-l-alanyl-l-glutamine (AG) was used as a model compound for protein. Based on the analysis of the results of ESI-MS and 13C-NMR of the reaction products between AG and formaldehyde prepared under different pHs, it was found that the pH has strong effects on these reactions. Under strong acid conditions, such as pH 1–3, the methylolation reaction mainly occurred at the aliphatic amino groups of AG. However, it was very difficult for the resulted methylolated AG to be further condensed. Under weak acid conditions, such as pH 5, both the methylolation reaction between amido groups and formaldehyde and the further condensation reaction were possible. However, the condensation reaction was still rather weak under these conditions. At alkaline conditions, the methylolation reaction occurred between all of the three types of amino groups of AG and formaldehyde with both methylene bonds and methylene–ether bonds present in the system. Methylene bonds were mainly from the reaction between AG methylols and amido groups and aliphatic amino groups of AG. Ether bridges were formed between two methylolated AG mainly from amido groups.