Precipitation of Protein in the Presence of Melanoidin

Abstract

A melanoidin was prepared by the reaction of glycine-d-glucose system. The fraction of melanoidin between 10,000 and 50,000 in molecular weight was mainly used to clarify its interaction with protein. Remarkable precipitation was observed between the isoelectric points of melanoidin (pH 2.5) and ovalbumin (pH 4.6). An increase of ionic strength restricted those interaction. As the concentration of melanoidin rose against protein in the reaction system, the molar ratio (protein/melanoidin) in precipitate fell towards 1:1. When the melanoidin was in excess beyond a certain level, precipitation hardly happened. The composition in the reaction system to attain the maximum rate of precipitation was affected by molecular weight of melanoidin. Precipitation proceeded according to second order reaction in respect to both ovalbumin and melanoidin. Based on the analysis of Arrhenius plots, precipitation was found to be promoted by the two subsequent steps. A mechanism involved in this reaction was proposed.