Amidation of methyl-esterified oligogalacturonides: examination of the reaction products using MALDI-TOF MS

Abstract

Amidation of methyl-esterified oligogalacturonides (oligoGalA) was studied to produce partly and fully amidated oligoGalA to be used as substrates and/or inhibitors for the characterization of pectolytic enzymes acting on the homogalacturonan backbone. The reactions were performed with varying concentrations of ammonia or methylamine and monitored in time using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) that allows sensitive monitoring of the reactions. MALDI-TOF MS reveals the degree of amidation (DAm) and extent of hydrolysis of methyl-esters. Using this technique the conditions for each of the reactions was optimized. Amidation was performed best under anhydrous conditions at a concentration of 4 M ammonia or methylamine at ambient temperature. Amidation using methylamine reached almost completeness (DAm 95) without hardly any hydrolysis of methyl-esters while amidation with ammonia reached a DAm of 70 on average. After an initial fast amidation, precipitation of the partly amidated oligoGalA reduced the reaction rate enormously. The use of ammonia in aqueous solutions instead off anhydrous ammonia resulted in 6–10% lower DAm values due to the hydrolysis of methyl-esters. Therefore, anhydrous conditions are preferred during amidation. Furthermore, methylamine is a better reagent for amidation of oligoGalA and pectins then ammonia, but also results in totally different products with other properties.