Hello researchers,

there is a phenomenon I have no proper explanation for: I have a pectin extract containing around 65% uronic acids according to the m-hydroxybiphenyl method (Blumenkrantz and Asboe-Hansen, 1973), mainly galacturonic acid. I seperated this extract by anion-exchange chromatography on DEAE Sephacel, eluting it with pure water, followed by 0,1M, 0,25M and 0,5M phosphate buffer. The neutral fraction eluted with pure water had 10% uronic acids, the 0,1M fraction had 83%, the 0,25M fraction 45% and the 0,5M fraction 27%. The 0,1M fraction appears to be quite pure homogalactornan according to NMR data, while the 0,5M seems to be mostly a rhamnogalacturonan-I with arabinogalactan sidechains, and the 0,25M is a mixture of these two.

Can anyone explain how it could be possible, that the uronic acid content obviously decreases with increasing acidity? I thought it could be the degree of esterification that decreases and unveils more acidic carboxyl groups in the more acidic fractions, thats why they have more acidity even tho they have a significantly lower content of uronic acids.

Could there be any other explanation?

Thank you for reading!