Conformational changes of humic substances induced by some hydroxy-, keto-, and sulfonic acids

We studied the effect of organic acids of plant, microbial, or anthropic origin on the molecular size distribution of dissolved humic acids (HAs). High Performance Size Exclusion Chromatography (HPSEC) was used to evaluate size changes in four different HAs upon addition of hydroxy- (glycolic and malic), keto- (glyoxylic), and sulfonic (benzenesulfonic and methanesulfonic) acids. All humic substances showed a decrease of peaks absorbance when the pH of HAs dissolved in HPSEC mobile phase was lowered from 7 to 3.5 by acid addition before analysis. This effect, combined with an increase of peaks elution volumes in most cases, was interpreted as a disruption of supramolecular humic associations into smaller-size but energy-richer conformations brought about by the formation of mixed intermolecular hydrogen bonding upon acid treatment. The extent of size variation was related to the pKa of acids and also to the chemical and stereochemical affinity of humic components with the chemical structure of the acids. Dicarboxylic malic acid was the most effective in modifying humic conformations in all HAs whereas the aromatic-rich superstructure of HA from an oxidized coal was effectively disrupted by the relatively small methanesulfonic acid and the chemically akin benzenesulfonic acid. These results suggest that the conformational association of humus dissolved in the soil solution may be systematically altered by organic acids present in the rhizosphere and might have effects on plant and microbial activities.