A. Cozzolino et al., Conformational changes of humic substances induced by some hydroxy-, keto-, and sulfonic acids, SOIL BIOL B, 33(4-5), 2001, pp. 563-571
We studied the effect of organic acids of plant, microbial. or anthropic or
igin on the molecular size distribution of dissolved humic acids (HAs). Hig
h Performance Size Exclusion Chromatography (HPSEC) was used to evaluate si
ze changes in four different HAs upon addition of hydroxy- (glycolic and ma
lic), 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 aci
d addition before analysis. This effect, combined with an increase of peaks
elution volumes in most cases, was interpreted as a disruption of supramol
ecular humic associations into smaller-size but energy-richer conformations
brought about by the formation of mixed intermolecular hydrogen bonding up
on acid treatment. The extent of size variation was related to the pKa of a
cids and also to the chemical and stereochemical affinity of humic componen
ts with the chemical structure of the acids. Dicarboxylic malic acid was th
e most effective in modifying humic conformations in all HAs whereas the ar
omatic-rich superstructure of HA from an oxidized coal was effectively disr
upted by the relatively small methanesulfonic acid and the chemically akin
benzenesulfonic acid. These results suggest that the conformational associa
tion 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. (C) 2001 Elsevier Science Ltd. All rights reserv
ed.