Rapp. Comm. int. Mer Médit., 36,2001
154
Introduction
Recently, melanoidins, sugar- amino acid condensation products,
have gained attention due to their chemical similarities to natural
humic substances. Although humic substances are often thought to be
formed from lignin polymers, in marine environments carbohydrates
are more abundant and are thus more likly precursors of humic acids
(1). Although, melanoidins are commonly prepared in laboratories by
prolonged heating (1) , they also may be formed under natural envi-
ronmental conditions in soil as well as in sea water. For example,
melanoidins have been identified in archeological plant remains from
excavations in Egypt and thus must have been produced by reactions
that occured in buried organic matter (2). In the present work we
examined and correlated the copper complexing capacity values of
twelve differently prepared melanoidins and two humic acids (HA).
Experimental
Voltammetric measurements were performed on aµ - Autolab (ECO-
CHEMIE ), connected to a Metrohm VA 663 stand. Complexing capac-
ity was determined as described in detail previously(3, 4, 5). Fulvic
acid (FA) was isolated from Cannet lagoon sediments (France) at the
University of Perpignan (6)(Table1). Aldrich HA is a commercially
available, well characterized humic material (Table1). Melanoidins
were prepared at SUNY (USA) from glucose and amino acids (glu-
tamic acid, lysine or valine) which were added in 1:1 ratio and heated
at 100°C for 2, 4,16 or 32 days.The products of the reactions were sol-
uble in water and were separated into two fractions :1) molecular mass
<10000 Da, and 2) molecular mass >10000 Da. For lysine and va
line,
only a 16-d reaction time with glucose was used. Thus 12 diff
erent
melanoidins were obtained (Table 2). The copper complexing capacity
was determined for 10 mg/L solutions of each melanoidin, FA or HA
in 0.55mol/L NaCl at pH 6 and pH 8.5.
Table1.Theelementalcompositionofhumicacidsand
melanoidins (1)
Results and Discussion
The complexing capacity of the inve
stigated melanoidins, HA and FA
are presented in Table 2. Among the melanoidins higher complex
ing
capacity values for copper ions are obtained for glu-lys, followed by
glu-val and the lowest values are for glu-glut.Lysine contains more
amino groups than glutamic acid or valine (1); basic amino acids ( i.e.,
those containing more amino groups than carboxylic groups) preferen-
tially condense with sugars to form nitrogen-rich polymers (1). T
hese
polymers are good complexing agents for copper ions. Glu-glut 1 and
glu-glut 5 show no complexing capacity towards copper ions rega
rdless
of their large molecular masses, which shows that two days of heating
is not enough to form structures capable of binding copper ions.
The complexing capacity for glu-glut 7 was determined at both pH
6 (shown in Table2) and at pH 8.5. The complexing capacity at pH 8.5
is much higher with: L
T
= 2.65x 10
-7
mol/L and Kapp= 9.8x10
7
L/mol.
This higher value is obtained at pH 8.5 because additional binding
sites for copper ions are available as more groups are dissociated.
There was no difference in complexing capacity between melanoidins
with molecular masses <10000 Da and >10000 Da (Table 2). FA iso-
lated from lagoon sediments has a complexing capacity in the same
range as melanoidins while HA isolated from soil (containing more
lignin type residues) has a complexing capacity four times higher than
the highest one for melanoidins.
Table 2. The composition, molecular mass, time of reaction,
complexing capacity and apparent stability constant deter-
mined at pH=6
Conclusion
Melanoidins complex copper ions, and the similarity in complexing
properties with FA isolated from marine lagoon sediments is obvi-
ous.The extent of complexation depends on basicity of the amino acids
used to make the melanoidin. A reaction time of two days was not
enough to form structures capable of complexing copper
ions.Complexing capacity values differed little between melanoidins
with molecular mass <10000 Da and >10000 Da. As melanoidins have
been found in nature (2), they may form under certain conditions
(e.g.,UV- light) and may represent a link in transformation of labile
organic matter (polysaccharides, amino acids) into more recalcitrant
humic material in seawater.
References
1- Hedges J.I., 1978. The formation and clay mineral reactions of
melanoidins.Geochim. Cosmochim. Acta, 42: 69-76.
2-Evershed R.P., Bland H.A., van Bergen P.
F., Carter J.F., Horton M.C.,
Rowley-Conwy P.A.,1997. Volatile compounds in archeological plant remains
and the Maillard reaction during decay of organic matter. Science, 278: 432-433.
3- Plavsic M., Krznaric D. and Branica M., 1982. Determination of the
apparent copper complexing capacity of seawater by anodic stripping
voltammetry.Mar. Chem., 11: 17-31.
4- Ruzic I.,1982. Theoretical aspects of the direct titration of natural waters and
its information yield for trace metal speciation. Anal. Chim. A
cta, 140: 99-113.
5- van den Berg C.M.G., 1982. Determination of copper complexation with
natural organic ligands in seawater by equilibration with MnO
2
, I Theory.
Mar. Chem., 11: 307-322.
6- Fauget D., 1982 . In?uence des substances humiques sur les formes
dissoutes et particulaires de quelques métaux ( Zn, Fe, Co, Mn.) dans les
milieux marin et lagunaire. PhD Thesis, Université de Perpignan.
THE COPPER COMPLEXING PROPERTIES OF MELANOIDINS AND MARINE HUMIC MATERIAL
Marta Plavsic
1
* , Bozena Cosovic
1
and Cindy Lee
2
1
Rudjer Boskovic Institute, Center for Marine and Environmental Research , Zagreb, Croatia - plavsic@rudjer.irb.hr
2
State University of NewYork, Marine Sci. Research Center, Stony Brook, NY, USA
Abstract
Melanoidins, condensation products of sugar and amino acids may represent a key link in the transformation of polysaccharides to humic
material in marine environment. Melanoidins prepared using a condensation time longer than two days exibit complexation properties
towards copper ions that appear to depend on the basicity of the amino acid precursor. Melanoidins of molecular mass <10000 and >10000
exibit similar complexing properties. The copper complexing properties of humic material isolated from marine lagoon sediments were
determined for a comparison.
Key words: electrochemistry, organic matter, metals
CHNOSAcidityMol. mass
(meq./g)distr. (Da)
FA(Canet)38.75.382.2551.641.975.82>1000–20 000
HA(Aldrich)50.175.430.8841.003.26-1000-5000
glu-glut (7 days)46.03.63.546.9-5.3not determined
glu-val (7 days)53.33.73.839.2-3.4not determined
glu-lys (7 days)54.47.48.030.2-1.3not determined
Melanoidins;Reaction timeMol. massL
T
K
ML
HA; FA(days)(Da)(mol Cu
2+
/L)(L/mol Cu
2+
)
[10 mg/L]
glu-glut 12<10 000not detectednot detected
glu-glut 24<10 0004.83
?
10
-8
1.4
?
10
7
glu-glut 316<10 0004.56
?
10
-8
2.4
?
10
8
glu-glut 432<10 0007.43
?
10
-8
7.2
?
10
7
glu-glut 52>10 000not detectednot detected
glu-glut 64>10 0002.03
?
10
-8
not determi-
ned
glu-glut 716>10 0008.01
?
10
-8
5.8
?
10
7
glu-glut 832>10 0006.86
?
10
-8
not determi-
ned
glu-val 9
16<10 0001.42
?
10
-7
3.3
?
10
7
glu-val 1016>10 0001.98
?
10
-7
4.6
?
10
6
glu-lys 1116<10 0009.92
?
10
-8
7.6
?
10
6
glu-lys 1216>10 0002.14
?
10
-7
1.4
?
10
7
FA
->1000-200008.68
?
10
-8
5.8
?
10
7
