The three-dimensional structure of humic substances and soil ...

~749 ~54
Fig. 10a-e. The enlarged section of a void in an organo-mineral com-
plex (V; geometry and energy optimization: bonds and angles, torsions,
van de Waals forces, hydrogen bridges) is shown in which atrazine is
trapped and fixed via a distinct hydrogen bond and SOM is bound to
the mineral matrix via hydrogen bonding in addition to three covalent
bonds, a Section (Sticks, Atoms, Number) is shown with the displayed
selected distances between atrazine I and II and III are as
follows:
Starting from C1 (i, I) (atom 1, molecule I) the dis-
tances to H (256, III) are 0.7947 nm (HyperChem ® out-
put 7.947477 Angstrom), to H (255, III) 1.0466 nm, to O
(225, III) 0.3737 nm, O (187, III) 0.3431 nm, O (100, III)
0.8717 nm and to the hydrogens of the long aliphatic
chain (see lower part of V in Fig. 9) H (1004, II)
0.6051 nm and H (1011, II) 0.8632 nm. From H (3, I) to
H (950, II) a distance of 0.3074 nm, to H (996, II) of
0.2160 nm and to H (1004, II) of 0.4177 nm was measured
for the lower part of the void. In the upper part, for in-
stance between H (17, I) and H (902, II) 0.3074 nm, H
(749, II) 0.4468 nm, H (745, II) 0.6537 nm, C (400, II)
0.7601 rim, O (577, II) 0.4885 nm were obtained. Despite
the apparent proximity between H (19, I) and H (251, III)
a distance of 1.1614 nm was found and clearly demon-
strated the necessity of exactly calculated coordinate
systems. Moreover, the distance between H (21, I) and
O (606, II) was of interest, because it narrowed with
progressing geometry optimization from >1rim to
0.2221 nm generating a hydrogen bond which immobi-
lized the atrazine molecule in the HA void. The more the
total energy content of the organo-mineral complex V is
/
1
6
4
6~4 "~
71
atoms labelled by numbers; b Section (Sticks, Atoms, Symbols) is
shown with atoms labelled by element symbols; c Section (Rendering;
Disks, Perspective) is shown as colour plot illustrating the space re-
quirements of the pesticide and the available three-dimensional trap-
ping space
lowered, the more shrinking of the hollow space and con-
sequently stronger bonding of the atrazine is observed. In
summary, one can assume that without operator interfer-
ence energy minimization simulates aging processes.
The results of the three-dimensional structure and dis-
tance measurements furthermore allow to evaluate the
space between the trapped molecule and its host, the
organo-mineral complex. In particular the space filling of
trapped biological or anthropogenic substances is sup-
posed to have an essential influence on their presumable
fate in SOM. Clearly air (N2, O2) and water (HzO) and
small organic acids (CH3COOH) can invade the gaps be-
tween the pesticide and organic or inorganic soil surfaces.
Thus, non-metabolic decomposition processes are easily
possible at this stage. Metabolic processes, however, can
be excluded as microorganisms such as bacteria (> 200 to
1000 nm) and fungi (> 10000 nm) have no access into the
described voids and even enzymes are too large to react
with the trapped materials when the front and back side
of the void is closed by recalcitrant SOM or inorganic
substances. The latter is much more likely, as the organic
matter in agricultural soils is only around 3°70. Thus, in
nature the inorganic matrix represented here by the
hypothetic model III should be larger by at least a factor
of 30.