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B. Liu et al. / Inorganic Chemistry Communications 10 (2007) 367–370
Y=1/(a-b)ln[(a-x)/(b-x)]
150
120
90
4
2
0
100
200
Time/h
c
60
b
a
30
480
520
560
600
Wavelength/nm
Fig. 2. Fluorescence changes with time (a ! c) for the mixture of 85 lM
[Cr(3-HNA)(en)2]+ with 850 lM EDTA at 37 ꢁC in 0.01 M Hepes, pH 7.4,
time: a, 10; b, 30; c, 50 h. kex, 280 nm. Inset: A plot of ln[(a ꢁ x)/(b ꢁ x)]/
(a ꢁ b) vs. time, a = 85 lM, b = 10a, x = a(F0 ꢁ Ft)/(F0 ꢁ Fꢂ), r = 0.995.
from 450 nm to 630 nm. The fluorescence peaks of
3-HNA at 530 nm increased with time gradually. The sec-
ond-order rate constant k is calculated to be (3.9 0.3) ·
10ꢁ3 Mꢁ1 sꢁ1. It indicated that the free 3-HNA is gradually
released from the complex. The results from the two meth-
ods are in good agreement. The control experiment for the
EDTA experiment with the complex alone and in the
absence of EDTA shows that the complex is stable in
Hepes buffer at pH 7.4 at 37 ꢁC. It can be concluded that
Cr(III) was combined by EDTA and the 3-HNA or en
ligands were competitively replaced. This procedure is
briefly illustrated by the following scheme:
Fig. 1. The structure of the title compound, with the atom-numbering
scheme. Displacement ellipsoids are drawn at the 30% probability level for
non-H atoms.
O atoms and two of N atoms occupy the equatorial posi-
tions and the remaining N atoms occupy the axial posi-
tions. There are four kinds of intermolecular hydrogen
bonds formed which stabilize the conformation with the
˚
distances in the ranges of 2.763 A for O–H ꢀ ꢀ ꢀ O,
˚
˚
3.148 A for O–H ꢀ ꢀ ꢀ Cl, 2.905–3.370 A for N–H ꢀ ꢀ ꢀ O
˚
and 3.282–3.659 A for N–H ꢀ ꢀ ꢀ Cl, respectively.
The characteristic absorption peak of 3-HNA occurs at
352 nm, while in the complex this peak exhibits consider-
able red shift to 372 nm. In addition, 3-HNA displays rela-
tively strong fluorescence intensity with a maximum
emission peak near 530 nm. When coordinated with Cr(III),
its fluorescence is quenched. In other words, the title com-
plex has hardly any fluorescence in the same condition.
Chromium(III) because of its d3 electronic configuration
forms stable and substitutional inert metal complexes,
which is usually required many days for the kinetics studies
[15]. In order to research the interaction of the complex
with transferrin, EDTA was employed as a simple compet-
itive ligand first. The interaction of 85 lM [Cr(3-
HNA)(en)2]+ with 850 lM EDTA in 0.01 M, pH 7.4 Hepes
buffer at 37 ꢁC was monitored by UV–Visible (UV–Vis)
spectra and fluorescence spectra. The changes of UV–Vis
spectra show that the absorption peaks at 372 nm for coor-
dinated 3-HNA decrease and the peaks at 352 nm for free
3-HNA increase gradually. There are three isosbestic
points at 325, 365 and 433 nm. The second-order rate con-
stant k was obtained using standard fitting procedures,
EDTA
½Crð3-HNAÞðenÞ2ꢂþ ! CrðEDTAÞ þ 3-HNA þ 2en
The iron-transport protein transferrin has been pro-
posed to serve as the major chromium transport agent. In
this paper, apoovotransferrin (apoOTf) was utilized in
place of serum transferrin because of its ready availability
in quantity and its cost; the binding properties of apoOTf
are nearly identical to serum transferrin [14]. ApoOTf
can bind Fe3+ and other metal ions tightly [16,17] in the
presence of synergistically bound anion that is usually car-
bonate. The UV–Vis spectra of Cr-OTf reported previously
are similar to that of Cr-saturated transferrin [15]. Further-
more, titration of CrCl3 to apoOTf monitored by fluores-
cence spectra shows the fluorescence intensity of apoOTf
decreased to about 50% [18]. The control experiment shows
that quenching was scarcely observed in the titration of
3-HNA to apoOTf in the same conditions.
To monitor the transfer of Cr(III)from [Cr(3-HNA)-
(en)2]+ to apoOTf, solution of 42 lM [Cr(3-HNA)(en)2]+
with 42 lM apoOTf in 0.01 M Hepes at pH 7.4 was stored
at 37 ꢁC for one week. Both the difference UV–Vis spectra
and the fluorescence spectra were monitored as a function
of time throughout the course. The fluorescence of apoOTf
at 336 nm (characteristic of tryptophan residue) is quenched
k = (3.8 0.1) · 10ꢁ3 Mꢁ1 sꢁ1
.
The fluorescence spectra are shown in Fig. 2. Sample
was excited at 280 nm and the emission was monitored