J. Luo et al. / Spectrochimica Acta Part A 90 (2012) 202–207
203
˚
2. Experimental
diffractometer using
273(2) K. The intensity data were collected by the
a
MoK␣ radiation (ꢁ = 0.71073 A) at
scan
w
mode within 1.94◦ < ꢂ < 25.5◦ for h k l (−9 ≤ h ≤ 9, −12 ≤ k ≤ 12,
−21 ≤ l ≤ 16) in the triclinic system. A light red crystal of 2
(0.27 mm × 0.23 mm × 0.21 mm) was also measured. Data were
2.1. Materials and instrumentation
All reagents and solvents were purchased commercially and
used without further purification unless otherwise noted. CT-DNA
and pBR322 DNA were obtained from Sigma Chemicals Co. (USA).
bromide (EB) were obtained from Huamei Chemical Co. (Beijing,
China). The concentration of DNA was determined spectrophoto-
metrically using a molar absorptivity of 6600 M−1 cm−1 (260 nm)
[22].
The melting points of the compounds were determined on a
Beijing XT4-100 microscopic melting point apparatus (the ther-
mometer was not corrected). Carbon, hydrogen, and nitrogen
were analyzed on an Elemental Vario EL analyzer. Infrared spec-
tra (4000–400 cm−1) were determined with KBr disks on a Therrno
Mattson FTIR spectrometer. The UV–Vis spectra were recorded on a
Varian Cary 100 UV–Vis spectrophotometer. The fluorescence spec-
tra were recorded on a Hitachi RF-4500 spectrofluorophotometer.
1H NMR spectra were measured on a Varian VR 300-MHz spectrom-
eter, using TMS as a reference. Mass spectra were performed on a
VG ZAB-HS Fast-atom bombardment (FAB) instrument and elec-
trospray mass spectra (ESI-MS) were recorded on a LQC system
(Finngan MAT, USA) using DMF as mobile phase.
˚
collected at 298(2) K using MoKa radiation (ꢁ = 0.71073 A). The
intensity data were collected by the Omega scan mode within
1.15◦ < h < 25.00◦ for h k l (−18 ≤ h ≤ 21, −13 ≤ k ≤ 11, −17 ≤ l ≤ 17)
in the monoclinic system. The structure was solved by direct
methods. The positions of non-hydrogen atoms were determined
placed in their geometrically calculated positions. The positions
and anisotropic thermal parameters of all non-hydrogen atoms
were refined on F2 by full-matrix least-squares techniques with the
SHELX-97 program package (G.M. Sheldrick, Bruker AXS, Madison,
WI, 2001) [23].
2.5. Spectroscopic studies on DNA interaction
The UV–Vis absorbance at 260 and 280 nm of the CT-DNA solu-
tion in 50 mM NaCl/5 mM Tris–HCl buffer (pH 7.2) give a ratio
of ∼1.9, indicating that the DNA was sufficiently free of protein
[24]. The DNA concentration was determined by measuring the UV
absorption at 260 nm, taking the molar absorption coefficient (ε260
)
of CT-DNA as 6600 M−1 cm−1 [22].
2.2. Preparation of the ligand (L)
2.5.2. Fluorescence spectra
An ethanol solution containing 2,4-dihydroxybenzaldehyde
(1.38 g, 10 mmol) was added dropwise to a suspension of 4-
aminobenzenesulfonic acid (1.73 g, 10 mmol) in water (150 ml).
The mixture was stirred and heated to reflux for 6 h. The yellow
precipitate was collected by filtration and washed with ethanol.
Recrystallistation from 1:1 (v/v) DMF/H2O gave L, which was dried
under vacuum. Yield, 85%. m.p. 209–211 ◦C. Elemental analysis:
Found (calculated) (%) for C13H11NO5S: C, 53.14 (53.24); H, 3.82
(3.75); N, 4.56 (4.78). 1H NMR (DMSO-d6 300 MHz, br, broad; s,
singlet; m, multiplet): ı (ppm) 8.45 (1H, s, CH N), 7.95–6.62 (7H,
m, Ph H). FAB MS: m/z = 294 (M+H). IR: ꢀmax (cm−1): ꢀ(C N):
The complexes at a fixed concentration (10 M) were titrated
with increasing amounts of CT-DNA. Excitation wavelength of
the samples were 338 nm, scan speed = 500 nm/min, slit width
bound to DNA, the intrinsic binding constants Kb of the three
compounds to DNA were obtained by the luminescence titration
method. The concentration of the bound compound was calculated
using the equation [25].
ꢀ
ꢁ
(F − F0)
Cb = Ce
(Fmax − F0)
1631 cm−1, ꢀ(SO3−): 1388 cm−1, 1232 cm−1, ꢀ( OH): 3058 cm−1
.
fluorescence emission intensity at a given DNA concentration, F0 is
the intensity in the absence of DNA, and Fmax is the fluorescence of
the totally bound compound. Binding data were cast into the form
of a Scathchard plot [26] of r/Cf versus r, where r is the binding
ratio Cb/[DNA]t and Cf is the free ligand concentration. All experi-
ments were conducted at 20 ◦C in a buffer containing 5 mM Tris–HCl
(pH = 7.2) and 50 mM NaCl.
Further evidence for complexes 1 and 2, and L binding to DNA
iment. EB is a common fluorescent probe for DNA structures and
has been employed in examinations of the mode and process of
metal complex binding to DNA [27]. A 2 mL solution of 10 M DNA
and 0.8 M EB (at saturation binding levels) [28] were titrated
by 0–6.5 M complexes 1 and 2, and 0–18 M L (ꢁex = 500 nm,
ꢁem = 520.0–650.0 nm).
2.3. Preparations of the complexes
A solution of ligand (0.29 g, 1.0 mmol) and NaOH (0.04 g,
1.0 mmol) in 1:1 (v/v) H2O/CH3CH2OH (8 ml) was stirred at room
temperature for 15 min. Then the mixture was heated to reflux
for 6 h after added a solution of KCl (0.150 g, 2.0 mmol) in water
(1 ml) to it. A red precipitate, the K(I) complex 1, was sepa-
rated from the solution by suction filtration, purified by washing
several times with water and ether, and dried for 24 h under
vacuum, yield 45%. The Ca(II) complex 2 was prepared by the
same method, yield 58%. Elemental Analysis: Found (calculated)
(%) for 1 C13H14NO7SK (%): C, 41.54 (41.60); H, 3.19 (3.16); N, 9.74
(9.69). IR for 1 (cm−1): ꢀ(C N): 1631 cm−1, ꢀ(SO3−): 1329 cm−1
,
1197 cm−1, ꢀ( OH): 3421 cm−1. ES-MS [DMF, m/z]: 1470.45 (M+H);
Elemental analysis. Found (calculated) for 2 C52H56N4O28S4Ca2 (%):
C, 44.67 (44.78); H, 4.22 (4.02); N, 4.01 (4.02). IR for complex
According to the classical Stern–Volmer equation [29].
2 (cm−1): ꢀ(C N): 1631 cm−1, ꢀ(SO3−): 1345 cm−1, 1223 cm−1
ꢀ( OH): 3419 cm−1. ES-MS [DMF, m/z]: 1358.2.
,
F0
= Kq[Q] + 1
F
where F0 is the emission intensity in the absence of quencher, F
is the emission intensity in the presence of quencher, Kq is the
quenching constant, and [Q] is the quencher concentration.
The shape of Stern–Volmer plots can be used to characterize the
quenching as being predominantly dynamic or static. Plots of F0/F
versus [Q] appear to be linear and Kq depends on temperature.
2.4. Crystallography
Table 1 summarizes the crystal data, data collection and
refinement parameters for 1 and 2. A light yellow crystal of 1
(0.26 mm × 0.23 mm × 0.23 mm) was selected for the X-diffraction
analysis. Data collection was performed on a Rigaku RAXIS-RAPID