The standard molar enthalpy of formation of a new copper(II) Schiff-base complex and its interaction with bovine serum albumin

Article abstract of DOI:10.1016/j.tca.2014.10.025

A new copper(II) Schiff-base complex [Cu(HL)·NO₃·MeOH] was prepared by using equivalent molar of Valen Schiff-base ligand [H₂LN,N′-ethylene-bis(3-methoxysalicylideneimine)] and Cu(NO₃)₂·3H₂O. The stru

Full text of DOI:10.1016/j.tca.2014.10.025

Thermochimica Acta xxx (2014) 7–15
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
The standard molar enthalpy of formation of a new copper(II)
Schiff-base complex and its interaction with bovine serum albumin
b
a
a
a
Jin-Qi Xie a, Chuan-Hua Li a, , Jia-Xin Dong , Wei Qu , Lan Pan , Meng-La Peng ,
*
Ming-An Xie a, Xu Tao a, Cheng-Mao Yu a, Yi Zhu a, Ping-Hua Zhang a,
a,**
Chun-Guang Tang a, Qiang-Guo Li
a
Hunan Provincial Key Laboratory of Xiangnan Rare-Precious Metals Compounds and Applications, Department of Chemistry and Life Science, Xiangnan
University, Chenzhou 423000, Hunan, PR China
School of Chemistry & Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, PR China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
A new copper(II) Schiff-base complex [Cu(HL)
ꢀ
NO3
ꢀ
MeOH] was prepared by using equivalent molar of
3H2O. The
Valen Schiff-base ligand [H2L
structure of the complex was confirmed by single-crystal X-ray diffraction. Based on an ideal and feasible
¼
N,N0-ethylene-bis(3-methoxysalicylideneimine)] and Cu(NO3)2
ꢀ
Received 5 September 2014
Received in revised form 27 October 2014
Accepted 29 October 2014
Available online xxx
thermochemical cycle, the standard molar enthalpy of formation of the complex was estimated to be:
u
Df Hm [Cu(HL)
ꢀ
NO3
ꢀ
MeOH(s), 298.15 K] = –(945.40 ꢁ 2.44) kJ molꢂ1 by an advanced solution-reaction
isoperibol calorimeter. In particular, the interaction between the complex and bovine serum albumin
(BSA) was investigated using the fluorescence quenching method. Fluorescence quenching data showed
that the quenching mechanism of BSA treated by the complex was static quenching, which was highly
accord with the non-radioactive energy transfer theory. And some relevant parameters such as binding
sites, binding distance and intermolecular forces between the complex and BSA were also obtained by
analyzing the fluorescence spectral data.
Keywords:
Valen Schiff base
Copper(II) complex
Standard molar enthalpy of formation
Bovine serum albumin
Fluorescence quenching
ã 2014 Elsevier B.V. All rights reserved.
1. Introduction
[4–6]. Researches have shown that some drugs have better effect
when they coordinate with some metal ions [7]. Especially, there
Derived from the condensation reaction of o-vanillin and
diamines, Valen Schiff-bases and their metal complexes have
played an important and popular role in the area of research due
to their simple synthesis, versatility, and diverse range of
applications such as catalysis, magnetism and pharmacology
[1,2]. As typical Salen-like ligands, hexdentate Valen Schiff-bases,
which display flexible coordination modes and multiple metal
coordination abilities, are usually used as the endogenous
bridging units to coordinate with various metal ions such as
lanthanide and transition metal ions [3]. Nowadays, the inves-
tigations on the medicinal values of Schiff-base complexes have
become a new trend, and some Schiff-base complexes have been
found to have excellent antibacterial and anti-tumor properties
are a great number of researches on copper(II) complexes with
diverse drugs [8–10], which is presumably because of the
biological role copper(II) plays and its synergetic activity with
drugs [11,12]. Many copper(II) complexes have been confirmed to
have a good inhibition effect on several pathogenic fungi and
bacteria [13,14].
It is very significant to investigate the synthesis methods,
characteristic of molecular structure and various aspects proper-
ties of Schiff-base complexes because they may well become one of
the most powerful drugs for combating disease of the human body
in the future. In this paper, we have prepared a new mononuclear
copper(II) Schiff-base complex derived from Valen Schiff-base and
Cu(II) salt. And then, its standard molar enthalpy of formation was
determined by using a solution-reaction isoperibol calorimeter.
Besides, the interaction between the complex and BSA was also
investigated by the fluorescence quenching method. These
properties of the title complex, we believe, can provide some
useful information for further research of Valen Schiff-base
complexes, especially in the field of medicine.
* Corresponding author. Tel.: +86 735 2278308; fax: +86 735 2278308.
** Corresponding author. Tel.: +86 735 2653353; fax: +86 735 2653353.
E-mail addresses: lichuanhua0526@126.com (C.-H. Li), liqiangguo@163.com
(Q.-G. Li).
http://dx.doi.org/10.1016/j.tca.2014.10.025
0040-6031/ã 2014 Elsevier B.V. All rights reserved.

8
J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
2. Experimental
dissolve all of the relevant samples rapidly and completely, but also
make two sides of the thermochemical cycle reaching a same final
state. The high overlapping UV spectra (Fig. 2) of Solution D0 and
Solution G0, which were obtained by diluting Solution D and
Solution G to 25 times with the colorimetric solvent S, and the
equal refractive indexes of Solution D and Solution G were the best
evidence that they had the same thermodynamic state.
In this way, according to Hess’s law, the standard molar
enthalpy change of the Reaction (1) can be calculated by means of
the dissolution enthalpies of relevant substances in the corre-
sponding colorimetric solvents, which can be determined by using
a solution-reaction isoperibol calorimeter. Finally, the standard
molar enthalpy of formation of the complex can be obtained by
combining the calculated enthalpy change of the Reaction (1) with
the literature values of other relevant substances.
2.1. Chemicals and instruments
The water used in this work was triply distilled. BSA was
purchased from Shanghai Bo’ao Biological Technology Co., Ltd.,
(Shanghai, China). Other chemicals used in the heat-measurement
experiment are listed in Table 1.
The dissolution enthalpies were determined by using
a
SCR-100 solution-reaction isoperibol calorimeter (constructed by
the Thermochemical Laboratory of Wuhan University, China). UV–
vis spectra and the fluorescence spectra were obtained on a
U-3010 UV–vis spectrophotometer and an F-4600 fluorescence
spectrophotometer (Hitachi, Tokyo, Japan), respectively. Refractive
indexes were measured by a WAY-IS digital Abbe refractometer
(Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai,
China). An elemental analyzer (PerkinElmer 2400CHN, USA) was
used to measure the C, H, and N contents of the complex. FTIR
spectra were recorded with an Avatar360 spectrometer using a KBr
pellet in the 400–4000 cmꢂ1 range (Nicolet, Madison, USA). X-ray
diffraction measurement was carried out on a Bruker SMART CCD
2.4. The determination of the dissolution enthalpies
The determination of relevant samples’ dissolution enthalpies
was conducted by a solution-reaction isoperibol calorimeter
(SRC-100). The principle, construction and calibration method of
the calorimeter have been elucidated in previous literature [16].
The calibration of the calorimeter was conducted by measuring the
dissolution of KCl (calorimetric primary standard, purity greater
than 99.99%) in triply distilled water at 298.15 K. The average value
of five parallel measurements is (17,557 ꢁ12) J molꢂ1. Comparing
with the published data (17,536 ꢁ 9) J molꢂ1 for KCl [16], the
eventual error was less than 0.5% and the present calorimeter was
very reliable.
diffractometer using graphite-monochromated Mo-K
a radiation
(
l
= 0.71073 Å).
2.2. Synthesis and single crystal cultivation of the copper(II) Schiff-
base complex
The ligand (H2L) was synthesized by reaction of o-vanillin and
ethanediamine with a molar ratio of 2:1 in ethanol at room
temperature [15]. Cu(NO3)2
ꢀ
3H2O (0.5 mmol, 0.121 g) was added to
All of the solid samples were dried thoroughly and fully ground.
The volume of calorimetric solvent was 100 mL for each time, and
the measurement was conducted at 298.15 K. The current of
electrical calibration was 21.813 mA and the resistance of the
a methanolic solution (50 mL) of the ligand (0.5 mmol, 0.164 g) at
50 ꢃC. After continuously stirred and refluxed for 2 h, the reaction
mixture was filtered, and black-green crystals were formed within
few days by slow evaporation of the green filtrate. The crystal
structure was characterized by the X-ray diffraction experiment.
Elemental analysis for the complex (C19H23CuN3O8), Calc. (Found):
C 47.06 (47.11), H 4.78 (4.85), N 8.66 (8.63) %.
heater was 1212.3
V. The sequence of measurements and the
corresponding solvents follow the designed thermochemical cycle.
All samples were measured for five times and the obtained results
were listed in Tables 2 and 3.
2.3. Design of thermochemical cycle and selection of calorimetric
solvent
2.5. The fluorescence quenching spectra
2.5.1. The preparation of solutions
Taking Hess’s law as the theoretical basis, we designed a
thermochemical cycle (as shown in Fig. 1). Obviously, the premise
to obtain the precise enthalpy change of Reaction (1) is a highly
consistent final state that two sides of the thermochemical cycle
must have, in other words, the components of Solution D must be
equal to those of Solution G. However, the key issue to reach that
state is the selection of colorimetric solvent. An ideal mixed
The preparation of BSA stock solution (5 g Lꢂ1): firstly, a
precursor solution of NaCl with molar concentration of 0.05 mol
Lꢂ1 was prepared. Then, the appropriate BSA powder was added to
the above solution and diluted it to the marked line in a 20 mL
volumetric flask. The prepared solution was stored at 1–4 ꢃC in a
refrigerator.
The preparation of the complex solution: the complex solution
ꢀ
ꢁ
calorimetric solvent S
V
DMF=VHNO ðwHNO : 32:5%Þ ¼ 1 : 1 was
(100 mL, 1 ꢄ10ꢂ4 mol Lꢂ1) was prepared by dissolving the complex
3
3
prepared through testing for many times, and it can not only
(0.01 mmol) in a DMSO–H2O mixed solventðVDMSO : VH O ¼ 1 : 4Þ.
2
Table 1
Chemical samples.
Chemical name
Source
Initial mass fraction purity
(%)
Purification
method
Final mole fraction Purity
(%)
Analysis method
The ligand (H2L)
The complex
Self-preparation
Self-preparation
–
–
Recrystallization
Recrystallization
99.6
99.4
HPLC
Complexometric
titration
[Cu(HL)
ꢀ
NO3 MeOH]
ꢀ
Cu(OAc)2
ꢀ
H2O
Sinopharm Chemical Reagent Co.,
Ltd.
Sinopharm Chemical Reagent Co.,
Ltd.
Sinopharm Chemical Reagent Co.,
Ltd.
98.5
99.7
99.8
Recrystallization
99.6
Complexometric
titration
HOAc
MeOH
NaNO3
NaOAc
Nanjing Kezheng Chemical Co., Ltd. 99.6
Nanjing Kezheng Chemical Co., Ltd. 99.5

J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
9
θ
m (1)
Δr H
H2L(s) + NaNO3(s) + MeOH(l) + Cu(OAc)2 H2O(s)
Cu(HL) NO3 MeOH(s) + NaOAc(s) + HOAc(l) +H2O(l)
+Calorimeric
(2)
+Calorimeric
solvent S
(6)
solvent S
(7)
Solution A
HOAc (aq)
Solution E
(3)
Solution B
(8)
(4)
Solution F
Solution C
(9)
(5)
Same Final State
Solution D
Solution G
UV–vis
UV–vis
spectrum
spectrum
Overlapping Spectra
Fig. 1. The designed thermochemical cycle.
The preparation of Tris–HCl buffer (500 mL): Tris–HCl buffer
was prepared with 5 mmol Lꢂ1 Tris and 50 mmol Lꢂ1 NaCl, and
adjusted to pH = 7.4 with hydrochloric acid.
different concentration (from 0 to 2.0 ꢄ10ꢂ5 mol Lꢂ1 at increment
of 2.0 ꢄ10ꢂ6 mol Lꢂ1) were prepared.
The excitation wavelength was fixed at 280 nm and the slit
width was set at 3 nm, the fluorescence emission spectra (in the
300–500 nm range) of the series of test liquids were determined
under the different temperature (T = 298.15 K, 302.15 K, 306.15 K,
310.15 K).
2.5.2. The determination of the fluorescence quenching spectra
The prepared BSA stock solution was diluted 10 times in
advance. Transferred 2.68 mL of this diluent to a 10 mL colorimetric
cylinder, and then added a certain amount of the complex solution
into the colorimetric cylinder. Finally, the resulting solution was
diluted with Tris buffer solution to the desired volume. Based on
the above preparation method, by changing the adding amount of
the complex solution, a series of test liquids containing BSA in
equivalent concentration (2.0 ꢄ10ꢂ6 mol Lꢂ1) and the complex in
3. Results and discussion
3.1. The crystal structure of the complex
X-ray crystallographic analysis shows that the complex
crystallizes in P ꢂ 1 space group of symmetry. As shown in
Fig. 3, the copper(II) ion is attached to the ligand in a ratio of 1:1. It
is coordinated with two imine nitrogen atoms and two phenolic
oxygen atoms from the ligand. One of the phenolic hydroxyl groups
removes its hydrogen atom after coordination while the other one
continues to exist in its original form. Besides, there exist two
uncoordinated groups in the complex molecule, one nitrate ion
and one methanol molecule, which are connected with the
coordination entity by ionic bond and hydrogen bond, respectively.
Crystal data and important refinement parameters for the complex
were summarized in Table 4. Selected bond lengths and angles
were listed in Table 5 (CCDC: 1021574). The Cu—O bonds involved
the phenolic oxygen atoms (from 1.918(2) to 1.948(2) Å). The Cu—N
bonds varied from 1.929(3) to 1.950(3) Å. The cisoid angles and
transoid angles varied between 83.38(11) and 92.81(10)ꢃ, 168.80
(11) and 176.10(10)ꢃ.
3.2. The IR spectra of the complex and its ligand
Compared with the infrared spectra of the complex and the
ligand (Fig. 4), it can be concluded that, after coordination, vC
¼N
shifted from 1635 cmꢂ1 to 1632 cmꢂ1 and nPh—O moved from
Fig. 2. UV–vis spectra of Solution D0 and G0 (obtained by diluting Solution D and
Solution G to 25 times with the colorimetric solvent S).
1252 cmꢂ1 to 1243 cmꢂ1, while the stretching vibration peak of

10
J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
Table 2
Dissolution enthalpies of relevant samples (the left side of the thermochemical cycle) in corresponding solvents (100 mL)a.
u
mb/g
tc/s
DsHm
System
No.
kJmolꢂ1
NaNO3(s) + calorimetric Sd ! Solution A
1
2
3
0.0427
0.0425
0.0427
13.53
13.74
13.23
13.186
13.520
13.583
4
0.0426
13.79
13.859
5
0.0426
13.50
13.59 ꢁ 0.099
13.004
Average
0.0426 ꢁ 3.74 ꢄ10ꢂ5
13.430 ꢁ 0.151e
ꢂ1
ꢂ1
ꢂ1
u
DsHm½NaNO3ðsÞ; 298:15Kꢅ ¼ ð13:430 ꢁ 0:151ÞkJmol
MeOH(l) + Solution A ! Solution B
1
0.0190
3.25
3.21
3.42
3.45
3.37
ꢂ3.852
ꢂ3.553
2
0.0153
3
0.0165
ꢂ3.627
4
0.0171
ꢂ3.893
5
0.0176
ꢂ3.682
Average
0.0171 ꢁ 6.11 ꢄ10ꢂ4
3.34 ꢁ 0.047
ꢂ(3.721 ꢁ 0.065)
u
DsHm½MeOHð1Þ; 298:15Kꢅ ¼ ꢂð3:721 ꢁ 0:065ÞkJmol
H2L(s) + Solution B ! Solution C
1
0.1642
216.71
222.95
222.43
222.39
222.56
221.41 ꢁ 1.178
ꢂ254.547
2
0.1642
ꢂ256.117
3
0.1643
ꢂ256.536
4
0.1640
ꢂ255.242
5
0.1644
ꢂ256.167
Average
0.1642 ꢁ 6.63 ꢄ10ꢂ5
ꢂ(255.722 ꢁ 0.363)
u
DsHm½H2LðsÞ; 298:15Kꢅ ¼ ꢂð255:722 ꢁ 0:363ÞkJmol
Cu(OAc)2
ꢀ
H2O(s) + Solution C ! Solution D
1
2
3
4
0.0999
0.0998
0.0999
0.1000
3.42
3.51
3.29
3.54
3.21
ꢂ4.347
ꢂ4.584
ꢂ4.724
ꢂ4.392
5
0.0998
ꢂ4.351
Average
0.0999 ꢁ 3.74 ꢄ10ꢂ5
3.39 ꢁ 0.063
ꢂ(4.480 ꢁ 0.075)
ꢀ
H2OðsÞ; 298:15Kꢅ ¼ ꢂð4:480 ꢁ 0:075ÞkJmolꢂ1
u
DsHm½CuðOAcÞ2
Standard uncertainties u are u(T) = 0.001 K, u(p) = 3 kPa.
a
T = 298.15 K, p = 99 kPa.
m: mass of sample.
t: heating period of electrical calibration.
b
c
d
!
0
1=2
Calorimetric S was prepared with equal volume proportion of N,N -dimethylformamide (DMF) and nitric acid (w = 32.5%).
n
X
2
e
1
nðnꢂ1Þ
uðxiÞ ¼ sðXiÞ ¼
ðXi;k ꢂ XiÞ
, where the standard uncertainty u(xi) to be associated with xi is the estimated standard deviation of the mean and n is the
k¼1
experimental number, xi is a single experimental value obtained from a series of measurements and Xi is the mean value of the results.
C—O bonds in the methoxyl groups have not moved at all
(1083 cmꢂ1), which indicates that the Cu2+ is not coordinate with
the oxygen atoms in the methoxyl groups. Furthermore, with its
relatively small ionic radius, the Cu2+ is more likely to coordinate
in the inner N2O2 cavity of the bicompartmental Schiff-base
ligand.
3.3. Calculation of the standard molar enthalpy of formation of the
complex
3.3.1. Calculation of the mixing enthalpy of (1HOAc + 1H2O)
According to Ref. [17]:
ꢂ1
u
There is a wide and weak absorption band at 3421 cmꢂ1 in the
IR spectrum of the ligand, and that is the stretching vibration peak
of phenolic hydroxyl groups affected by intramolecular hydrogen
bonds in the structure. However, the stretching vibration peak of
hydroxyl groups from the complex appears at 3459 cmꢂ1, this is
because the methanol molecule was brought into the system so as
to form intermolecular hydrogen bonds. Besides, the emergence of
a new strong absorption band at 1384 cmꢂ1 suggests that nitrate
ion exist in the structure of the complex molecule.
Df Hm½HOAcð1Þ; 298:15Kꢅ ¼ ꢂ484:5kJmol
ꢂ1
u
Df Hm½1 : 1HOAcðaqÞ; 298:15Kꢅ ¼ ꢂ483:846kJmol
The mixing enthalpy of (1HOAc + 1H2O) can be obtained as
following:

J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
11
Table 3
Dissolution enthalpies of relevant samples (the right side of the thermochemical cycle) in corresponding solvents (100 mL)a.
u
mb/g
tc/s
DsHm
System
No.
kJmolꢂ1
NaOAc(s) + calorimetric Sd ! Solution E
1
2
3
4
0.0409
0.0408
0.0410
0.0409
12.44
12.39
12.71
12.35
12.37
12.45 ꢁ 0.066
ꢂ13.360
ꢂ12.810
ꢂ12.940
ꢂ12.870
5
0.0409
ꢂ12.010
Average
0.0409 ꢁ 3.16 ꢄ10ꢂ5
ꢂ(12.798 ꢁ 0.219)e
ꢂ1
u
DsHm½NaOaAcðsÞ; 298:15Kꢅ ¼ ꢂð12:798 ꢁ 0:219ÞkJmol
1:1 HOAc(aq) + Solution E ! Solution F
1
2
3
4
0.3903
0.3905
0.3910
0.3890
4.29
4.37
4.40
4.53
4.33
ꢂ0.624
ꢂ0.653
ꢂ0.661
ꢂ0.608
5
0.3907
ꢂ0.631
Average
0.3903 ꢁ 3.45 ꢄ10ꢂ4
4.384 ꢁ 0.041
ꢂ(0.635 ꢁ 0.010)
ꢂ1
u
DsHm½1 : 1HOAcðaqÞ; 298:15Kꢅ ¼ ꢂð0:635 ꢁ 0:010ÞkJmol
Cu(HL)
ꢀ
NO3
ꢀ
MeOH(s) + Solution F ! Solution G
1
2
3
4
0.2430
0.2429
0.2430
0.2431
176.55
176.78
176.36
176.31
176.65
176.53 ꢁ 0.088
ꢂ226.631
ꢂ224.921
ꢂ225.637
ꢂ227.287
ꢂ226.808
5
0.2431
Average
0.2430 ꢁ 3.74 ꢄ10ꢂ5
ꢂ(226.257 ꢁ 0.428)
ꢀ
MeOHðsÞ; 298:15Kꢅ ¼ ꢂð226:257 ꢁ 0:428ÞkJmolꢂ1
u
DsHm½CuðHLÞ
ꢀ
NO3
Standard uncertainties u are u(T) = 0.001 K, u(p) = 3 kPa.
a
T = 298.15 K, p = 99 kPa.
m: mass of sample.
t: heating period of electrical calibration.
b
c
d
Calorimetric S was prepared with equal volume proportion of N,N-dimethylformamide (DMF) and nitric acid (w = 32.5%).
!
1=2
n
X
2
e
1
uðxiÞ ¼ sðXiÞ ¼
ðXi;k ꢂ XiÞ
, where the standard uncertainty u(xi) to be associated with xi is the estimated standard deviation of the mean and n is the
nðnꢂ1Þ
k¼1
experimental number, xi is a single experimental value obtained from a series of measurements and Xi is the mean value of the results.
u
u
DdHmð7Þ ¼ Df Hm½1 : 1HOAcðaqÞ; 298:15Kꢅ
u
ꢂ
Df Hm½HOAcð1Þ; 298:15Kꢅ
¼ 0:654kJmolꢂ1
3.3.2. The standard molar enthalpy change of the Reaction (1)
As shown in the thermochemical cycle (Fig. 1), the standard
molar enthalpy change of the Reaction (1) can be calculated
according to the relationship between reaction enthalpy and
dissolution enthalpies in Eq. (1):
S
B
u
u
D
rHm ¼ ꢂ vBDsHm;T;B
(1)
u
where
D
rHm represents the standard molar enthalpy change of a
u
chemical reaction;
D
sHm;T;B is the dissolution enthalpy of a
substance B when it is dissolved in corresponding solvent at
temperature T and vB is the stoichiometric coefficient of the
corresponding substance B in a chemical reaction equation, which
is negative for reactants and positive for products.
Plugging relevant data into the Eq. (1), the enthalpy change of
the Reaction (1) can be obtained:
Fig. 3. Molecular structure of the complex.

12
J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
Table 4
Crystallographic data for the complex.
Crystal data
CCDC No.
1021574
C19H23CuN3O8
967.87
293(2)
0.71073
Triclinic
P ꢂ 1
V (Å3)
1974.22(11)
2
1.628
1.161
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
Z
Dcalc (g/cm3)
Absorption coefficient (mmꢂ1
Crystal size (mm)
Reflections collected
)
0.05 ꢄ 0.03 ꢄ 0.02
16621
Independent reflections (Rint
)
7745
99.9%
8.9241(3)
10.1845(4)
22.3874(5)
98.820(2)
90.750(2)
100.653(3)
Completeness to theta = 26.00ꢃ (%)
Maximum and minimum transmission
Data/restraints/parameters
b (Å)
0.9771 and 0.9442
7745/6/569
1.050
R1 = 0.0417, wR2 = 0.1069
R1 = 0.0494, wR2 = 0.1126
c (Å)
a
b
g
(ꢃ)
(ꢃ)
(ꢃ)
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
s
(I)]
u
u
u
u
u
u
Df H ½NaOAcðsÞ; 298:15Kꢅ ¼ ꢂ708:81kJmolꢂ1
u
Dr
H
mð1Þ ¼ ½Ds
H
mð2Þ þ Ds
H
H
mð3Þ þ Ds
H
mð4Þ þ Ds
H
mð5Þꢅ ꢂ ½Ds
H
mð6Þ
m
u
u
u
Df H ½H2Oð1Þ; 298:15Kꢅ ¼ ðꢂ285:830 ꢁ 0:040ÞkJmolꢂ1
u
þ
Dd
H
mð7ÞþDs
H
mð8ÞþDs
ð9Þꢅ¼ ½13:43 þ ðꢂ3:72Þ
þðꢂ255:72Þþðꢂ4:48Þꢅꢂ½ðꢂm12:80Þ þ 0:65 þ ðꢂ0:64Þ þ ðꢂ226:26Þꢅ
m
Df H ½H2LðsÞ; 298:15Kꢅ ¼ ðꢂ517:75 ꢁ 2:36ÞkJmolꢂ1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
2
2
2
2
2
2
2
m
ꢁ
ð0:15Þ þð0:07Þ þ ð0:36Þ þ ð0:08Þ þ ð0:22Þ þ ð0:01Þ þ ð0:43Þ
Df H ½CuðOAcÞ2ꢀH2OðsÞ; 298:15Kꢅ ¼ ꢂ1189:1kJmolꢂ1
u
¼ ꢂð11:44 ꢁ 0:63ÞkJmolꢂ1
m
Df H ½NaNO3ðsÞ; 298:15Kꢅ ¼ ꢂ467:85kJmolꢂ1
u
m
Df H ½MeOHð1Þ; 298:15Kꢅ ¼ ꢂ238:4kJmolꢂ1
u
m
3.3.3. The standard molar enthalpy of formation of the complex
The standard molar enthalpy of formation of the complex can
be calculated as following:
The standard molar enthalpy change of a chemical reaction can
also be described by the standard molar enthalpy of formation of
reactants and products in the reaction:
u
DrHm ¼ ½CuðHLÞ
ꢀ
NO3MeOHðsÞ; 298:15Kꢅ ꢂ ðꢂ11:44Þ
u
u
ꢂ ½ðꢂ708:81Þ þ ðꢂ484:5Þ þ ðꢂ285:830Þꢅ
DrHm
¼
S
v
BDf Hm;T;B
(2)
B
þ ½ðꢂ517:75Þ þ ðꢂ1189:1Þ þ ðꢂ467:85Þ þ ðꢂ238:4Þꢅ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
ꢁ
ð0:63Þ þ ð0:040Þ þ ð2:36Þ
u
¼ ꢂð945:40 ꢁ 2:44ÞkJmolꢂ1
where Df Hm;T;B is the standard molar enthalpy of formation of a
substance B at temperature T. Therefore, for the Reaction (1), there
is the following relationship:
u
u
DrHmð1Þ ¼ Df Hm½CuðHLÞ
ꢀ
NO3
ꢀMeOHðsÞ; 298:15Kꢅ
u
3.4. The interaction between the complex and BSA
u
þ
þ
ꢂ
ꢂ
Df Hm½NaOAcðsÞ; 298:15Kꢅ þ Df Hm½HOAcð1Þ; 298:15Kꢅ
u
u
3.4.1. The fluorescence quenching mechanism of BSA
Df Hm½H2Oð1Þ; 298:15Kꢅ ꢂ Df Hm½H2LðsÞ; 298:15Kꢅ
u
Serum albumins are one of the most abundant proteins in the
plasma, and as an important drug delivery carrier, they play a
significant role in organisms. Bovine serum albumin (BSA), a
homologous protein of human serum albumins, is a kind of
bioluminescent protein that can emit strong fluorescence with its
intrinsic fluorophores such as tryptophan, tyrosine and phenylal-
anine. When it is excited by an ultraviolet light at 280 nm, the
Df Hm½CuðOAcÞ2ꢀH2OðsÞ; 298:15Kꢅ
u
u
Df Hm½NaNO3ðsÞ; 298:15Kꢅ ꢂ Df Hm½MeOHð1Þ; 298:15Kꢅ
Combing the standard molar enthalpies of formation of other
relevant substances [18–22]:
Table 5
Selected bond lengths (Å) and bond angles (ꢃ) for the complex.
Bond distances (Å)
Cu(1)–O(1)
Cu(1)–N(1)
Cu(2)–O(5)
Cu(2)–N(3)
1.948(2)
1.950(3)
1.920(2)
1.929(3)
Cu(1)–O(2)
Cu(1)–N(2)
Cu(2)–O(6)
Cu(2)–N(4)
1.918(2)
1.943(3)
1.933(2)
1.938(3)
Bond angles (ꢃ)
O(2)–Cu(1)–N(2)
O(2)–Cu(1)–O(1)
N(2)–Cu(1)–O(1)
O(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
92.81(10)
90.90(9)
176.10(10)
168.80(11)
83.38(11)
92.75(10)
O(5)–Cu(2)–N(3)
O(5)–Cu(2)–O(6)
N(3)–Cu(2)–O(6)
O(5)–Cu(2)–N(4)
N(3)–Cu(2)–N(4)
O(6)–Cu(2)–N(4)
92.54(10)
90.80(9)
175.43(13)
174.28(10)
83.95(12)
92.46(10)
Fig. 4. The IR spectra of the complex and its ligand.

J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
13
Table 6
Quenching rate constants (kq) at different temperaturesa.
Temperature (K)
Kq (L molꢂ1 sꢂ1
)
Rb
298.15
302.15
306.15
310.15
(1.08 ꢁ 0.009) ꢄ 1013
(9.21 ꢁ 0.019) ꢄ 1012
(8.17 ꢁ 0.013) ꢄ 1012
(7.44 ꢁ 0.013) ꢄ 1012
0.995 ꢁ 0.002
0.989 ꢁ 0.001
0.990 ꢁ 0.001
0.993 ꢁ 0.001
Standard uncertainty u is u(p) = 3 kPa, u(T) = 0.006 K.
a
p = 99 kPa.
The correlation coefficient.
b
concentration and KSV is the dynamic quenching constant (also
called Stern–Volmer quenching constant), kq is the bimolecular
quenching rate constant, which reflects biological macromole-
cules’ fluorescence lifetime decay rate under the influence of
intermolecular diffusion and collisions, t0 is the average lifetime of
the fluorophore in the absence of quencher and it is about equal to
10ꢂ8 s in this experiment [23].
The fluorescence data measured at different temperatures were
analyzed according to the Stern–Volmer equation. In general, the
frequency and strength of dynamic collision should be more
intense with increasing temperature. However, it can be seen
Fig. 5. The fluorescence quenching spectra of BSA treated by the complex at
298.15 K. lex
/
lem = 280 nm/340 nm, c(BSA) = 2.0 ꢄ10ꢂ6 mol Lꢂ1
.
fluorescence emission of BSA is caused by the total emission effect
of tryptophan and tyrosine. In this experiment, the concentrations
of BSA were stabilized at 2.0 ꢄ10ꢂ6 mol Lꢂ1, while the concen-
trations of the complex varied from 0 to 2.0 ꢄ10ꢂ5 mol Lꢂ1 at
clearly from Fig. 6 that the Stern–Volmer quenching constant (KSV
)
decreases with the increase of temperature, which indicates that
the most probable quenching mechanism is not dynamic quench-
ing but static quenching [24]. Furthermore, the quenching rate
constants kq (see in Table 6) are far above the limiting diffusion rate
constant (2.0 ꢄ10l0 L molꢂ1 sꢂ1) [23], which is a further reason why
the dynamic collision quenching is not the main quenching
mechanism.
increment of 2.0 ꢄ10ꢂ6 mol Lꢂ1
.
As shown in Fig. 5, BSA has a strong fluorescence emission band
at 340 nm and the fluorescence intensity of BSA decreases regularly
with the increasing of the complex concentration. Generally
speaking, the quenching processes can be classified into two
categories: dynamic quenching and static quenching. Static
quenching refers to formation of a nonfluorescent fluorophore-
quencher complex while dynamic quenching is closely related to
the intermolecular diffusions and collisions. Supposing the
quenching process is caused by dynamic quenching, we can
confirm the quenching mechanism by the dynamic Stern–Volmer
equation (Eq. (3)):
3.4.2. The mode of interaction between the complex and BSA
Static quenching can be seen as the formation of a nonfluores-
cent complex composed by fluorophore and quencher. If there are
n binding sites the drug molecule has on the protein (B), then
nQ þ B ! QnB
(4)
and
F0
F
¼ 1 þ KSV½Qꢅ ¼ 1 þ kqt0½Qꢅ
(3)
½QnBꢅ
KA
¼
(5)
n
½Qꢅ ½Bꢅ
where F0 and F are the fluorescence intensities in the absence and
presence of quencher, respectively, [Q] is the quencher
where [B] and [Q] are the protein and quencher concentration,
respectively, [QnB] represents the concentration of non-fluorescent
fluorophore-quencher complex and KA is the binding constant. Set
the total protein concentration as [B0], then the binding constant
(KA) can be represented as the following equation:
2.5
T=298.15K; Slope=1.0753
T=302.15K; Solpe=0.9208
T=306.15K; Solpe=0.8167
T=310.15K; Solpe=0.7444
2.0
1.5
1.0
0.5
0.0
½B0ꢅ ꢂ ½Bꢅ
KA
¼
(6)
n
½Qꢅ ½Bꢅ
The fluorescence intensity is proportional to the concentration
of the unquenched protein, which is described in Eq. (7):
½Bꢅ
F
¼
(7)
½B0ꢅ F0
According to Eqs. (6) and (7), it can easily be deduced the
following equation:
ꢂ
ꢃ
F0 ꢂ F
lg
¼ lgðKAÞ þ nlg½Qꢅ
(8)
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
[Q]/(10-5mol L-1)
F
Based on Eq. (8), the logarithmic plot of fluorescence quenching
of BSA treated with different drug concentrations was drawn by
Fig. 6. The Stem–Volmer plots for the fluorescence quenching of BSA by the
complex at different temperatures.

14
J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
T=298.15K; Slope=1.0789;
Intercept=5.4151; R=0.998
T=310.15K; Slope=1.2935;
Intercept=6.2977; R=0.997
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-5.8
-5.6
-5.4
-5.2
-5.0
-4.8
-4.6
lg [Q]
Fig. 8. The overlap of the fluorescence spectrum of BSA (a) and the absorbance
spectrum of the complex, (b) at 298.15 K, c(the complex) = 2.0 ꢄ10ꢂ6 mol Lꢂ1
.
Fig. 7. Double-log plots of the fluorescence quenching effect of the complex on BSA
u
u
u
at temperatures of 298.15 K and 310.15 K.
D
G ¼
D
H ꢂ T
D
S
(11)
The relevant thermodynamic parameters for the interaction of
the complex with BSA were listed in Table 7. The free energy
using the collected data under the temperatures of 298.15 K and
310.15 K (Fig. 7). The binding constant KA and the number of
binding sites n can be calculated from the slope and intercept of the
correspondent straight line in Fig. 7. Results listed in Table 7 show
that either KA or n at 310.15 K are greater than those at 298.15 K,
which indicates that an increase in temperature of the experiment
will be beneficial to the binding between the complex and BSA.
Moreover, the number of binding sites n is approximately equal to
1, that is, the complex combines with BSA in the proportion of
about 1:1.The binding force between drug molecules and biological
macromolecules such as protein, mainly include hydrophobic
forces, van der Waals interactions, hydrogen bonds, and electro-
static interactions. In order to elucidate which type the binding
force between the complex and BSA is, we calculated the
thermodynamic parameters by means of Eqs. (9)–(11). As the
u
change (
DG ) is a negative number, which suggests the spontaneity
u
u
of the binding process. The positive
DH and DS values indicate
that the binding between the complex and BSA mainly attributes to
the hydrophobic forces [25], and accompanies with the weak
electrostatic interactions [26].
3.4.3. Energy transfer and binding distance between the complex and
BSA
Assuming that the fluorescence quenching of BSA treated by the
complex accords with the non-radioactive energy transfer theory,
according to the Forster theory, the energy transfer efficiency (E) is
related to the binding distance (r) and the critical distance (R0)
when the energy transfer efficiency is 50% between the donor and
the acceptor [27,28]:
temperature changes a little during the experiment, the variation
u
of enthalpy change
constant.
D
H is not obvious and it can be regarded as a
F
F0
R06
R60 þ r6
E ¼ 1 ꢂ
¼
(12)
ꢂ
ꢃ
u
K2
K1
1
1
D
R
H
ln
¼
ꢂ
(9)
T1 T2
where F0 and F are the fluorescence intensity of donor in the
absence and presence of equal amount of acceptor, respectively.
The critical distance (R0) can be calculated according to Eq. (13):
u
D
G ¼ ꢂRTlnK
(10)
R60 ¼ 8:79 ꢄ 10ꢂ25K2nꢂ4
fJ
(13)
In the above equation, K2 is the orientation factor,
F
is the
where K1 and K2 are the binding constants at the experiment
temperatures T1 and T2, respectively, R is the gas constant
fluorescence quantum yield of donor in the absence of acceptor
and n is the refractive index of the medium. These parameters were
u
(8.314472 J Kꢂ1 molꢂ1). The free energy change (
D
G ) and the
u
applied to the following values [29]: K2 = 2/3,
F=0.15, n = 1.36.
enthalpy change (
D
H ) can be calculated from the binding
constants (KA) and correspondent experiment temperature.
Besides, J denotes the overlap integrals between the fluorescence
emission spectrum of the donor and the UV–vis absorption
spectrum of the acceptor (Fig. 8), it can be calculated by the
u
Furthermore, the entropy change (DS ) can be calculated by the
following equation:
Table 7
Binding constants (KA), number of binding sites (n), and relevant thermodynamic parameters of the interaction between the complex and BSAa.
u
u
u
Temperature (K)
KA(L molꢂ1
)
n
D
H
(kJ molꢂ1
130.1 ꢁ 0.03
)
D
G
(kJ molꢂ1
)
DS )
(J molꢂ1 Kꢂ1
298.15
310.15
(2.60 ꢁ 0.0012) ꢄ 105
1.0789 ꢁ 0.0144
1.2935 ꢁ 0.0345
ꢂ30.9 ꢁ 0.01
ꢂ31.4 ꢁ 0.02
539.9 ꢁ 0.11
520.8 ꢁ 0.16
(1.98 ꢁ 0.0015) ꢄ 106
Standard uncertainty u is u(p)= 3 kPa, u(T)= 0.006 K.
a
p = 99 kPa.

J.-Q. Xie et al. / Thermochimica Acta xxx (2014) 7–15
15
following integral equation [29]:
Listeria monocytogenes, and Salmonella enterica, J. Food Prot. 66 (2003)
1811–1821.
R
1 Fð
l
Þ
e
ð
l
Þ
l
4d
l
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0
R
J ¼
(14)
1 Fð
lÞdl
0
where F(
of the acceptor at
acceptor at
l
) is the fluorescence intensity of the donor in the absence
l, e(l
) is the molar absorption coefficient of the
l.
According to Eqs. (12)–(14), the results was calculated to be:
J = 1.11 ꢄ10ꢂ14 cm3 L molꢂ1, R0 = 2.57 nm, E = 0.15 and r = 3.43 nm.
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quenching is highly accord with the non-radioactive energy
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In summary, a new copper(II) Schiff-base complex [Cu(HL)
ꢀ
NO3 MeOH] was prepared and its detail molecular structure was
ꢀ
obtained by the X-ray diffraction method. Based on an ideal and
feasible thermochemical cycle, the standard molar enthalpy of
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ꢀ
NO3
ꢀ
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Immobilization of
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interaction of the complex with BSA was also investigated by using
the fluorescence quenching method. The experimental results
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complex was static quenching, and the complex combined with
BSA in the proportion of about 1:1. The binding process between
the complex and BSA was spontaneous, and the binding forces
mainly attributed to the hydrophobic forces and accompanied with
the weak electrostatic interactions. Moreover, the binding distance
(r) between donor (BSA) and acceptor (the complex) was 3.43 nm
and the quenching mechanism was accord with the non-
radioactive energy transfer theory.
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The authors thank the National Natural Science Foundation of
China (21273190, 21103027), Science and Technology Plan Projects
of Hunan Province (No. 2014FJ3011), the College Student Innova-
tive Experimental Project of Xiangnan University (The Design for
Drainage-oil Test Pen), the Hunan Provincial Colleges and
Universities “twelve-five planning” Professional Comprehensive
Reform Pilot Project (No. 2012-266), the Construction Project of
Hunan Provincial Colleges and Universities Teaching Practice (No.
2013–295), and the Construct Program of the Key Discipline in
Hunan Province for financial support.
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