L. Yang et al. / Journal of Molecular Liquids 151 (2010) 134–137
135
Fig. 1. The structure of quercetin.
and sodium hydroxide (NaOH) were purchased from Chengdu Kelong
Chemical Factory (Chengdu, China), and various concentrations of
aqueous sodium hydroxide (from 5.0×10−3 to 1.0 molL−1) were
prepared. All reagents and solvents are analytical reagent grade, and
the water used throughout was doubly distilled.
Fig. 2. Time resolved absorption spectra of quercetin reacting with sodium hydroxide.
A deuterium lamp was used as continuous light source, and
the light emitted from the lamp enters a quarts cell with 1×1-cm
cross-section which contains the sample solution. The light trans-
mitted through the sample is dispersed with the grating spectro-
graph (Acton Research Corp., SpectraPro-275), and detected by
the intensified CCD camera (Princeton Instruments, ICCD PI-Max
1024RB). The resolution of spectra can reach 0.2 nm. The synchro-
nization of the measuring system is achieved using a digital delay
generator (Stanford Research Systems, DG535) which sequentially
generates electric pluses to trigger the addition of NaOH and the
opening of the camera's gate, and to control the width of the gate.
Spectra were recorded at different delay times after the addition of
NaOH to quercetin solution with the moment when two solutions
contacted as start time t=0.
The concentration of quercetin in ethanol is kept constant at
5.0×10− 5 molL− 1 in all experiments. However, concentrations
of sodium hydroxide in deionized water are 1.0, 2.5×10− 1, 2.4×
10− 2, 1.0×10− 2 and 5.0×10− 3 molL− 1. Immediately after add-
ing 1 mL of sodium hydroxide solution into 3 mL quercetin in
ethanol, the reaction process was monitored by acquiring time
resolved absorption spectra of the reaction solution over a wave-
length range of 200–500 nm. A total number of 200 spectra with the
same exposure time of 0.1 ms for each spectrum but the different
time interval between two neighboring spectra were recorded for
each experiment; the first 50 spectra have the time interval of
20 ms, the next 50 have 40 ms, and the last 100 have 1 s. All spectra
were measured at 298 K.
at 374 nm completely disappeared, and the absorption peak at
427 nm reached the maximum, which was suggested as the
characteristic for the intermediate product; Fig. 3 (d) shows that at
time t=440 ms, the intensity of absorption at 427 nm is decreased,
and the absorption peak at 314 nm of final product appeared. Fig. 3 (e)
shows that after a relatively long time of 980 ms, only the absorption
band of the final product existed.
The disappearing of the typical bands centered at 254 nm and
374 nm of quercetin, the growing and disappearing of the new band
centered at 427 nm of the intermediate product, and the growing of
the new band centered at 314 nm of the final product can be clearly
seen from Figs. 2 and 3.
Though the changes of absorption bands are similar for quercetin
reacting with different concentrations of sodium hydroxide, the
times at which the changes happened are different. In the present
work, as shown in Table 1, the reaction times to form the maximal
concentration of the intermediate product are in the range from
80 ms to 4.00 s, and the total reaction times to form the final product
are in the range from 360 ms to 45.0 s, which are dependent on
concentrations of sodium hydroxide.
Based on our experiment, the reaction mechanism of quercetin in
sodium hydroxide solution was proposed as Scheme 1, this is also
consistent with the conclusions of Lei et al. [14–16]. Deduce from our
experiment data, we found that OH− plays a very important role in
the formation B. Then the generated B by autoxidation reacted with
oxygen on the surface of solution or dissolve in mixture solution to
produce C, which generates new band centered at 314 nm.
3. Results and discussion
Time resolved absorption spectra of quercetin reacting with so-
dium hydroxide have been recorded. The results show that the
spectra are similar when different concentrations of sodium hydrox-
ide are added. One typical result is shown in Fig. 2. It can be seen from
Fig. 2 that absorption bands of the reaction solution changed during
the process, which indicates that the reaction between quercetin and
sodium hydroxide took place and there were intermediate and final
products formed. For further discussion, five typical spectra selected
from Fig. 2 are shown in Fig. 3.
Fig. 3 (a) shows the absorption bands centered at 254 nm and
374 nm of pure quercetin in ethanol at the time t=0 when no
reaction took place; Fig. 3 (b) shows at the time 80 ms, the intensity of
absorption peak of quercetin at 374 nm decreased, and a new
absorption peak at 427 nm emerged; Fig. 3 (c) shows that when the
reaction time of 140 ms had passed, the absorption peak of quercetin
4. Conclusion
Time resolved spectroscopy is a suitable technique for observ-
ing the chemical changes of quercetin in basic medium. Present
results show that quercetin can react with sodium hydroxide
easily, there is an intermediate product generated during the re-
action, and its absorption band centered at 427 nm. Chemical
changes in the reaction are similar for different concentrations of
sodium hydroxide, but the chemical change rate is dependent on
the concentration.
Since no other transient spectroscopic data are currently available
on quercetin reacting with sodium hydroxide, results obtained in this
paper are valuable for understanding the microscopic mechanism of
reaction between quercetin and sodium hydroxide. The information
obtained is also helpful for putting quercetin to good use as a dietary