Four cyclodextrins (CD) including β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), heptakis-O-(2-hydroxypropyl)-β-cyclodextrin (HP-β-CD), and heptakis-O-(2, 6-di-O-methyl)-β-cyclodextrin (DM-β-CD) were used as solubilizer to study the solubility enhancement of myricetin. The results of the phase solubility study showed that the presence of CDs could enhance the solubility of myricetin by forming 1:1 complexes. Among all CDs, HP-β-CD had the highest solubilization effect to myricetin. The concentration of myricetin could be 1.60 × 10−4 moL/L when the presence of HP-β-CD reached 1.00 × 10−2 moL/L, which was 31.45 times higher than myricetin’s aqueous solubility. Subsequently, the HP-β-CD:myricetin complex was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). In order to get an insight of the plausible structure of the complex, molecular docking was used to study the complexation process of HP-β-CD and myricetin. In the complex, the A ring and C ring of myricetin were complexed into the hydrophobic cavity of HP-β-CD, while the ring B was located at the wide rim of HP-β-CD. Four hydrogen bonding interactions were found between HP-β-CD and -OH groups of the guest in the HP-β-CD: myricetin complex. The complexation energy (△E) for the host-guest interactions was calculated with a negative sign, indicating the formation of the complex was an exergonic process. A 30-ns molecular dynamics simulation was conducted to the HP-β-CD: myricetin complex. Calculation results showed that no large structural deformation or position change were observed during the whole simulation time span. The average root-mean-square deviation (RMSD) changes of the host and guest were 2.444 and 1.145 Å, respectively, indicating the complex had excellent stability.
Generating
ultrafine charged droplets using electrospray is crucial
for attaining high ionization efficiency for mass spectrometry. The
size of the precursor charged droplets depends on the spray flow rate,
and conventional wisdom holds that an electrospray of a nL/min flow
rate (nanoelectrospray) is only possible using narrow capillaries
with an inner diameter of ∼1 μm or smaller. Here, the
electrospray of aqueous solutions with high electric conductivities
generated from a large off-line capillary of 0.4 mm i.d. has been
performed using a high-pressure ion source. The electric discharge
is avoided by operating the ion source at 2.5 bar gauge pressure.
The highly stable Taylor cone can be tuned to a near-hydrostatic state
that exhibits the “true nanoelectrospray” properties,
i.e., high salt tolerance and minimal ion suppression. The Q
1/2 scaling law describing the electrospray
current I and flow rate Q is found
to be valid down to the nanoflow regime under a condition that is
free of electric discharge. For a given solution, the flow rate and
the size of the initial droplets and ionization species can be controlled
with the spray current as the indicator for the instantaneous flow
rate without changing the emitter capillary of different sizes. In
regard to the application, the nanoelectrospray with a large micropipette
tip is easy to use, free of clogging when dealing with viscous and
high-salt buffer solutions, and with reduced surface interaction with
the emitter inner surface. An acquisition of very clean mass spectra
of proteins from concentrated solutions of nonvolatile salts such
as phosphate-buffered saline is demonstrated.
On-demand tunable oxidation is performed during the nanoESI-MS analysis by varying the nanoflow rate and the initial droplet size. The oxidation is initiated when the electric field of the droplet reaches ∼1.3 V nm−1.
The small charged droplet generated from the nanoelectrospray
ionization
(nanoESI) source at nL/min flow rate gives its unique feature of high-performance
ionization. A continuous scan of the flow rate in this regime can
trace the effect of droplet size in greater detail for a better understanding
of the ionization process. To date, such practical implementation
is hindered by the lack of a suitable liquid pump and the reproducibility
of microcapillaries-based systems. Here, offline nanoESI mass spectrometry
with a continuously varying flow rate in a dynamic range of several
hundred pL/min to ∼100 nL/min was performed by the precision
scanning of ESI high voltage (HV). The principle is based on the new
paradigm of generating nanoelectrospray from a large Taylor cone with
a known spray current–flow rate relationship. The instantaneous
flow rate controlled by the HV was determined by simultaneous measurement
of the spray current. The system is successfully applied to reveal
the role of nanoflow rate on the average charge state of proteins,
analysis of analyte mixture, and desalting effect. With the use of
a buffer solution with high electric conductivity, a highly controllable
oxidative modification was also observed by tuning the flow rate below
a threshold of ∼5 nL/min, a finding that has potential application
to on-demand oxygen labeling.
A bipolar
ESI source is developed to generate a simultaneous emission
of charged liquid jets of opposite polarity from an electrodeless
sprayer. The sprayer consists of two emitters, and the electrosprays
are initiated by applying a high potential difference (HV) across
the counter electrodes facing each emitter. The sprayer and the liquid
delivery system are made of all insulators without metal components,
thus enabling the total elimination of electrochemical reactions taking
place at the liquid–electrode interface in the typical electrosprayer.
The bipolar electrospray has been implemented using an online configuration
that uses a syringe pump for flow rate regulation and an offline configuration
that relies on HV for adjusting the flow rate. The voltage–current
and flow rate–current relationships of bipolar electrospray
were found to be similar to the standard electrospray. The application
of bipolar ESI to the mass spectrometry of protein, peptide, and metallocene
without electrochemically induced oxidation/reduction is demonstrated.
An electrospray operated in the steady cone-jet mode
is highly
stable but the operating state can shift to pulsation or multijet
modes owing to changes in flow rate, surface tension, and electrostatic
variables. Here, a simple feedback control system was developed using
the spray current and the apex angle of a Taylor cone to determine
the error signal for correcting the emitter voltage. The system was
applied to lock the cone-jet mode operation against external perturbations.
For a pump-driven electrospray at a regulated flow rate, the apex
angle of the Taylor cone decreased with increasing voltage. In contrast,
for a voltage-driven electrospray with low flow resistance, the angle
was found to increase with the emitter voltage. A simple algorithm
based on iterative learning control was formulated and implemented
using a personal computer to automatically correct the emitter voltage
in response to the error signal. For voltage-driven electrospray ionization
(ESI), the feedback control of the spray current can also be used
to regulate the flow rate to an arbitrary value or pattern. Electrospray
ionization-mass spectrometry (ESI-MS) with feedback control was demonstrated
to produce ion signal acquisition with long-term stability that was
insusceptible to the emulated external disturbances.
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