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.
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