In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.
Placentation, which is critical for maternal-fetal exchange of nutrients and gases, is a complicated process comprising stepwise vasculogenesis and angiogenesis. Hypoxia caused by impaired trophoblast invasion may cause various angiogenic abnormalities in human placenta. The Notch1 signaling pathway plays an important role in the regulation of angiogenesis. The angiogenesis of human umbilical vein endothelial cells (HUVECs) under normal/hypoxic conditions and the mRNA/protein level of Notch1/Dell4/Jagged1 were investigated in this study. The effects of DAPT/JAG-1 on the migration of HUVECs were also assessed by cell wound healing assay, so as to discover the possible role of notch1 signaling pathway in the angiogenesis of human placenta. The results showed that angiogenic ability of HUVECs was seriously reduced under hypoxic conditions. The mRNA and protein levels of Notch1/Dell4/Jagged1 were decreased in the hypoxic group compared to the control one. In addition, the migration capability of HUVECs was significantly obstructed when treated with DAPT and under hopoxic condition, but promoted when treated with JAG-1. The above results demonstrate that hypoxia downregulates the angiogenesis in human placenta via Notch1 signaling pathway.
The photoionization and photodissociation of L-valine are studied by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry at the photon energy of 13 eV. The ionization energy of L-valine and the appearance energies of major fragments are measured by the photoionization efficiency spectrum in the photon energy range of 8-11 eV. Possible formation pathways of the major fragments, NH(2)CHC(OH)(2)(+) (m/z=75), NH(2)(CH(3))(2)(CH)(2)(+) (m/z=72) and NH(2)CHCO(+) (m/z=57), are discussed in detail with the theoretical calculations at the B3LYP/6-31++G (d, p) level. Hydrogen migration is considered as the key way for the formation of NH(2)CHC(OH)(2)(+) (m/z=75) and NH(2)CHCO(+) (m/z=57). Furthermore, other fragments, NH(2)CHCOOH(+) (m/z=74), (CH(3))(2)(CH)(2)(+) (m/z=56), C(4)H(7)(+) (m/z=55), NH(2)CHOH(+) (m/z=46), NH(2)CH(2)(+) (m/z=30) and m/z=18, species are also briefly described.
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