Chemical Studies About Three Kinds Of Natural Antioxidants

Naturally occurring antioxidants, compared to the synthetic antioxidants, have greater potentials in terms of their good safeties as well as potential nutritional and pharmaceutical values. In essence, their usage into the chemoprevention or treatments of ROS-induced diseases, such as carcinogenesis, cardiovascular diseases and senescence, has emerged as a viable alternative approach. The present dissertation aimed at searching natural antioxidants with stronger potencies and doing further researches, in chemical means, about their in vitro activities.1. The first chemical studies of Vitis wilsonae led to the isolation of 15 resveratrol oligomers that were elucidated by the detailed spectroscopic analysis and the comparison with literatures. Among them,3 ones were new and named as wilsonols A-C, whose relative configurations were determined by their own NOESY spectra. In addition, the biomimetic reactions of these oligomers were established by the catalysis of enzymes, photoirradiation and inorganic acids, not only to ascertain their absolute configurations but also prove their conceivable biogenetic relationships. Meantime, a newly putative biomimetic tetramer called diviniferin B was isolated and identified. Using the well-established ROS (OH, O2-,1O2) scavenging models by EPR spin-trapping technique, the systematic assessments about the inhibitory effects of these oligomers were carried out, getting a generally suitable rule:they showed less weak scavenging effects on OH and O2- whereas selective and much stronger quenching properties on 1O2 were possessed. HPLC-ESI-MS2 technique and PM3 semiempirical calculations were performed, indicating the 1O2 quenching mechanism was the participation of both chemical and physical quenchings. Herein, chemical quenching meant that 1O2 was cycloadded into the resorcinol rings of quenchers which then underwent an endoperoxide intermediate form to quinones, resulting in the irreversible destruction of quenchers and the definite loss of total antioxidant potencies. During physical quenching, quenchers quenched 1O2 by a very efficient and dominant electronic energy transfer to molecular oxygen, while quenchers were regenerated and total antioxidant potencies didn't reduce.2. The separation procedures of Ampelopsis heterophylla caused the yields of two flavan-3-ol isomers 1 and 2 that were corroborated to be cinchonain la and Ib. Their scavenging activities on'OH were evaluated and a unique phenomena was observed. When the concentrations of 1 and 2 were low, their scavenging behaviors were general and identical while the individual IC50 value was approximate. As the concentrations rising, the inhibition of 2 still enhanced and on the contrary, the curve of 1 significantly declined about 25%. Considering the similar IC50, structures and the same experimental conditions of 1,2 as well as the presence of Fe2+ and Fe3+ in the used systems, the generation of OH by Fenton reactions and the scavenging actions of'OH by cinchonains were alike whereas the pro-oxidant production of OH induced by the chelations of Fe2+ and Fe+ with cinchonains, which only depended on the single structures and were strengthened with the increasing concentrations, revealed the crucial reason for the inconsistency between 1 and 2. UV and MS experiments demonstrated the stronger chelating effects of 1 with Fe2+ than those of 2 and also their equivalent effects with Fe3+. Besides, MS data showed most of 1 chelated Fe2+ at a mole ratio of 2:1 and least 2 chelated at the same ratio. Thus it deduced compared with 2,1 was easier to be chelated by Fe2+. Once the concentrations growing up, the pro-oxidant potency for 1 rose to be predominant and at that time, the produced amount of OH which contains the formed by the pro-oxidant reactions plus Fenton reactions was higher than the consumed. So the inhibition curve of 1 fell.3. Online and high-throughput structural elucidation of mango peels by HPLC-ESI-MSn was accomplished, suggesting the implication of many gallate type antioxidants. Then it caused the direct separation of two valuable constituents confirmed to be ethyl gallate and penta-O-galloyl-glicoside, whose contents were quantified by an HPLC external standard method. EPR results indicated that they presented potent scavenging properties on each ROS species, which was common for this kind of natural antioxidants and greatly different from the cases of resveratrol oligomers. In view of hydrogen atom transfer as the OH-scavenging mechanism, the related structure-relationship studies were conducted to obtain the prevalent rules:the OH-scavenging potencies were positively reinforced rather than proportional to an increase in the numbers of phenol groups; the compounds with o-OH groups and intramolecular reactions exhibited more exceptional OH-scavenging potencies. Otherwise, the synergistic effects between these two constituents were observed.