Chemical Constituents And Radical Scavenging Activity Of The Marine Red Algae Polysiphonia Urceolata And Rhodomela Confervoides

Marine organisms have ability to produce structurally interesting and biologically active secondary metabolites. In recent years, natural product chemists have paid more and more attention to the halogenated secondary metabolites from marine red algae of Rhodomelaceae family.In the present dissertation, the structures of 100 compounds were characterized from two Rhodomelaceae species of Polysiphonia urceolata and Rhodomela confervoides. The separation was performed by repeated column chromatography (CC) on silica gel, RP-18, Sephadex LH-20, and semi-preparative HPLC. The structures of the isolated compounds were elucidated by spectroscopic analysis including IR, UV, EI-MS, FAB-MS, HR-ESI-MS, CD, 1D-NMR (1H-NMR, 13C-NMR, DEPT) and 2D-NMR (1H-1H COSY, HSQC, HMBC).From the extracts of P. urceolata, a total of 38 compounds were isolated and structurally elucidated. Among them, 7 bromophenols were found to be new structures (P1-P7) with one compound (urceolatin, P1) possessing a previously not reported phenanthro[4,5-bcd]-furan moiety in natural products. From a biogenetic point of view, P1 is likely generated by intermolecular free radical reactions involving two molecules P12 and P13, which were also isolated from P. urceolata. P2 and P3 possess a 9, 10-dihydrophenanthrene skeleton and this is only the second example of such kind of compounds isolated from a marine source, while P4 possesses an unusual 5,7-dihydrodibenzo[c, e]oxepine structural moiety.From the extracts of R. confervoides, a total of 62 compounds were isolated and structurally elucidated. Among them, 18 bromophenols were found to be new chemicals (R1-R18). Of these compounds, R1 and R2 possess oxopyrrolidin-ureido structural unit, while R3-R6 possessγ- ureidobutanoic acid moiety. In addition, five bromophenols (R7, R8, R9, R13, and R14) possess amino-4-oxobutanoic acid units. The other new compounds including one methylsulfonylmethyl bromophenol (R12), one xanthene derivatives bromophenol (R10), and five simple bromophenols (R11, R15, R16, R17, R18). To the best of our knowledge, this is the first report of natural products containing ureido-pyrrolidinone moiety (R1 and R2), while R10 is the first example of a bromo-xanthene natural derivative.Radical scavenging activities of the isolated compounds were evaluated by scavenging DPPH (1,1-diphenyl-2-picryl-hydrazyl) radical assay and ABTS (2,2'-Azinobis-(3- ethylbenzothiazoline-6-sulfonic acid)diammonium salt) radical assay. In general, bromophenols displayed scavenging activity in various degrees with R3 displayed significant DPPH radical-scavenging activity (IC50 value 3.3μM) which is 24-fold more potent than positive control BHT (IC50 82.1μM). R2 showed moderate ABTS free radical-scavenging activitiy with a TEAC = 5.2 mM, which is 5-fold potent than positive control ascorbic acid (TEAC = 1.0 mM). Structure-activity analysis indicated that our result is in accordance with the former conclusions that the DPPH radical scavenging efficiency of monohydroxyl phenols is strongly enhanced by the introduction of second hydroxyl group and is increased by one or two methoxyl substitutions in the ortho position. Special substitutions such as ureido and pyrrolidinone had significant influence on ABTS radical scavenging activities. The bromination and polyhydroxyl are also important factors responsible for ABTS radical scavenging activities.The results from this investigation provided new entities to the marine algal derived halogenated natural products and may provide new idea for the use of P. urceolata and R. confervoides.