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    Antioxidant and Tyrosinase Inhibitory Activities of Flavonoids from Trifolium nigrescens Subsp petrisavi
    (Amer Chemical Soc, 2013) Demirkiran, Ozlem; Sabudak, Temine; Ozturk, Mehmet; Topcu, Gulacti
    Trifolium nigrescens was researched for its chemical constituents for the first time. Bioassay-guided isolation of the EtOAc extract from the leaves of T. nigrescens resulted in the purification of a new biflavone, 4',5,5 '',7,7 ''-pentahydroxy-3',3'-dimethoxy-3-O-beta-D-glucosyl-3 '',4'-O-biflavone (1) along with eleven known compounds consisting of three phenolics (2-4), and eight flavonoid glycosides (5-12). Their structures were determined by extensive ID and 2D NMR, and MS data analyses. The isolated compounds were evaluated for their antioxidant activity and inhibitory activity on mushroom tyrosinase. Highly potent inhibitions Were found by compounds 7 (IC50 = 0.38 mM), 8 (IC50 = 0.19 mM), and 12 (IC50 = 0.26 mM) when compared with standard tyrosinase inhibitors kojic acid (IC50 = 0.67 mM) and L-mimosine (IC50 = 0.64 mM). The antioxidative effect of the isolated compounds and the extracts were determined by using, beta-carotene-linoleic acid, DPPH center dot scavenging, ABTS(+center dot) scavenging, superoxide scavenging, and CUPRAC assays. The experimental findings indicated that all the compounds demonstrated activity in all antioxidant activity tests employed except for the compounds 2-6. Compounds 2-6 showed moderate activity only in ABTS(+center dot) scavenging assay. The new compound 1 exhibited better activity than standard alpha-tocopherol in DPPH center dot scavenging, and ABTS(+center dot) scavenging assays. The results show that T. nigrescens can be regarded as a potential source of antioxidant compounds and tyrosinase inhibitors of significance in both the pharmaceutical and food industries.
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    Lignans From Taxus Species
    (Springer, 2007) Topcu, Gulacti; Demirkiran, Ozlem
    Lignans are widely distributed in nature and exhibit various activities, including antitumor, antiviral, hepatoprotective, antioxidant, antiulcer, antiallergen, antiplatelet and antiosteoporotic activities. So far, Taxus species have received a great deal of interest in regard to their taxane diterpenes rather than their lignans. This chapter will review lignan biosynthesis and recent strategies in the synthesis of lignans and provide an overview of isolation and structural elucidation studies of all Taxus lignans, along with their biological activities. About 50 lignans, including neolignans and a few terpenolignans, isolated from eight Taxus species, are presented herein. Recent studies on the activities of lignans, particularly Taxus lignans, are outlined.
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    Phenolic compounds from Trifolium echinatum Bieb. and investigation of their tyrosinase inhibitory and antioxidant activities
    (Pergamon-Elsevier Science Ltd, 2013) Sabudak, Temine; Demirkiran, Ozlem; Ozturk, Mehmet; Topcu, Gulacti
    Two bischromones, 3,3'-dimethoxy-2'-oxychromone (1), 3,3'-dihydroxy-2,2'-oxychromone (2) and a biflavone, 5,7,4',5 '',3''',4''''-hexahydroxy-3 ''-O-beta-glucosyl-3',7 ''-O-biflavone (3) have been isolated from whole plant of Trifolium echinatum Bieb. together with five known fiavonoids. The structures of the compounds were elucidated by 1D and 2D NMR analysis as well as HRESIMS. The isolated compounds were investigated for their antioxidant activity and tyrosinase inhibitory activity. Highly potent inhibition was found for compounds 1 (IC50 = 0.41 mM), 5 (IC50 = 0.47 mM) and 8 (IC50 = 0.45 mM) compared to those of standard tyrosinase inhibitors kojic acid (IC50= 0.67 mM) and L-mimosine (IC50 = 0.64 mM). The antioxidative effect of the extracts was determined by using beta-carotene-linoleic acid, DPPH. scavenging, ABTS(+-) scavenging, and CUPRAC assays. The experimental findings indicated that the compounds 2 and 8 were found to be active in radical scavenging and CUPRAC assays. (C) 2013 Elsevier Ltd. All rights reserved.
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    Structure elucidation of two new unusual monoterpene glycosides from Euphorbia decipiens, by 1D and 2D NMR experiments
    (Wiley-Blackwell, 2011) Demirkiran, Ozlem; Topcu, Gulacti; Hussain, Javid; Ahmad, Viqar Uddin; Choudhary, M. Iqbal
    Two new unusual monoterpene glycosides, (Z)-3,6-dimethyl-3-(beta-D-O-glucosylmethylene)cyclohept-4-ene-1-one (1) and 3,6-dimethyl-3-(beta-D-O-glucosylmethylene) cycloheptanone (2) have been isolated along with five known compounds, 3-hydroxy-4-methoxybenzoic acid, 6,7-dihydroxycoumarin, luteolin, apigenin 5-O-alpha l-L-rhamnoside, and pinocembrin-7-O-rutinoside from ethyl acetate extract of Euphorbia decipiens. The structures of the isolated compounds were elucidated by extensive 1D- and 2D-NMR, and mass spectroscopic analyses. Copyright (C) 2011 John Wiley & Sons, Ltd.
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    Tyrosinase Inhibitory Activity of Chemical Constituents of Euphorbia macrostegia
    (Springer, 2014) Demirkiran, Ozlem; Topcu, Gulacti; Azarpira, Ali; Choudhary, M. Iqbal
    The chloroform extract of the aerial parts of E. macrostegia has yielded a new compound, 2-(4-hydroxyphenyl)-ethylhentriacontanoate (1), along with hentriacontan-1-ol (2), lupenone (3), cycloart-22-ene-3,25-diol (4), 3,3',4'-tri-O-methylellagic acid (5), cleomiscosin (6), and glucoclioniasterol (stigmast-5-en-3-O-beta-glucopyranoside) (7), and from the ethyl acetate extract of aerial parts of the plant isoquercetin (8), quercetin-3-O-alpha-arabinofuranoside (avicularin) (9), and rutin (10) have been isolated. Compounds 1-10 were tested for their tyrosinase inhibitory activity, and 2 exhibited an IC50 value of 71.4 mu M, almost comparable to standard kojic acid (IC50 58.2 mu M). Compounds 1-4 also showed good activity with IC50 values of 77.2, 78.6, 71.4, and 77.5 mu M, respectively.