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It is shown that the relationship between the alkyl chain length of 3-alkyl-3',4',5,7 tetrahydroxyflavones (FnH) bearing alkyl chains of n=1, 4, 6, 10 carbons and their capacity to counter oxidative damage varies markedly with the nature of the biological system. In Cu(2+)-induced lipid peroxidation of low-density lipoprotein (LDL), the less hydrophobic short-chain F1H and F4H are probably located in the outer layer of LDL and parallel the reference flavonoid antioxidant, quercetin (Q) as effective inhibitors of lipid peroxidation. A marked inhibition of haemolysis induced in red blood cells (RBC) suspensions by the membrane-permeant oxidant, tert-butylhydroperoxide (t-BuOOH), is observed with F4H and F6H present at concentration in the micromolar range. However, F10H the most hydrophobic FnH is even more effective than Q against both haemolysis and lipid peroxidation as measured by malondialdehyde (MDA) equivalents. In oxidation of RBC by H(2)O(2,) at least 50 times more F6H and F10H than by t-BuOOH are required to only partly inhibit haemolysis and MDA production. The F1H, F4H and Q are found rather inactive under these conditions. At concentrations in the micromolar range, a marked protection against the cytotoxic effects of the t-BuOOH-induced oxidative stress in human skin NCTC 2544 keratinocytes is also exhibited by the four FnH antioxidants and is comparable to that of Q. Thus, the four FnH species under study may be considered as potent antioxidants which manifest complementary anti-oxidative actions in biological systems of markedly different complexity.

Four polyhydroxylated 2-styrylchromones, structurally related to flavones and cinnamic acid, have been studied. An SC derivative with OH groups only at positions 3' and 4' on the styryl moiety and another SC bearing an additional OH group at position 5 on the benzopyrone ring were more potent inhibitors of the Cu2+-induced peroxidation of LDL than the flavonoid quercetin. Fluorescence and absorption spectroscopies suggested that one LDL particle may bind 40 SC molecules. A pulse radiolysis study in pH 7 buffered micellar solutions of neutral TX100 and positively charged CTAB demonstrated that one-electron oxidation by *Br2-, *O2- and tryptophan radicals (8Trp) depends strongly on the micellar microenvironment. All SCs were readily oxidized by *O2- in CTAB micelles (rate constants: 6-18 x 10(6) M(-1) s(-1)). In TX100 micelles only the SC derivative with OH groups in position 3' and 4' reacted with *O2- (rate constant: 1.1 x 10(6) M(-1)s(-1)). In CTAB, electron transfer to *Trp radicals was observed for all SCs with rate constants > or =3.2 x 10(7) M(-1) s(-1). In TX100 micelles, this reaction occurred solely with the derivative bearing OH groups only at positions 3' and 4'.