Insights about α-tocopherol and Trolox interaction with phosphatidylcholine monolayers under peroxidation conditions through Brewster angle microscopy

• Langmuir monolayers with phosphatidylcholines were applied as model of biomembranes.
• Brewster angle micrographs and π–A isotherms were examined after peroxidation.
• Trolox, α-tocopherol and peroxidation products induced changes on lipid domains.
• π–A isotherms showed different profile upon peroxidation conditions.

Membranes are major targets to oxidative damage, particularly due to lipid oxidation, which has been associated to aging. The role, efficacy and membrane interaction of antioxidants is still unclear, requiring further understanding of molecular interaction. Hence, the objective of this work was to evaluate the interaction between antioxidants (α-tocopherol and its aqueous soluble analog Trolox) and the monolayer formed by phosphatidylcholine molecules at air/liquid interface upon peroxidation conditions, promoted by peroxyl radicals from thermal decomposition of 2,2′-azobis(2-methylpropionamidine) (AAPH). The interaction with three different monolayers, containing (i) 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), (ii) DDPC + α-linolenic acid, or (iii) egg yolk l-α-phosphatidylcholine (EPC), was ascertain by surface pressure (π)–molecular area (A) isotherms and by monitoring monolayer features through Brewster angle microscopy (BAM). The interaction of antioxidants with DPPC monolayers was confirmed by modifications on DPPC domain shape for α-tocopherol and through the maintenance of typical multilobed domain shape during an extended surface pressure interval for Trolox. Under peroxidation conditions, BAM images showed a clear interaction between components of AAPH subphase with the monolayer through changes on DPPC domain shape and appearance of white dots, located mainly at the frontier between the condensed and expanded liquid phases. White branched structures were also observed whenever both α-linolenic acid and α-tocopherol were present, indicating the segregation of these components within the monolayer, which is highly significant in biological systems. For EPC monolayers, no information from BAM was obtained but π–A isotherms confirmed the existence of the same interactions observed within the other two monolayers.

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