Enhanced Antioxidant Activity under Biomimetic Settings of Ascorbic Acid Included in Halloysite Nanotubes.

Andrea Baschieri,Luca Valgimigli,Laura Mazzocchetti,Riccardo Amorati,Emanuele D’Angelo,Tiziana Benelli

doi:10.3390/antiox8020030

Abstract

Antioxidant activity of native vitamin C (ascorbic acid, AH2) is hampered by instability in solution. Selective loading of AH2 into the inner lumen of natural halloysite nanotubes (HNT) yields a composite nanoantioxidant (HNT/AH2), which was characterized and investigated for its reactivity with the persistent 1,1-diphenyl-2-picrylhydrazyl (DPPH•) radical and with transient peroxyl radicals in the inhibited autoxidation of organic substrates, both in organic solution (acetonitrile) and in buffered (pH 7.4) water in comparison with native AH2. HNT/AH2 showed excellent antioxidant performance being more effective than native ascorbic acid by 131% in acetonitrile and 290% (three-fold) in aqueous solution, under identical settings. Reaction with peroxyl radicals has a rate constant of 1.4 × 106 M−1 s−1 and 5.1 × 104 M−1 s−1, respectively, in buffered water (pH 7.4) and acetonitrile, at 30 °C. Results offer physical understanding of the factors governing HNT/AH2 reactivity. Improved performance of HNT/AH2 is unprecedented among forms of stabilized ascorbic acid and its relevance is discussed on kinetic grounds.

Highlights

L-Ascorbic acid (AH2 ) is, possibly, the most important water-soluble radical-trapping antioxidant in biological systems [1,2]
The composite solid halloysite nanotubes (HNT)/AH2 was characterized by thermogravimetric analysis (TGA) with the aim to assess the real content of the antioxidant within the modified halloysite, as reported in the recent literature for similar products [33,43,44,45]
Since halloysite tends to adsorb water, whose actual amount depends on the storage condition, each sample was preliminarily heated under nitrogen atmosphere from room temperature up to 130 ◦ C and held at this temperature for 40 min

Summary

Introduction

L-Ascorbic acid (AH2 ) is, possibly, the most important water-soluble radical-trapping antioxidant in biological systems [1,2]. Humans cannot biosynthesize it from glucose due to inactivation of the gene encoding the enzyme L-glucono-γ-lactone oxidase; ascorbic acid is an essential nutrient, known as vitamin C [3]. Following isolation in 1928, its fundamental biological function became progressively clear, starting from the prevention of scurvy [8]. Ascorbic acid is essential to the biosynthesis of collagen and other biomolecules, and it protects from several chronic diseases including neurodegenerative conditions, heart disease, eye disease, and can serve as an adjuvant in the treatment of cancer [8,9]. Emerging evidence highlights its role in regulating the epigenome [10]

Methods

Results

Conclusion