Antioxidant Radical Scavenging Properties of Phenolic Pent-4-En-1-Yne Derivatives Isolated From Hypoxis Rooperi. A DFT Study in vacuo and in Solution

B3LYP/6-311+G* calculations were performed on five phenolic pent-4-en-1-yne derivatives, namely rooperol, dehydroxyrooperol I and II, bis-dehydroxyrooperol, and hypoxoside, to investigate and compare their antioxidant radical scavenging properties. The main objectives were to model, for the first time, the antioxidant properties of dehydroxyrooperol I and II and bis-dehydroxyrooperol, to clarify the possible role of hypoxoside as radical scavenger and to provide insight into molecular geometry factors influencing the radical scavenging properties of phenolic pent-4-en-1-yne derivatives. The study was conducted by checking the molecules' ability for two main antiradical mechanisms, hydrogen atom transfer and electron transfer. The results indicated that the antiradical properties of rooperol and the two dehydroxyrooperols are predominantly exerted through the hydrogen atom transfer mechanism and are the outcome of the stabilization of their neutral radical species through the interplay of hydrogen bond(s) and spin density distributions stabilizing their neutral radical species. The antiradical properties of bis-dehydroxyrooperol may be explained only in terms of the spin density distribution of its neutral radical species. The antiradical properties of hypoxoside, with O-H bond dissociation enthalpy considerably greater than that of phenol, could be considered to be minor and mainly governed by electron transfer mechanism.