Hydroperoxyl Radical Scavenging Activity of Bromophenols from Marine Red Alga Polysiphonia urceolata: Mechanistic Insights, Kinetic Analysis, and Influence of Physiological Media.

Bromophenols (BPs), particularly those derived from marine sources, are known for their potent radical scavenging activity, effectively neutralizing reactive oxygen species (ROS). However, their exact mechanism of action remains largely unexplored, limiting our understanding of their potential as natural antioxidants. In this study, the antiradical mechanisms of two BP derivatives (1 and 2), previously isolated from the marine red alga Polysiphonia urceolata, were systematically investigated using thermodynamic and kinetic calculations. Both compounds demonstrated potent hydroperoxyl radical (HOO(•)) scavenging activity in polar and lipid environments, with rate constants surpassing those of the well-known antioxidant standards Trolox and BHT. In lipid media, BP 2 exhibited approximately 600-fold greater activity than BP 1, with rate constants of 9.75 × 10(5) and 1.64 × 10(3) M(-1) s(-1), respectively. In contrast, both BPs showed comparable activity in aqueous media, with rate constants of 3.46 × 10(8) and 9.67 × 10(8) M(-1) s(-1) for 1 and 2, respectively. Mechanistic analysis revealed that formal hydrogen atom transfer (f-HAT) is the predominant pathway for radical scavenging in both lipid and polar environments. These findings provide critical insights into the antiradical mechanisms of natural BPs and underscore the potential of BP 1 and BP 2 as highly effective hydroperoxyl radical scavengers under physiological conditions.