Gum Arabic Interaction with Excited States of Riboflavin and Reactive Oxygen Species

BOIERO, LAURA; VALLE, LORENA; CRIADO, SUSANA; MONTENEGRO, MARIANA; GARCÍA, NORMAN A.; BORSARELLI, CLAUDIO

Mendoza

Conferencia; 21ST Inter American Photochemical Society; 2011

Inter American Photochemical Society

Gum arabic (GA) is the exudates obtained from acacia trees, Acacia senegal and Acacia seyal, which grows mainly in Sudan, Africa. Structural studies showed that GA is a group of macromolecules characterized by a high proportion of carbohydrates (97%) and low proportion of protein (1-3%).  In biological milieu, GA plays antioxidant activities, influences on lipid metabolism, and contributes positively on kidney, cardiovascular, and gastrointestinal diseases. On the other hand riboflavin (Rf) is present in many biological systems and can act as a photosensitizer to generate reactive oxygen species (ROS) producing photoinduced damage or photo-oxidation of biological molecular targets. In this work, we studied the antioxidant capacity of GA towards ROS generated chemically and by Rf photosensitization. In particular, we analyzed the GA interaction  with the triplet excited state of Rf, 3RF, and ROS such as superoxide anion radical, O2·-, hydroxyl radical, ·OH, and singlet molecular oxygen, 1O2. Laser flash photolysis experiments were performed using a laser pulse excitation at 355 nm to generate the 3RF, with transient absorption at 720 nm. 3Rf was efficiently quenched by GA, with a rate quenching constant kqRF-GA = 1.6´109 M-1s-1. Additionally, the total quenching and reactive rate constant of 1O2 by GA, i.e. kt,D = 2.7 x 107 M-1s-1 and kr,D = 7.6 x 104 M-1s-1, respectively were obtained by actinometry, and indicated that GA is a good catalytic (physical) quencher of 1O2. Finally, the GA ability to scavenging both ·OH and, O2·- was investigated. The Fenton reaction was used to generate ·OH and detected by UV-Vis spectroscopy at 532 nm due to formation of an adduct between malonaldehyde with thiobarbituric acid. The O2·- scavenging ability determination was based on GA ability to inhibit the Monoformazan (MF+) formation (lmax = 530 nm) by reducing Nitrotetrazolium Blue (NBT2+). In both cases, the GA inhibition percentage using Trolox as reference antioxidant, yielded Trolox Equivalent Antioxidant Capacity (TEAC) values of 0.71 and 1.82 for deactivation of ·OH and O2·- respectively. The present results show that GA may act as an effective antioxidant, particularly in Rf photo-sensitized processes, since prevent the formation of ROS species by quenching of 3RF, and also deactivates ROS (1O2 and O2·-) efficiently. This antioxidant functionality is associated with the small protein moiety of GA by the action of the amino acid residues such as histidine, tyrosine, and proline.