GE) Marco Landi, Laura Estevez, Ricardo Mosquera, Lucia Guidi

Marco Landi 1 , Laura Estevez 2 , Ricardo Mosquera 2 , Lucia Guidi 1

1 Departimento di Scienze Agrarie, Alimentari e Agro-Ambientali, Università di Pisa, Pisa, Italia 2 Departamento de Química Física, Universidade de Vigo, Galicia, Spain

In previous experiments, “Red Rubin” basil plants resulted more tolerant to B stress. Among other tolerance mechanisms, the possibility of anthocyanins (ACNs) to chelate B ions, thereby boosting their sequestration into the vacuole and reducing their hazardous effect on cellular metabolism, was proposed. However, while the metal-chelation capacity of ACNs has been already described, the possibility of ACNs to chelate metalloid ions (ME) have been never tested before.

In the present work, cyanidin-based ACNs isolated from “Red Rubin” leaves were added to ME salts (H 3 BO 3 for B; GeO 2 for Ge) at different pH (5, 6 and 7) and different ACN-ME stoichiometric ratios (1:1, 1:10, 1:100 and 1:500) to evaluate possible bathochromic and hypochromic shift with UV-vis transmittance spectroscopy, as an evidence of ACN:ME complex formation. In order to provide further information on the molecular structure of the ACN:ME complexes, density functional theory (DFT) and time-dependent functional theory (TD-DFT) calculations were performed. For each of the computed ACN-ME complex, energy of formation has been calculated to estimate the most stable one and to compare Ge and B complexes.

According to the literature, GeO 2 in solution is present as Ge(OH) 4 at the pH values explored experimentally, while boric acid is as B(OH) 3 . Possible complexes of cyanin-based ACNs with B and Ge ions have been explored bearing in mind the following considerations: i) cyanin acts as a bidentate ligand through the catechol moiety at the B-ring (O3’ and O4’); ii) only 1:1 complexes have been explored; iii) only those processes where neutral water is released were considered.

Addition of both Ge and B salts to ACNs solutions caused both bathocromic shift and hyperchromic effects on UV-vis spectra. The effect is particularly pronounced at pH 5 and 1:500 (ACN:ME) ratio. Raising pH from 5 to 7, λ max is shifted to longer wavelengths. Overall, Ge showed a higher capability to bind to ACNs than B ions.

Complexation reactions considered here between ACNs and B or Ge ions led to different a1-a3/b1-b3 complexes. Calculated ΔE solv and ΔG solv values indicate that complexes labelled as a1-a3 displays positive DE values, which would indicate that formation of these complexes are not expected. In contrast, b2 and b3 and b1 formations are exothermic and exergonic processes. Also, it is clearly shown that complexation with Ge ions is highly favored over B ones when comparing same processes.

In conclusion, the present experiment highlights for the first time the possibility of ACNs to bind ME ions. This is supportive for the possibility that these flavonoids contribute in ameliorating the performances of B-stressed “Red Rubin” plants through complexing excessive B ions.