Probing the stability of the food colourant R-phycoerythrin from dried Nori flakes

Abstract

The high content of vitamins, minerals, antioxidants, and proteins makes red algae Porphyra sp. (Nori) superfood with exceptional health-promoting benefits. Its intense colour originates from R-phycoerythrin (R-PE), phycobiliprotein containing covalently attached tetrapyrrole chromophores: red phycoerythrobilin and orange phycourobilin. The present study aims to characterize the stability of R-PE, a natural colourant with a high potential for application in the food, cosmetic, and pharmaceutical industries. We purified R-PE from dried Nori flakes with a high purity ratio (A560 /A280 ≥5). Far-UV CD spectroscopic showed that α-helix is the dominant secondary structure (75%). The thermal unfolding of α-helix revealed two transitions (Tm1 and Tm2 at 56 and 72°C, respectively), ascribed to the different subunits of R-PE. Absorption measurements showed that high pressure (HP) induces dissociation of R-PE into subunits followed by subunit unfolding. Contrary to temperature, HP treatment showed a significant advantage under applied conditions: the protein unfolding is partly reversible, and the R-PE colour bleaching is minimized. Based on the fluorescence quenching approach, R-PE's binding affinities for Cu2+ and Zn2+ ions were 6.27x105 and 1.71x103 M-1, respectively. Absorption and near-UV/VIS CD spectroscopy suggested conformational changes in protein chromophores upon metal ions binding. Far-UV CD spectroscopy did not reveal that metal binding affects R-PE structure. The obtained results give new insights into the stability of R-PE with a good use-value in replacement of toxic synthetic dyes, preservation of R-PE red colour in fortified food and beverages by HP processing, and as a biosensor for Cu2+ in aquatic life systems. Acknowledgments: This study was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, Contract number: 451-03-9/2021-14/200168 and the European Commission, under the Horizon2020, FoodEnTwin Project, GA No. 810752.