*2.1. Characterization of the Phlorethols Isolated from Costaria costata*

Phlorotannins constitute an extremely heterogeneous group of molecules, which differ by the types of bonds between the phloroglucinol units. Fucols have only phenyl bonds in the polymer molecule, phlorethols and fuhalols have ether bonds, and fucophlorethols have both of these bond types. The presence of a dibenzodioxin element in the structure is specific for eckols. Within each class, the binding of monomers to each other can take place at different positions of the phloroglucinol ring, resulting in the formation of structural isomers. The separation of longer, condensed phlorotannins is currently a challenge. [11].

The phlorotannins were isolated from an aqueous ethanol extract of the brown alga *C. costata.* Earlier the structural characteristics of phlorethols from *C. costata* were determined [7]. In the present work, NMR analysis showed that the phlorotannins of *C. costata* belong to the class of phlorethols (**CcPh**). The average molecular weight of **CcPh** was 2520 Da, as measured by ESI-MS in negative ionization mode. The degree of polymerization (DP) of the obtained phlorethols ranged from 12 to 25 phloroglucinol units with the most abundant phlorethols containing between 16 and 20 phloroglucinol units. [7] (Figures S1 and S2, and Table S1).
