Conformational analysis of ulvans from Ulva fasciata and their anticoagulant polycarboxylic derivatives

This study investigates conformational aspects of ulvans (F2) and their polycarboxyl derivatives obtained through periodate–chlorite oxidation (C3) followed by DEAE-Sephacel fractioning (C3b and C3c). Size exclusion chromatography coupled with laser light scattering and viscometric detection, in addition to circular dichroism (CD) and molecular modeling analyses, suggested that F2 had a compact sphere conformation with a helical motif as secondary structure. In contrast, all the polycarboxyl ulvans showed a random coil conformation, although C3c (NaSO₃⁻ 21.0%; COO⁻ 1.81 mmol·g⁻¹; M_w 18 kg·mol⁻¹) had a more rigid and constrained backbone than C3 (NaSO₃⁻ 21.0%; COO⁻ 1.81 mmol·g⁻¹; M_w 49 kg·mol⁻¹), largely due to its higher sulfate and carboxyl content. Despite the higher ionic character of C3c, its anticoagulant activity (ACA), determined by activated partial thromboplastin time (APTT) assay, was not improved compared to that of C3. Moreover, C3b (NaSO₃⁻ 14.1%; COO⁻ 1.23 mmol·g⁻¹; M_w 8.1 kg·mol⁻¹) had higher activity than F2 (NaSO₃⁻ 20.6.%; COO⁻ 0.36 mmol·g⁻¹; M_w 123 kg·mol⁻¹), even with its lower sulfate content and molar mass. These results suggest that the ACA of polycarboxyl ulvans relies on carboxyl and sulfate content and may depend, in addition, on a proper flexible conformation.