Extraction, characterization, and antimicrobial potential of Kappa-Carrageenan from Kappaphycus alvarezii with HasA-targeted molecular docking validation.

Kappa-carrageenan is a hydrocolloid from red algae and has bioactive properties for food, pharmaceutical, and biomedical uses. The present study comprises efficient extraction of κ-carrageenan from Kappaphycus alvarezii with a yield of 52.3 ± 2.1% (dry weight) using a mild alkaline method (CA) with superior viscosity (0.285 ± 0.018 Pa·s) compared to commercial carrageenan (CC) (0.198 ± 0.012 Pa·s) with gel strength (85 ± 16.4 g/cm²). Sulfate analysis revealed lower content in the extracted sample (11.02 ± 0.90 mg/g, 3.0 wt% S) than the commercial carrageenan (14.87 ± 0.70 mg/g; 4.5 wt% S). Whereas higher total phenolic content (25.79 ± 0.31 mg GAE/g) was observed in the extracted carrageenan. Structural characterization confirmed slightly higher crystallite size in the extracted carrageenan (~57.3 nm), which improved molecular ordering. Morphological and particle size analyses showed larger but more uniform particles in CA (154.25 ± 26.6 μm) from SEM analysis, dynamic light scattering size (687.6 nm), and greater colloidal stability (-32.9 mV). Antibacterial evaluation demonstrated strain-specific bactericidal activity against S. marcescens and Staphylococcus aureus with protein leakage and SEM-based membrane damage. Molecular docking further revealed binding of κ-carrageenan to the HasA heme-binding protein (-6.9 kcal/mol), suggesting a putative secondary mechanism involving computationally predicted interference of bacterial iron acquisition, which needs experimental validation.