Chitosan-alginate polyelectrolyte complexes for encapsulation of low molecular weight fish bioactive peptides

Encapsulation of low molecular weight (M_w) bioactive peptides (BAPs) in electrostatically mediated polyelectrolyte complexes (PECs) of sodium alginate (ALG) and chitosan (CS) was studied, formed via a simple one-step mixing process. PECs were characterized via dynamic light scattering (DLS), mixed-mode phase analysis light scattering (M3-PALS), static light scattering (SLS), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The encapsulation efficiency (EE) and in vitro release of low M_w antihypertensive LKPNM and LKP BAPs (derived from fish hydrolysates) were measured, under both gastric and intestinal pH conditions, via high performance liquid chromatography (HPLC). Two different ALG were tested (M_w ≈ 8 and 21 kDa, with mannuronic: guluronic ratios 5.1 and 1.4, respectively), whilst the M_w and degree of acetylation (DA) of the CS were ≈111 kDa and ≈10 %, respectively. At the pH (5.5) of PEC formation the BAPs were positively charged. As the molar charge ratio of alginate to chitosan (n⁻/n⁺) was increased from 0.1 to 0.6 the PEC size reduced from ca. 800 to 300 ± 50 nm, indicating more compact structures, but increased again significantly to >10 μm around charge neutralisation (n⁻/n⁺ = 1) and net PEC ζ-potential swapping from +ve to –ve. The size then decreased again to between 1 and 10 μm as n⁻/n⁺ → 10, as expected if the more prevalent polysaccharide (ALG) coated the surface of the PECs. However, higher M_w ALG gave significantly smaller (more compact) and more highly negatively charged PECs for n⁻/n⁺ > 1. The PECs showed high (≈80%) EE but low (10–20%) release in gastrointestinal conditions, highlighting their potential as effective carriers of such BAPs.