Stabilizing Effect of Chitosan on Curcumin from the Damaging Action of Alkaline pH and Ultraviolet Light

The naturally available biopolymer, chitosan (MW = 10 kD) binds to yellow herbal spice, curcumin with high affinity (K_a ~ 400 μM) and moderate capacity (n∼20 Mole/Mole of Chitosan) at considerably high pH (pH∼10.5, 0.1 M NaBO₃). Binding is pH sensitive and reduces nearly to zero at the acidic range (pH∼5.0). The presence of high salt (0.1 M-2.0 M NaCl) does not alter the binding affinity (K_a) rather increases the capacity ∼12 %. Interestingly, the affinity (K_a) remains unaltered even when the chitosan was coated on powdered Talc surface causing ∼ 40% reduction in binding capacity (n). The chitosan-curcumin complex formed at high pH shows remarkable stability at pH 7.0 - 10.5 and in high salt concentrations (1.0 M - 4.0 M NaCl) showing least dissociation effect. Lowering the pH (<7.0) enables the complex to dissociate efficiently. The bound curcumin remains chemically unaltered when analyzed by high-pressure liquid chromatography (HPLC). As per logical expectation, the glucosamine unit within chitosan molecule participates in the binding process as evidenced by dose-dependent enhancement of optical density (O.D.) (λ = 440 nm) of curcumin in the presence of glucosamine at pH - 8.5. The thin layer chromatography (TLC) of glucosamine-curcumin complex confirms the chemically unaltered state of curcumin within the complex. The enhancement of O.D. in basic environment by glucosamine was unnoticed in acidic condition, pH ∼ 4.8. Perhaps, protonation (-NH₃⁺) of the - NH₂(s) within sugar moiety in acidic environment hinders curcumin to interact. As for interest, the complexed curcumin molecule in chitosan or glucosamine acquires substantial stability from the alkaline pH (∼10.5) or UV damage (λ∼240 nm).