Binding of lambda carrageenan to bovine serum albumin and non-equilibrium effects of complexation

Binding of lambda carrageenan (lCG) to bovine serum albumin (BSA) and the effect of the formation path, namely mixing (M) at specific pH versus titration (T) from a neutral mixture, on complexation have been studied using turbidimetry, phase analysis, dynamic light scattering, confocal laser scanning and bright-field optical microscopy, circular dichroism and fluorescence measurements. The sizes and shape of the complex particles formed under T-conditions differ significantly from those obtained under M-conditions, and at maximal complexation T-complexes contain significantly less lCG as compared to M-complexes. At a pH below pI(BSA) phase analysis reveals two domains of q corresponding to an approximately constant lCG/BSA weight ratio in the complex phase q* (domain I), and a decreasing q* (domain II). Binding stoichiometries at pH 4.2 and 4.7 are 1:77 and 1:119 mole lCG/mole BSA. The yield of BSA in the complex phase is higher than 98% at pH 4.2 and 4.7, and the complexes form and are stable in the presence of 1.0M-1.5M NaCl. The interaction did not lead to protein denaturation or significant changes in the tertiary and secondary structure and illustrates an enhancement of the hydrophobicity of the protein within the complex particles. At pH 5.2 (pH on the wrong side of PIBSA) up to 37.6% BSA was bound to lCG, and the weight ratio of lCG/BSA in the complex particles (q*) was weakly dependent on q in the complete q range studied. The molecular origin of the unusually strong interaction between lCG and BSA is discussed.

Automated summary

This study investigated the binding of lambda carrageenan (lCG) to bovine serum albumin (BSA) and the effect of the formation path on complexation. The results showed that the formation path significantly influenced the size and shape of the complexes. pH also played a role in the formation and stability of the complexes.