Application of PRODAN fluorescent probe to measure surface hydrophobicity of proteins interacting with κ-carrageenan

The fluorescent probe, 6-propionyl-2-(N-N-dimethylamino)-naphthalene or PRODAN, was applied to measure surface hydrophobicity (S0) of proteins interacting with polysaccharides. Whey protein isolate (WPI), β-lactoglobulin (BLG), and bovine serum albumin (BSA) were used as model systems in combination with κ-carrageenan (KCG) at KCG:protein ratios of 1.0:1.2 (high ratio), 1.0:6.0 (medium ratio), and 1.0:62.5 (low ratio). Heating (80°C for 30 min), pH (3.0, 5.0, 7.0 and 9.0) and addition of KCG at low, medium or high ratio had significant effects (P≤0.05) on S0 value of each protein. Interactions between pH, heating and KCG addition also had significant effects on S0. In general, the proteins had higher S0 at pH 9.0 and were more sensitive to the effects of heating and KCG addition at pH 9.0 than at other pH. For WPI, KCG increased S0 only when added at high ratio at pH 9.0 or at high or medium ratio at pH 5.0. In contrast, decreases in S0 of BLG and BSA at pH 9.0 were more pronounced at the lower KCG ratio. Heating KCG:protein mixtures generally increased S0 at pH 3.0 for high ratio mixtures, and decreased S0 at higher pH, especially pH 9.0. The S0 values of heated mixtures differed from proteins heated alone under some conditions; however, the nature of these changes depended on the protein, pH and KCG:protein ratio. This study demonstrates the usefulness of PRODAN to monitor changes in surface hydrophobicity resulting from interactions of proteins with κ-carrageenan under varying pH and heat treatments.