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 (S-0) of proteins interacting with polysaccharides. Whey protein isolate (WPI), beta -lactoglobulin (BLG), and bovine serum albumin (BSA) were used as model systems in combination with K-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 degreesC 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 less than or equal to 0.05) on S-0 value of each protein. Interactions between pH, heating and KCG addition also had significant effects on S-0. In general, the proteins had higher So 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 So only when added at high ratio at pH 9.0 or at high or medium ratio at pH 5.0. In contrast, decreases in So of BLG and BSA at pH 9.0 were more pronounced at the lower KCG ratio. Heating KCG:protein mixtures generally increased S-0 at pH 3.0 for high ratio mixtures, and decreased So at higher pH, especially pH 9.0. The So 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 K-carrageenan under varying pH and heat treatments. (C) 2001 Published by Elsevier Science Ltd. All rights reserved.