Interaction of chlorogenic acid with milk proteins analyzed by spectroscopic and modeling methods

Polyphenols are powerful antioxidants implicated in reducing the risk of human cancer and cardiovascular disease. Chlorogenic acid is a principal polyphenol in coffee, a beverage consumed worldwide on an immense scale. In many countries, coffee is consumed with milk, which has been shown to affect the bioavailability of polyphenols. Here we assessed the interactions of chlorogenic acid with five major milk proteins (alpha-casein, beta-casein, kappa-casein, alpha-lactalbumin, and beta-lactoglobulin) by spectroscopy and molecular docking calculations. The data showed that the number of binding sites in each chlorogenic acid-milk protein complex was close to 1. Fluorescence quenching of milk proteins by chlorogenic acid occurred through a static mechanism and the binding distance was smaller than 8 nm. Binding constant for chlorogenic acid-beta-lactoglobulin was larger than those for chlorogenic acid-alpha-lactalbumin, chlorogenic acid-alpha-casein, chlorogenic acid-beta-casein, and chlorogenic acid-kappa-casein. Thermodynamic parameters revealed that Van der Waals forces and hydrogen bond interactions predominated in chlorogenic acid-alpha-casein and chlorogenic acid-alpha-lactalbumin complexes; hydrophobic interactions were predominant in chlorogenic acid-beta-casein, chlorogenic acid-kappa-casein, and chlorogenic acid-beta-lactoglobulin. Molecular docking calculations identified chlorogenic acid was located near Pse66, Ile65, and Pro29 in the chlorogenic acid-alpha-casein adduct, Leu138, Thr126, Gln123, Ser124, Gln167, Ser166, Ser168 in the chlorogenic acid-beta-casein adduct, Glu147, Asn123, Val143, Pse149, Pro120, Leu79, Ala148 in the chlorogenic acid-kappa-casein adduct, Glu49, Tyr103, Gln54, His32, Trp104, Leu110 in the chlorogenic acid-alpha-lactalbumin adduct, and Cys66, Leu22, Lys60, Trp61, Ser21, Lys69, Gln59 in the chlorogenic acid-beta-lactoglobulin adduct. Notably, the antioxidant activity of chlorogenic acid decreased significantly on interaction with each of the milk proteins (p< 0.05). This study reveals the binding behaviors of chlorogenic acid with five milk proteins and provides basic data that can help to clarify the binding mechanism.