The Kinetics of the Acid-Induced Conformational Change of Influenza Virus Hemagglutinin Can Be Followed Using 1,1′-bis(4-Anilino-5-naphthalenesulphonic Acid)

1,1′-Bis(4-anilino-5-naphthalenesulphonic acid) (bis-ANS) has been shown by fluorescence spectroscopy to bind to bromelain-cleaved influenza haemagglutinin (BHA). The fluorescence intensity of 1.2μM bis-ANS in the presence of BHA in its low-pH conformation is twenty-fold higher than in the presence of BHA in its neutral-pH conformation. The use of this probe provides a sensitive method for investigating the kinetics of the irreversible conformational change of BHA induced by low pH. At pH5.0 the reaction is described by a rapid burst followed by a double exponential increase in the fluorescence of bis-ANS, with rate constants of 5.2±0.9×10−3sec−1 and 6.7±1.9×10−4sec−1. This reaction is sensitive to the presence of tert-butylhydroquinone, an inhibitor of the conformational transition of BHA. The dependence of the reaction rate on pH indicates that the acid-induced conformational change is dependent upon the multiple protonation of the neutral-pH conformation of BHA.