Chemical Modification of the Tryptophan Residue in a Recombinant Ca²⁺-ATPase N-domain for Studying Tryptophan-ANS FRET

ANS binds to the Ca²⁺-ATPase recombinant N-domain. Fluorescence spectra display a FRET-like pattern upon excitation at a wavelength of 295 nm. NBS-mediated chemical modification of Trp quenches the fluorescence of the N-domain, which leads to the absence of energy transfer (FRET) between the Trp residue and ANS.