Use of fluorescence polarization to probe the structure and aluminum complexation of three molecular weight fractions of a soil fulvic acid

A soil fulvic acid isolated from a northern coniferous forest (NCFA) was fractionated by ultrafiltration into three different molecular sizes ranging from less than 500 to 10 000 daltons. The conformation changes of this soil fulvic acid, NCFA, and its molecular weight (MW) fractions were studied using fluorescence polarization as a function of pH and concentration of added ionic inorganic aluminum. Polarization values versus aluminum concentration increased for the lowest molecular weight fraction, whereas they decreased for the higher molecular weight fractions. The change in the polarization values suggests that the lower molecular weight fraction of the fulvic acid molecule does not change conformation when complexed to aluminum, whereas the higher molecular weight fractions changes conformation when complexed to aluminum. The fluorescence intensity versus aluminum concentration increased for all the fractions indicating that the rigidity of the fulvic acid molecule increases as aluminum binds to it. Stability constants were calculated for the aluminum-fulvate complexes using anisotropy values calculated from the polarization data. The log of the stability constants for the aluminum-fulvate complex varied between 4.9 to 5.2 over a pH range of 3.0 to 4.0. The log of the stability constants for the aluminum-fulvate MW fractions varied between 4.2 to 5.1 at a constant pH of 3.5.