Fluorescence polarization studies of the conformation of soil fulvic acid

The conformation of soil fulvic acid was studied by fluorescence polarization as a function of pH, concentration, and ionic strength. Rotational relaxation times were measured from the slopes of plots of polarization vs. the ratio of temperature to viscosity. The rotational relaxation time did not change over the pH range 5–8 or over the concentration range 3.3 X 10^-5–3.3 × 10^-4 M fulvic acid. This suggests that fulvic acid does not aggregate or change conformation. Changes in ionic strength also do not cause a measurable change in rotational relaxation time. A net rotational relaxation of 2.0 ns was calculated for fulvic acid. This value is based on a fluorescence lifetime of 2.1 ns which is the best single exponential fit to the observed fluorescence decay. A molecular weight of 2400 was calculated for fulvic acid assuming that fulvic acid is spherical. The discrepancy between this value and the true number-average molecular weight of 990 suggests that fulvic acid exists in a flat extended conformation.