The Effects of Conformational Changes on the Native Fluorescence of Aqueous Humic Materials

Aims: To elucidate the effect of induced conformational changes on the native fluorescence of aqueous humic materials.

Study Design: The conformation of dissolved humates was changed by adjustment of a variety of environmental factors and the resulting fluorescence emission, excited at 240nm was monitored in the 300-465nm range.

Place and Duration of Study: Department of Chemistry, University of Idaho, Moscow, ID, USA; April to August, 2012

Methodology: The fluorescence spectra of a number of humic and fulvic acids in different solution environments were measured with a photon counting fluorimeter. Attention was focused on the emission range centered on 400 nm and the intensity of the peak observed in this region was interpreted in terms of conformational changes.

Results: The addition of a multivalent cations produced distinct changes in the native fluorescence of dissolved humic materials that had otherwise broad and featureless emission spectra. A series of divalent cations were found to produce these emission changes. Microaggregation brought about by alternative causes, such as changes in pH, concentration, and solvent, produced similar outcomes. Chain length and rigidity of the humic polyanions also had significant effects.

Conclusion: The appearance and variation of a 400-nm emission peak was rationalized by invoking the formation of pseudomicellar structures that incorporated the emitting entity and provided limited access to water.