title: Protein encapsulation by humic substances.
abstract: Protein encapsulation by natural organic matter is hypothesized to preserve the activity of proteins in terrestrial and aquatic environments. Direct molecular-level evidence for encapsulation of net positively charged proteins lysozyme, trypsin, and ribonuclease A by a diverse set of humic substances (HS) in nanostructured films was collected using a combination of optical waveguide lightmode spectroscopy and quartz crystal microbalance measurements. The results suggest that protein-HS electrostatic attraction drives encapsulation of positively charged lysozyme by a soil humic acid at pH 5 to 8 and by six additional humic and fulvic acids from terrestrial and mixed terrestrial aquatic sources at pH 5 and 6. Encapsulation of trypsin and ribonuclease A, which had negatively charged surface patches under the studied conditions, suggested that localized protein-HS electrostatic repulsion is overcompensated by attractive forces, likely including contributions from the hydrophobic effect. Evidence is provided showing that encapsulation of lysozyme at pH 8 and of ribonuclease A at pH 5 and 6 involved partial disassembly of HA supramolecular associations. This work advances a molecular-level picture of protein encapsulation by HS and presents a novel approach to study the effects of encapsulation on protein enzymatic activity and susceptibility to abiotic and biotic transformations.
authors.affiliation: Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Zurich, Switzerland.
authors.forename: Jeanne E
authors.initials: JE
authors.lastname: Tomaszewski
authors.forename: René P
authors.initials: RP
authors.lastname: Schwarzenbach
authors.forename: Michael
authors.initials: M
authors.lastname: Sander
mesh_terms: D001711:Biotransformation; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D009113:Muramidase; D049329:Nanostructures / Q000737:chemistry*; D011506:Proteins / Q000737:chemistry*; D058525:Quartz Crystal Microbalance Techniques; D012259:Ribonuclease, Pancreatic; D013057:Spectrum Analysis; D055672:Static Electricity; D014357:Trypsin
chemical_list: D006812:Humic Substances; D011506:Proteins; D012259:Ribonuclease, Pancreatic; D009113:Muramidase; D014357:Trypsin