Hydrophobic Interactions between Spin-Label 5-SASL and Humic Acid As Revealed by ESR Spectroscopy

• Published in: Environmental science & technology Vol. 35; no. 4; pp. 761 - 765
• Main Authors: Ferreira, Julieta A, Nascimento, Otaciro R, Martin-Neto, Ladislau
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.02.2001
• Subjects: Acids; Applied sciences; Biological and physicochemical phenomena; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemistry; Earth sciences; Earth, ocean, space; Electron Spin Resonance Spectroscopy; Engineering and environment geology. Geothermics; Exact sciences and technology; Humic Substances - chemistry; Hydrogen-Ion Concentration; Natural water pollution; Pollutants; Pollution; Pollution, environment geology; Scientific imaging; Soil and sediments pollution; Soils; Spectrum analysis; Stearic Acids - chemistry; Water treatment and pollution; Organic matter; Nitroxyl; Pollutant behavior; Inorganic free radical; EPR spectrometry; Hydrophobic interaction; Soil pollution; Intermolecular interaction; Carboxylic acid; Saturated fatty acid; Medium effect; Spin label; Model compound; Reaction time; pH; Water pollution; Stearic acid; Humic acid; Organic compounds
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es0010251

The spin-label probe 5-SASL (stearic acid spin-label with nitroxide free radical in position 5 of hydrocarbon chain), detectable by electron spin resonance (ESR), was tested to evaluate pH and reaction time dependencies of hydrophobic interactions with humic acid (HA). Strong changes were observed in 5-SASL ESR spectra in the presence of HA suspensions below pH 5, with disappearance of the three isotropic narrow hyperfine lines of the nitroxide group (typical of free spin-label) and formation of “immobilized” 5-SASL spectra. These changes were interpreted as due to 5-SASL bonding with hydrophobic groups of HA, by van der Waals forces and/or hydrogen bonds, in very hydrophobic sites (probably water-protected) existent in HA below pH 5. However, such sites are absent above pH 5, as demonstrated by a specific experiment to check 5-SASL spectra reversibility. On the other hand, the HA suspension was more efficient in dissolving 5-SASL than water above pH 5. This fact also suggests the existence of “surface” hydrophobic sites, where the spin-label binds to HA while maintaining the nitroxide group in contact with water, as evidenced by the typical free spin-label spectrum and hyperfine interaction splitting (a 0= 1.574 mT). Also experiments checking 5-SASL reversibility bonding with HA were consistent with the supramolecular association model to HA.