Investigating the Location of Propyl Gallate at Surfaces and Its Chemical Microenvironment by ¹H NMR

Abstract

The location and the resulting chemical microenvironment of the antioxidant propyl gallate (PG) was studied in micellar solutions using the cationic emulsifier cetyl trimethyl ammonium bromide (CTAB), the anionic emulsifier sodium dodecyl sulphate (SDS) and the non-ionic emulsifier Brij 58 (polyoxyethylene-20-cetyl ester). T (1) relaxation time of the aromatic protons of PG was investigated in micellar solutions and compared with that in aqueous solution in the absence of emulsifier. The relaxation time of the PG portion that is solubilized in the micelle (T (1,eff)) was calculated from the partition behavior of PG in micellar solution. From the 1D-(1)H spectrum, the alteration in the electron density of the aromatic protons and the alteration in the peak shape of the emulsifier headgroup and alkyl chain proton signals were indicative of the location of propyl gallate in the different micelles. Nuclear Overhauser effects (NOE) made it possible to deduce the exact location of PG by calculation of the relative NOEs. Marked differences were found for the location of PG in CTAB, SDS and Brij 58 micelles. PG was found to be located in the palisade layer of CTAB micelles, in the region of the polyoxyethylene chain of Brij micelles and in the Stern layer of SDS micelles. For careful study of the location of antioxidants and therefore to be able to characterize the chemical microenvironment of the antioxidants is crucial for understanding differences in antioxidant activities as a function of lipid surfaces. The application of spectroscopic methods may help to optimize the antioxidant activity to inhibit lipid oxidation at surfaces that are formed in a wide range of foods (emulsions), cosmetics, pharmaceuticals (emulsions and carrier systems) and of biological membranes (LDL-particles).

The Location of Phenolic Antioxidants and Radicals at Interfaces Determines Their Activity

“…As we found in studies investigating the location of antioxidants in dispersed systems [25,31] the main proportion of these gallates partitioned into the CTAB emulsifier pseudophase and was solubilized in the palisade layer of CTAB (see caption of Fig. 14).…”

“…After 5 min the reaction had reached completion [11] and the spectra were able to be recorded under the following conditions: centerfield: 3464.45 G, sweep width: 30.0 G (40.0 G in ethanol; The ethanolic solution was not deoxygenated for better handling, therefore the spectrum was getting broader), microwave frequency: approx. 9.73 GHz, microwave power: 2.01 mW, modulation frequency: 100 kHz, modulation amplitude: 1.07 G, receiver gain: 8 · T 1 Relaxation Time T 1 relaxation time were performed on a Bruker DRX500 (B 0 = 11.75 T) spectrometer in a 5 mm multinuclear inverse probe and at a constant temperature of 300 ± 0.1 K. Performance of NMR-measurements is previously described in detail [25]. Experimental errors were calculated for each proton signal and were set on 10%.…”

“…To obtain additional information on the location of Fremy's radical, the T 1 relaxation time of the individual emulsifier proton signals was measured in the presence of Fremy's radicals by NMR [25] (Fig. 5).…”

“…Gallates were found to be solubilized in the Stern layer of SDS micelles [25,31] and the galvinoxyl is located in the micellar interior (Figs. 12, 13).…”

Reaction of Alkyl Gallates With Diphenyl Picrylhydracyl Radicals in SDS Micelles

“…In micellar solutions, it can be considered that DPPH is mostly associated to the micellar pseudophase, while the distribution and location of the phenolyc compound will depend upon their characteristics, mostly its hydrophobicity. In particular it can be assumed that an increase in the solute hydrophobicity will increase its incorporation to the micelles, particularly in the micelle hydrophobic core 5,6 . Hydrophobicity of alkyl gallates can be tuned by changing the length of the alkyl chain 7,8 .…”