Carbon-centered radicals in isolated soy proteins

All natural mussel-inspired bioadhesives from soy proteins and plant derived polyphenols with marked water-resistance and favourable antibacterial profile for wound treatment applications

HYPOTHESIS

Implementation of tissue adhesives from natural sources endowed with good mechanical properties and underwater resistance still represents a challenging research goal. Inspired by the extraordinary wet adhesion properties of mussel byssus proteins resulting from interaction of catechol and amino residues, hydrogels from soy protein isolate (SPI) and selected polyphenols i.e. caffeic acid (CA), chlorogenic acid (CGA) and gallic acid (GA) under mild aerial oxidative conditions were prepared.

EXPERIMENTS

The hydrogels were subjected to chemical assays, ATR FT-IR and EPR spectroscopy, rheological and morphological SEM analysis. Mechanical tests were carried out on hydrogels prepared by inclusion of agarose. Biological tests included evaluation of the antibacterial and wound healing activity, and hemocompatibility.

FINDINGS

The decrease of free NH2 and SH groups of SPI, the EPR features, the good cohesive strength and excellent underwater resistance (15 days for SPI/GA) under conditions relevant to their use as surgical glues indicated an efficient interaction of the polyphenols with the protein in the hydrogels. The polyphenols greatly also improved the mechanical properties of the SPI/ agarose/polyphenols hydrogels. These latter proved biocompatible, hemocompatible, not harmful to skin, displayed durable adhesiveness and good water-vapour permeability. Excellent antibacterial properties and in some cases (SPI/CGA) a favourable wound healing activity on dermal fibroblasts was obtained.

Electron spin resonance (ESR) spectroscopy study of cheese treated with accelerated electrons

The generation, accumulation and decay of free radicals in six varieties of cheese, irradiated (0-4 kGy) in an electron accelerator, have been studied by electron spin resonance (ESR) spectroscopy. Remarkably, the ESR spectra of all untreated cheeses showed only one singlet signal with a g-factor of 2.0064 ± 0.0005. Surprisingly, the ESR spectra of irradiated samples presented a new signal with g-factor of 2.0037 ± 0.0003 which was independent of the type of cheese, and which might be due to free radicals from the radiolysis of proteins. Surface regression models (P < 0.0001) established the relationship among signal intensity, absorbed dose (0, 1, 2 and 4 kGy) and storage time (0-180 days) for the different types of cheese. Results suggested that the analysis by ESR (or electron paramagnetic resonance, EPR) is suitable to evaluate, either qualitatively or quantitatively, the irradiation treatment of different types of cheese.

Thermally stimulated luminescence in powdered soy proteins

Heating powder isolated soy proteins (ISPs) in a N₂ environment produced thermally stimulated luminescence (TSL), in 2 major temperature regions, 50 to 250°C (region R1) and 250 to 350°C (region R2). In soy protein 7S fraction, strong TSL was detected in both regions with glow peak maximum (T(m)) at 150 ± 15°C and at 300 ± 10°C. Two additional satellite or shoulder peaks were detected from the ISP and 7S protein fraction within region R1 at T(m) = 90°C and T(m) = 210°C. The soy protein 11S fraction produced a broad, poorly defined TSL peak in the low-temperature region. Electron paramagnetic resonance spectroscopy data from the control ISP sample, deuterium sulfide-treated ISP, ISP stored in either N₂ or O₂, and defatted soy flour, indicated that the trapped radicals present in ISP is associated with the production of the primary TSL peak at 150 ± 15°C. Activation energies required to release the trapped charges (for luminescence to occur) are approximately 0.70, 0.78, 1.50, and 1.8 eV for TSL at Tm = 100, 150, 200, and 300°C, respectively. The reaction mechanism that leads to the release of the trapped charges for TSL to occur followed a mixed order kinetic, between 1.5 and 1.8. The frequency factor varied between 10⁷/s and 10¹⁷/s.

Effect of sequestering intrinsic iron on the electron paramagnetic resonance signals in powdered soy proteins

This investigation examined iron in powdered soy protein products using electron paramagnetic resonance (EPR) spectroscopy, and the effect that selectively binding free iron in isolated soy protein (ISP) had on the occurrence of metastable radicals in powdered soy proteins. EPR analyses of soybean defatted flour, commercial ISP and laboratory ISP samples revealed a peak at g = 4.3 characteristic of high-spin ferric iron in a rhombic-coordinated environment. Commercial ISP samples examined contained higher levels of the rhombic ferric iron than laboratory-prepared ISP samples. During the first 6 wk of storage the primary singlet EPR signal at g = 2.0049 in the commercial ISP samples approximately doubled, and the laboratory prepared samples increased by about 9-fold. The EPR signal was initially about 4-times higher in the freshly prepared commercial samples compared to the corresponding laboratory ISP. Laboratory ISP samples prepared with added deferoxamine to sequester endogenous iron exhibited a large increase in the high-spin ferric iron EPR signal at g = 4.3. ISP treated with deferoxamine also exhibited a multiple-line EPR signal at about g = 2.007, instead of the typical singlet signal at g = 2.0049. The power at which the signal amplitude was half-saturated also changed from about 1 mW in the control ISP to about 20 mW in the deferoxamine treated ISP. The multiple-line EPR spectrum from the ISP treated with deferoxamine increased during storage over a 6-wk period by about 6-fold. The observed changes in EPR line-shape, g-value, and power saturation with the deferoxamine treatment indicate that the primary free-radical signal in powdered ISP samples may be from stabilized tyrosine radicals with spin densities distributed over the aromatic ring.

Effect of moisture, lipids, and select amino acid blocking agents on the formation and stability of metastable radicals in powdered soy proteins

Incremental increases in the moisture content of powdered soy protein products from 4.4% to 13.4% produced an inverse effect on the ability of soy proteins to maintain metastable free radicals. The corresponding reduction in electron paramagnetic resonance signal was not due to dielectric loss in the range of moisture contents examined. Subsequent evaluations of various treatments were conducted after drying soy proteins with molecular sieve to a water activity below 0.085 in order to minimize the influence from variations in moisture. Isolated soy protein (ISP) samples, prepared with "defatted flour" that had been further extracted with chloroform/methanol (2: 1), had a 96% reduction in total lipids compared to the control ISP samples. The initial rate of radical accumulation in the "reduced-lipid" ISP for the first 3 wk was not significantly different from the initial rate of radical increases in the control ISP. After 3 wk, radical accumulation in the "reduced-lipid" ISP continued to increase, but at a rate that was less than the control. These findings indicate that the initial reactions contributing to the formation of metastable radicals in the powdered ISP are not strongly dependent on associated lipids. Blocking sulfhydryl groups during ISP preparation with N-ethylmaleimide did not significantly slow the rate of radical accumulation compared to the control ISP. Blocking arginine residues in ISP samples with phenylglyoxal caused an increased rate of radical accumulation for the first 4 wk.

PRACTICAL APPLICATION

Levels of metastable radicals in powdered soy protein products typically range from 10 to 100 times greater than the free radicals in other food protein sources. This current research examines various compositional and treatment parameters that might be used to minimize the content of free radicals in foods containing soy proteins.

Metastable radicals and intrinsic chemiluminescence from soy proteins

Chemiluminescence from various powdered food proteins were examined without the addition of any external source of free radicals or luminescent agents. In the solid-state, soy and whey proteins produced more intrinsic chemiluminescence than casein, sodium caseinate, or egg albumin. However, when these same food proteins were hydrated, intrinsic chemiluminescence from soy proteins was about 4- to 8-times greater than other source proteins. Quenching the alkyl-radicals in the powdered soy proteins with hydrogen sulfide reduced the typical electron paramagnetic resonance spectra from soy proteins below detectable levels, and reduced the chemiluminescence from the hydrated soy proteins by about 65%. Antioxidants also reduced chemiluminescence in hydrated soy proteins by about 50% to 92%, with ellagic acid being the most effective. The reduction in chemiluminescence from both quenching radicals in the solid state, and by the addition of antioxidants to aqueous mixtures, indicate that the chemiluminescence produced when soy proteins are hydrated is a free radical catalyzed event. Based on the production of chemiluminescence, the radicals from soy protein were largely released within 30 min of hydration at 23 °C. Elevating the hydration temperatures increased chemiluminescence by as much as 280% at 70 °C, and decreased the half-life of the light-emitting reaction by about 9-fold.

PRACTICAL APPLICATIONS

Levels of metastable radicals in powdered soy proteins typically range from to 10 to 100 times greater than free radicals from other food protein sources. This research focuses on the types of reactions these radicals catalyze when soy proteins are hydrated, and the radicals suddenly become reactive. The findings suggest that a portion of the energy released from metastable radicals when powdered soy proteins are hydrated is involved in the generation of chemically-induced light.

Distribution of stable free radicals among amino acids of isolated soy proteins

Application of deuterium sulfide to powdered isolated soy proteins (ISP) was used to quench stable free radicals and produce a single deuterium label on amino acids where free radicals reside. The deuterium labels rendered increases of isotope ratio for the specific ions of radical-bearing amino acids. Isotope ratio measurements were achieved by gas chromatography/mass spectrometry (GC/MS) analyses after the amino acids were released by acidic hydrolysis and converted to volatile derivatives with propyl chloroformate. The isotope enrichment data showed the stable free radicals were located on Ala, Gly, Leu, Ile, Asx (Asp+Asn), Glx (Glu+Gln), and Trp but not on Val, Pro, Met, Phe, Lys, and His. Due to the low abundance of Ser, Thr, and Cys derivatives and the impossibility to accurately measure their isotope ratios, the radical bearing status for these amino acids remained undetermined even though their derivatives were positively identified from ISP hydrolysates. The relative isotope enrichment for radical-bearing amino acids Ala, Gly, Leu, Ile, Asx (Asp+Asn), Glx (Glu+Gln), and Trp were 8.67%, 2.96%, 2.90%, 3.94%, 6.03%, 3.91%, and 21.48%, respectively. Isotope ratio increase for Tyr was also observed but further investigation revealed such increase was mainly from nonspecific deuterium-hydrogen exchange not free radical quenching. The results obtained from the present study provide important information for a better understanding of the mechanisms of free radical formation and stabilization in "dry" ISP.