The temperature window for the extrusion of glycerol-plasticized wheat gluten was increased by the use of salicylic acid, a known scorch retarder and radical scavenger. It was possible to extrude 30 wt % glycerol-wheat gluten films with a die-head temperature as high as 135 degrees C, rather than 95 degrees C, by incorporating only 1 wt % salicylic acid. Small effects of shear-induced heating during extrusion at the higher temperatures suggested that the acid acted as a lubricant and viscosity reducer. The latter was suggested to originate primarily from the salicylic-acid-induced reduction in the degree of protein aggregation/cross-linking, as indicated by size-exclusion high-performance liquid chromatography and chemiluminescence. Electron paramagnetic resonance spectroscopy on extruded films indicated that the beneficial effect of salicylic acid was due to its radical scavenging effect. Tensile tests on extrudates revealed that the materials produced at the substantially higher processing temperature were still ductile. The complex shear modulus increased more slowly with increasing salicylic acid content above 110-120 degrees C, indicating that the aggregation/cross-linking rate was slower with salicylic acid, that is, that it did have a scorch-retarding effect, besides yielding a lower final degree/complexity of aggregation.

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Ribonucleotide reductase catalyses the reduction of ribonucleotides to the corresponding deoxyribonucleotides needed for DNA synthesis. This review describes recent studies on the iron/tyrosyl free radical site in the R2 protein of iron-containing (class I) ribonucleotide reductases. The active enzyme is composed of two homodimeric proteins, R1 and R2. Active protein R2 contains a diiron-oxygen site and a neighboring free radical on a tyrosyl residue per polypeptide chain. The properties of the different redox states of the diiron center in protein R2 are discussed, as well as the formation of the iron/radical site and its possible involvement in long range electron transfer from the substrate binding site in protein R1. The EPR properties of oxidized neutral tyrosyl free radicals are described, and also of tryptophan free radicals found in studies of a mutant of the R2 protein, which lacks the tyrosyl radical site. NMR studies on protein R2 include observations of paramagnetically shifted resonances. Structural NMR studies have been performed on its highly mobile C-terminal domain as well as the corresponding oligopeptide which interacts with protein R1.