Free radicals of coniferyl alcohol and isoeugenol

Abstract

E.s.r, spectra have been obtained from the free radicals formed by photolysis of coniferyl alcohol and isoeugenol in low temperature ice matrices. Analysis of these spectra indicates that the unpaired electrons are delocalized over the entire radicals and the spin densities at the β carbon atoms are about a third of an electron.

Coniferyl Alcohol Radical Detection by the Dirigent Protein AtDIR6 Monitored by EPR

“…This result is in agreement with previous theoretical calculations supporting a radical delocalization on the whole CA molecule [4] . Because of its fugacious character, no signal of a phenoxyl radical should be directly observable at room temperature during the constant UV irradiation of an aqueous solution of CA [15] . However, irradiation of a CA solution at 100 % of the light power eventually generated a broad EPR feature ( g =2.005) corresponding to signals of a mixture of radicals in different environments typical of unterminated branched phenolic polymers [19] .…”

“…A signal of CA . obtained from UV irradiation (310 nm) of the coniferyl alcohol has been observed by low‐temperature EPR in the mid‐1970s [15] . From this study comes that the unpaired electron is delocalized over the entire molecule and that the half‐life of the radical is approximately 10 s at 223 K. Therefore, one can easily predict a lifetime too short to enable a direct observation of a CA .…”

“…As set, no signal of a radical is observable from a CA solution under constant UV irradiation at room temperature (Figure 1 dotted blue line). To the contrary, when CA and At DIR6 were mixed and photo irradiated together a signal centred at g =2.004 (peak to peak signal width 11 G) typical for a phenoxyl radical [15,17,20] was recorded (Figure 1 solid blue line). Moreover, both the shape and the g value of this signal are similar to those we observed for a photo generated coniferyl alcohol radical adsorbed on SBA‐15 silica after 10 min of irradiation (Figure SI2).…”

“…The demonstration of the dirigent activity of DIRs has required the use of a chemical (e. g. ammonium persulfate) or an enzymatic (e. g. laccase) oxidant [5] . In order to simplify the study, we decided to replace the oxidative part of the system by a simple UV light irradiation (in fact an unfiltered white light) which provides as well a clean formation of phenoxyl radicals [15] . This approach enables to minimize the number of paramagnetic species and to control the triggering of the process in the EPR cavity.…”

“…In order to assess the formation of a CA . under irradiation of a CA solution, preliminary spin trapping experiments were performed with 5,5‐dimethyl‐1‐pyrroline N ‐oxide (DMPO) which, contrary to spin‐traps used in previous studies, [15,18] traps efficiently both carbon‐ and oxygen‐centred radicals. After 10 min irradiation, examination of the EPR spectrum revealed only the addition of carbon‐centred radicals onto DMPO (HFC a H =21.1 and 24.3 G) which confirms the radical delocalization on the vinyl moiety (Figure SI1 in the Supporting Information).…”