Coumarins in Phormium (New Zealand flax) fibers: their role in fluorescence and photodegradation.

The genus Phormium (New Zealand flax) has fiber possessing a high content of lignin and, like other lignocellulosic materials, it is subject to photodegradation. Photoproducts in the fiber absorbing over a broad spectral region from ∼370 to 600nm are observed as a result of exposure to near-UV radiation from 350 to 400 nm. Irradiation was shown to produce hydrogen peroxide and this can account, at least in part, for the photo-oxidation manifested in changes in the reflectance spectra of the fibers. Unirradiated solid fibers and their aqueous extracts exhibit fluorescence with excitation maxima at 350-360nm and emission maxima at 440 nm. The fluorescence spectra of the fibers change following exposure to near-UV radiation with the major fluorophore being substantially photodegraded, evidenced by a substantial loss of emission between 410 and 480nm, which is largely responsible for the yellow and duller appearance of the fiber. Analysis of the aqueous extracts of the fibers, using electron-spray ionization mass spectroscopy of aqueous extracts, showed the presence of coumarin, hydroxycoumarin and a number of substituted hydroxycoumarins. The spectral distributions of the fluorescence associated with the unirradiated fibers and their aqueous extracts are consistent with them originating from a number of 7-hydroxycoumarins present.