Abstract:
Dihydroquercetin [(2,3-trans)-3,3',4',5,7-pentahydroxyflavanonol]
was shown to be the precursor to the phenolic
stains that often develop in Douglas-fir [Pseudotsuga
menziesii (Mirb.) Franco] sapwood. Douglas-fir sapwood was
extracted with methanol and the methanol solubles were
fractionated by solvent partition, thin-layer chromatography,
silica-gel column chromatography, and Sephadex gelpermeation
chromatography. Tyrosinase, a commercially
available catechol oxidase enzyme, was used to locate and
trace the precursors to color formation in the solubilized
materials. Careful control of the separations and careful
testing of the fractions for color formation resulted in the
identification and isolation of dihydroquercetin as the
precursor of the stains.
It was established that neither oregonin, the color
precursor of stains in red alder wood and bark, nor
catechin, a flavonoid that has been associated with stains in western hemlock wood, were involved in the staining of
Douglas-fir sapwood.
The inhibitors, ethylenedinitrillotetraacetic acid
disodium salt (EDTA), phenylthiourea, mercaptobenzothiazole,
and Chloramine-T proved ineffective in preventing color
formation when the methanol-extracted materials from
Douglas-fir sapwood were treated with tyrosinase.
An improved method of isolating dihydroquercetin from
Douglas-fir bark was developed. This pure crystalline
dihydroquercetin was reacted with tyrosinase to simulate the
reactions that might lead to color formation in Douglas-fir
sapwood. The reaction mixture became brown in color and a
dark brown polymeric precipitate formed. The brown polymer
was recovered and was shown to have a number of characteristics
similar to the stains in Dotglas-fir sapwood by
ultraviolet and infrared spectroscopy. The compounds 3,4-
dihydroxybenzoic acid (protocatechuic acid), 3,4-dihydroxyphenylacetic
acid, and quercetin were found in the reaction
products of dihydroquercetin and tyrosinase. They were
considered to be degradation products of dihydroquercetin
and possible intermediates in polymeric stain formation.
Thus, a relationship between the presence of dihydroquercetin,
enzyme action, and the phenolic stains that often
develop in Douglas-fir sapwood was established.
