Isolation and identification of extracted components from Wenge (Millettia laurentii) heartwood and the mechanism of color change

The heartwood of Wenge (Millettia laurentii) has been used as decorative fine furniture owing to its luxurious color and regular fine grain. However, over time, heartwood turns from purple-brown to dark brown and eventually fades, reducing its wood value. The structures of the pigment compounds in wood and the mechanism underlying this discoloration are unclear. Using nuclear magnetic resonance (NMR) spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), fourteen compounds (1–14), including nine new dye compounds (2–5, 8, 11–14), were identified in the methanol extract of Wenge heartwood. Among them, 2, 4, and 5 are orange isoflavane quinones; 8 and 11 are yellow flavonols; 12 is a brown pterocarpan; and 14 is a purple benzofuran quinone, which are considered characteristic of Wenge. The absolute stereoconfigurations of 2, 3, 5, and 12 were identified by comparing the calculated electronic circular dichroism (ECD) spectra with the measured values. To investigate the color-change mechanism of Wenge, the structural changes under room fluorescent light of 2, the main dye compound, was determined using NMR and MALDI-TOF-MS analysis. These results indicate the formation of dark colored pterocarpane ortho-quinone, which causes the darkening of the wood surface.