Effect of Wavelength on the Photoisomerization of Vitamin K₂

Abstract

trans-Vitamin K₂(trans-VK₂), widely used as a medicine for hemostasis, is manufactured by chemical processes as its mixture with cis-VK₂, among which trans-VIC, is isolated and cis-VK₂ is discarded. During the course of our investigation on the isomerization of unsaturated bonds by a tunable laser instrument, we found that laser excitation successfully converted cis-VK₂ into trans-VK₂.
cis-VK₂ exhibits absorp tion in 200-500 nm. The irradiation of its diisopropyl ether solution (1.5 wt%, 3 ml) under nitrogen with lasers of various wavelength in 200-800 nm (10 Hz, 2 mi. /pulse, fwmh/5 ns) resulted in the isomerization to trans-VK₂. Among the lasers examined, the most efficient isomerization was accomplished with lasers in the region of 355435 nm (355, 366, 375, 404, 414 and 435 nm). Excitation of either cis- or trans-VK 2afforded their photostationary mixture comprising of trans- and cis-VK₂ in 7: 3..
It is noteworthy that the isomerization between cis- and trans-VIC, takes place highly selectively at the unsaturated bond closest to the quinone ring among four unsaturated bonds in the tetraprenyl chain. Furthermore, from the irradiation mixture, a small amount of menachromenol, a cyclized product, was isolated as a by-product. We have proposed a reaction mechanism on the photoisomerization of cis- and trans-VK₂.