Limitations of the photostationary approximation in the photochemistry of provitamin D: The ambiguous role of the irreversible degradation channel

Abstract

The limitations of the generally accepted photostationary approximation in the photochemistry of provitamin D resulting from the strong spectral dependence of the effectiveness of the irreversible channel were established theoretically by a simplified model. The results show clearly that disregard of the irreversible channel with low quantum yield in a system of reversible photochemical reactions over a wide spectral range is not always justified. As a result the approximation according to which the concentration of the main photoisomers of provitamin D is constant only holds in a specific region of the spectrum, and this must be taken into account during concentration analysis of the photoisomeric mixture.The wide range of monomolecular photoisomerizations in the synthesis of vitamin D has attracted the attention of investigators over many years. It is generally accepted that reversible photoreactions play a determining role in the kinetics of a photoreaction [scheme (1)]. In such a case it is clear that UV irradiation of the initial provitamin D (Pro) at a temperature excluding the formation of vitamin D leads to the formation of a mixture of the four principal photoisomers, between which a dynamic equilibrium -the so-called photostationary state (PS) -is established after a specific time.In spite of the fact that irreversible phototransformations of previtamin D (Pre) are also known [1, 2], it is usually considered that due to the low quantum yield (j » 0.039) [2] the products of the irreversible photoreactions (toxisterols, Tox) only accumulate in the mixture of photoisomers during prolonged exposure after complete conversion of the initial Pro. Therefore, the irreversible channel does not as a rule appear on the reaction scheme, but the presence of Tox is disregarded during concentration analysis of the photoisomeric mixture, and the total concentration of the four main photoisomers is taken as 100% [3][4][5][6].It was within the scope of the PS approximation [4] that a dramatic change was found in the calculated quantum yield of the photocyclization Pre®L within the limits of the narrow spectral range of 300-305 nm. Later on the "surprising" increase in the quantum yields of photocyclization and decrease in the quantum yield of cis-trans isomerisation of previtamin D received several different interpretations [7-9], but none of them was considered entirely satisfactory [10].Our spectral observations of the kinetics of the photoreaction during laser irradiation of provitamin D also revealed substantial differences in the kinetics in relation to the irradiation wave length [11]. During photoreaction initiation with the radiation from an XeCl excimer laser (l = 308 nm) it was found that the usual range of reversible photoreactions was blocked in favor of irreversible photoreactions with a lower quantum yield. Here careful analysis of the possible participation of two-quantum processes showed that they are not involved in the discovered "anomaly." 286 0040-5760/08/4405-0286

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: The influence of solvent on excited state dynamics

“…The photochemistry of DHC and production of vitamin D 3 is complicated by a web of photochemical pathways and byproducts. 19,20 Most of these byproducts are conjugated dienes or trienes that absorb in the UV between 300 and 250 nm. Thus the desired yields and photostationary states are wavelength dependent.…”

“…Thus the desired yields and photostationary states are wavelength dependent. 6,[20][21][22][23] In addition the solvent environment plays a significant role in the conformational equilibrium of previtamin D 3 and on the various photochemical pathways. 24,25 The electronic absorption spectrum of DHC is located in the UV, with the lowest allowed transition peaking at 282 nm (see Fig.…”

The influence of the optical pulse shape on excited state dynamics in provitamin D3

“…[2][3][4][5][6] The photochemical scheme for the formation of vitamin D 3 from DHC in vivo involves UV photoexcitation of a 1,3-cyclohexadiene chromophore embedded in the much larger molecule. 7 Excitation of this chromophore results in an electrocyclic ring-opening reaction as illustrated in Fig. 1.…”

“…Production of vitamin D 3 from DHC is complicated by competing photochemical reactions producing byproducts containing conjugated dienes or trienes that absorb in the same general region between 300 nm and 250 nm. 7 This system presents an interesting opportunity for optical control of complex molecular dynamics. The goal is selective formation of the previtamin and concurrent suppression of the competing side-reactions.…”

The role of tachysterol in vitamin D photosynthesis – a non-adiabatic molecular dynamics study

“…Another factor in the quenching of Vita production is the degradation of DPIs to toxisterols. 31 Furthermore, since the reaction naturally takes place in the cellular membrane of the epidermis, also the steric interactions between DPI and the biological membrane might play a role. 32,33 The membrane is thought to enhance the isomerization from Pre to Vita, by enforcing the cZc conformation necessary to allow for the [1,7]-sigmatropic hydrogen shift.…”