Nanosecond laser photolysis of provitamin D

Terenetskaya, I. P.; Gundorov, S I; Kravchenko, V. I.; Berik, E.B.

doi:10.1070/qe1988v018n10abeh012519

Cited by 14 publications

(10 citation statements)

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“…Therefore, besides the VD3 thermal pathway, the irradiation of 7DHC gives a complex dynamic equilibrium between four photoisomers. The optimal irradiation range is in the absorption band of 280–295 nm . Below this, T3 has quantum preference giving irreversible and undesired reactions, and above 305 nm the reaction becomes “anomalous” or unpredictable, as quoted by Terenetskaya et al., also due to the irreversible degradation of 7DHC .…”

Section: Introductionmentioning

confidence: 91%

“…The latter is most efficiently ensured by irradiation in the region of 250 nm, with decreasing yield for longer excitation wavelengths. [4] It is especially relevant that (i) both L3 and T3 are not desired because they are not physiologically active, and (ii) VD3 can be obtained only by a thermally activated hydrogen shift from P3. Also, Toxisterols (Tox) can be generated from P3 or even 7DHC with prolonged high-power irradiations, [5] as a result of two-photon reactions which fully supress VD3 synthesis.…”

Section: Introductionmentioning

confidence: 99%

“…Also, Toxisterols (Tox) can be generated from P3 or even 7DHC with prolonged high-power irradiations, [5] as a result of two-photon reactions which fully supress VD3 synthesis. [4] Therefore, besides the VD3 thermal pathway, the irradiation of 7DHC gives a complex dynamic equilibrium between four photoisomers. The optimal irradiation range is in the absorption band of 280-295 nm.…”

Section: Introductionmentioning

confidence: 99%

“…Due to the low stability of P3 to UV irradiation in the same wavelength range as 7DHC and L3, a reversible ring closure can follow as an equilibrium reaction and the subsequent 7DHC and L3 recovery, or formation of Tachysterol (T3) by a cis‐trans isomerization. The latter is most efficiently ensured by irradiation in the region of 250 nm, with decreasing yield for longer excitation wavelengths . It is especially relevant that (i) both L3 and T3 are not desired because they are not physiologically active, and (ii) VD3 can be obtained only by a thermally activated hydrogen shift from P3.…”

Section: Introductionmentioning

confidence: 99%

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Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow

et al. 2018

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A new reactor design for photosynthesis in microflow is presented based on spatial overlap of a laser beam through a capillary tube. A conical Archimedean spiral facilitates exposure of the entire volume of the reaction solution to the irradiation beam. In this fashion, photo‐high‐p,T intensification of the photolysis of 7‐dehydrocholesterol is achieved using either a pulsed ultraviolet (UV) laser excitation (in the order of femtoseconds) alone or in tandem with a conventional UV lamp. The goal is to control the kinetics of the side photoreactions under pulsed photolysis, as well as to reduce the reaction time and enhance the vitamin D3 yield. Three kinds of effects are targeted: a) Utilizing a very large and spatially homogeneous photon flux inside the flowing solution; b) the novel process window of combining photo‐ with thermal intensification to promote the final, irreversible thermal step to vitamin D3; and c) the use of sub‐ps laser pulses to supply excitation energy on a timescale shorter than the formation of side photoproducts. Finally, photo‐processing with the laser alone is compared to the tandem irradiation by the laser‐UV lamp.

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Section: Introductionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow

et al. 2018

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“…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.…”

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confidence: 89%

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

Terenetskaya

2008

Theor Exp Chem

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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

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Asymmetric photolysis of biomolecules under high-intensity UV laser irradiation

et al. 1990

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Nanosecond laser photolysis of provitamin D

Cited by 14 publications

References 1 publication

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow

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

Asymmetric photolysis of biomolecules under high-intensity UV laser irradiation

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Nanosecond laser photolysis of provitamin D

Cited by 14 publications

References 1 publication

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D3 Synthesis in Continuous Flow

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D3 Synthesis in Continuous Flow

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

Asymmetric photolysis of biomolecules under high-intensity UV laser irradiation

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Product

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Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D₃ Synthesis in Continuous Flow