Isomerization, light-induced

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

Alicyclic ew-dialkyldiazenes are very thermolabile when compared to the corresponding tram-isomers, often having only transient existence under typical reaction conditions. It has been proposed49 that the main light-induced reaction of the dialkyldiazenes is tram-cis isomerization. Dissociation to radicals and nitrogen is then a thermal reaction of the cis-isomer (Scheme 3.19),... 

It has been found that cyclohexa-2,4- and -2,5-dienones undergo a light-induced valence isomerization reaction in strong acid analogous to the alkylbenzenes, to yield 2-hydroxy-bicyclo[3,l,0]hexenyl cations. The hydroxybenzenium ion (34), for example, underwent a clean photoisomerization to 35 at temperatures below — 60° (Paxrington and Childs, 1970). Cation 35 was also produced upon similar irradiation of 36. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

Milanowska, J. and W. I. Gruszecki. 2005. Heat-induced and light-induced isomerization of the xanthophyll pigment zeaxanthin. J. Photochem. Photobiol. B Biol. 80 178-186. 

However, an interpretation of this experiment is not straightforward in view of the experimental conditions, which imply a monolayer on the substrate surface covered by the solution of molecules. Cis-trans isomerization for an observed molecule is supposed to promote desorption, so that a pattern change includes absorption-desorption equilibria. It would be important therefore, to observe a light induced change of a 2D pattern only for adsorbed molecules. This is, indeed, possible for C9(DIA)C8ISA... 

In lipid metabolism, ds-trans isomerism is particularly important. For example, double bonds in natural fatty acids (see p.48) usually have a as configuration. By contrast, unsaturated intermediates of p oxidation have a trans configuration. This makes the breakdown of unsaturated fatty acids more complicated (see p. 166). Light-induced cis-trans isomerization of retinal is of central importance in the visual cycle (see p.358). 

Signaling substances and many proteins are involved in visual processes. Initially, a light-induced cis-trans isomerization of the... 

I and II. At very low temperatures a transient form photorhodopsin with a wavelength maximum at 580 nm may precede bathorhodopsin.461b,501-502a Furthermore, nanosecond photolysis of rhodopsin has revealed a blue-shifted intermediate that follows bathorhodopsin within 40 ns and decays into lumirhodopsin.500,503,504 The overall result is the light-induced isomerization of the bound 11-czs-retinal to all-fraus-retinal (Eq. 23-38) and free opsin. Tire free opsin can then combine with a new molecule of 11-czs-retinal to complete the photochemical cycle. 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

The attempted synthesis of [njcyclophenacenes has also been reported. Kuwata-ni and coworkers reported the synthesis of a pentabenzo[20]annulene, as a possible precursor to [10]cyclophenacene (Scheme 2.3) [16]. Light-induced electrocycli-zation of the precursor was unsuccessful because of isomerization of the (Z)-olefin into the (E)-olefin, which undergoes an undesirable transannular reaction [17]. St. Martin and Scott tried to synthesize [12]cyclophenacene, but attempts at the macrocyclic oligomerization of substituted naphthalene and phenanthrene building blocks by flash vacuum pyrolysis method were unsuccessful [18]. 

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

Figure 22-2. Photobilirubins. Light-induced isomerization of bilirubin at the 4- and 15-double bonds produces photoisomers that are water soluble. In addition, bilirubin can be photochemically cyclized to water-soluble lumirubin.

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

The photoprotonation of cycloalkenes, described in this procedure, is believed to proceed via initial light-induced cis —> trans isomerization of the alkene.4 The resulting highly strained trans isomer undergoes facile protonation. This procedure permits the protonation of cyclohexenes and cycloheptenes under neutral or mildly acidic conditions.5 Since the process is irreversible, high levels of conversion to addition products can be achieved. 

The complexes cis- and /rn s-[RuCl2(dppm)2] undergo isomerization reactions induced by light, heat or oxidation to Ru111. The conversion trans - cis occurs thermally while cis -> trans occurs photochemically apparently by irradiation of the lowest lying d-d transition.1598 In water/alcohol mixtures photolysis of cis- or tranj-[RuCl2(dmpe)2] gives rro/u-[RuCl(OH2)(dmpe)2]+, while trans-[RuCl(DMSO)(dmpe)2]1 is always observed in DMSO. These results are consistent with the... 

Barrell, Campana, von Delius, Geertsema, and Leigh [50] combined the features of the above type of linear molecular motor with light induced E —> Z and Z —> E alkene isomerization. They synthesized and studied a walker-on-track-like light-driven molecular motor, where positions 2 and 3 of the track are connected through a CH = CH unit (Fig. 12). It can operate in either direction, depending on the order in which the stimuli are applied to it. 

The new structural insights have also posed new problems. One of these concerns the first and critical deprotonation/protonation event that involves the Schiff base and Asp-85. What is the direction of the Schiff base N-H bond in L, i.e., after the light-induced isomerization of the retinal but before its deprotonation and the protonation of Asp-85 Does its orientation allow direct proton transfer from the Schiff base to Asp-85, or does it now point toward the cytoplasmic direction as expected from... 

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