The retinylic cation

A retinylic cation (Amilx = 598 nm) was proposed by Blatz et al. [212] to explain the long wavelength absorption maxima of visual pigments. This model was, however, rejected by the same group later [213] on the basis of further experimental data collected on the absorption spectral properties of polyenyl cations. 

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Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

The initial stages of the reaction between anhydroretinol (112) and anhydrous HC1 in 1,2-dichloroethane have been studied by stopped-flow spectrophotometry, and a five-step mechanism has been proposed, involving two carbonium ions, with the anhydroretinylic cation (113) forming much more rapidly than the more stable retinylic cation (114) of which it may be a precursor.51... 

Isomerization takes place in a free or ion-paired retinylic cation, R+, where the double-bond character in the polyene chain is markedly reduced. 

Photodehydroxylation of all tro/is-retinol 2 in nitrogen-saturated acetonitrile by 355-nm pulsed laser photolysis gave the all-trans-retinyl cation 3 as identified by an intense absorption at 580 nm [18]. The transient cation 3 was then photoexcited with a pulse from a flashlamp-pumped dye laser at 590 nm [Eq. (3)]. Laser excitation of alcohol 2 in cyclohexane yielded an oxygen-sensitive major band with = 380-400 nm, identified as the alcohol triplet-triplet absorption. 

The singlet-excited trans-retinyl cation 3 also undergoes electron transfer quenching by aromatic donors [18]. Two laser photolysis of all-fw/ts-ret-... 

V) but not in the presence of anisole = 1.72 V) or biphenyl (Egjj = 1.82 V). Direct evidence for an electron transfer process was obtained by irradiating the tra/is-retinyl cation in the presence of tro/u-stilbene. Transient absorption spectroscopy showed bleaching of the cation signal and enhanced absorption at 470 nm, corresponding to formation of the stilbene radical cation. 

In this study, the product selectivity and product distribution following photooxidation was found to depend on the zeolite used. All of the zeolites that exhibited a loss of product selectivity for toluene photooxidation are susceptible to acid site formation during cation exchange and subsequent pretreatment or calcination. The colorimetric test described by Ramamurthy and coworkers was utilized to detect Bransted acid sites in the zeolites used in this study (10, 11). The colorimetric test is based on differences in the electronic absorption properties of protonated and unprotonated forms of probe molecules, such as retinol and retinol acetate. Both retinol and retinol acetate form a blue retinyl cation in an acid solution. The zeolites were activated by heating to 300°C under vacuum for approximately 12 hrs. Samples of BaZSM-5 (12) and CaY both turned dark blue when a dilute solution of retinol was added to the activated zeolite indicating the presence of Bronsted acid sites. Activated NaZSM-5 turned blue and activated BaY zeolite turned light blue... 

Lipofuscin harbours two unusual retinoids, the lipophilic cations N-retinyl-N-retinylidene ethanol-amine and its isoform, iso-N-retinyl-N-retinylidene ethanolamine, first isolated from the eyes of old... 

The interaction of Lewis acids with retinol initially involves the extraction of the hydroxyl moiety, leaving a retinylic cation (X x which forms a... 

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