Type I mechanism

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

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The type I mechanism is a radical process, and involves the excited state of the photosensitizer in electron-transfer processes, as indicated in Scheme 1. The reactions there are essentially photochemically stimulated autoxidation processes. 

In other cases fullerene antibacterial action takes place after photoirradiation of fulleropyrrolidinium salts. It is not yet clear if the photodynamic action implies the participation of superoxide and hydroxyl radicals (type I mechanism) or singlet oxygen (type II mechanism) but the efficacy is really interesting with the death of more than 99.9% of bacterial and fungal cells and a special selectivity for microbes over mammalian cells (Tegos et al., 2005). Also a sulfobutyl fullerene derivative is able to inhibit environmental bacteria after photoirradiation and it exerts its action on E. coli even if incorporated in coated polymer (Yu et al., 2005). 

T(—H) can also be formed in a two-step process involving radical cation formation produced by photosensitizers operating through a type I mechanism, followed by deprotonation. Cadet has also developed the photolabile phenyl sulfide precursor that upon irradiation at 254nm generates T(—H). " The corresponding phenyl selenide" and methoxy-substituted sulfides" were developed by the Greenberg... 

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As exemplified in Figure 2, Type I mechanism, electron transfer from L to 3 sens yields two radicals, the substrate radical, L, and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions131-132. The sensitizer radical anion may also start a sequential one-electron reduction of 3C>2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

Photochemical oxidation of 1,1-diphenylethylene on silica produced ben-zophenone as the major product as well as diphenylmethane and oxidized diphenylethanes as minor products as shown in Scheme 10 [40]. The oxidation of 1,1-diphenylethylene has been proposed to proceed by a type I mechanism [41]. 

A comparison of the photodynamic properties of H2(4-TRPyP) and its zinc metalated derivative, Zn(4-TRPyP) with the methylene blue and riboflavin photosensitizers also was carried out (240) using 2 -deoxyguanosine as a model compound (169, 241, 242). Riboflavin is a typical type 1 photosensitizer, while methylene blue exhibits a type If behavior. The selectivity measured by the ratio of the amount of photoproducts generated by type Il/type I mechanisms was 0.4 for riboflavin, and 2.3,3.6, and 5.6 for H2 TRPyP, methylene blue, andZn(4-TRPyP), respectively, showing that Zn(4-TRPyP) is the most specific type If photosensitizer of the series. 

The free radical(s) so formed are then scavenged by molecular oxygen forming the peroxy radical or anion (Reaction 5) thus leading to the oxidation of the acceptor AH. This is called the Type I mechanism of photooxidation. 

Taking into account [81,82] that photodegradation of poly(4,4-dimethyl-l-penten-3-one) [poly(BVK)] and poly(3-methyl-3-buten-2-one) [poly(MIK)] proceeds predominantly through a Norrish type I mechanism via the triplet state (Scheme 15), the above homopolymers have been studied as initiators in the photoinduced polymerization of vinyl monomers such as MMA, St, AN and VAc [83]. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

There is one very important feature of the type II system that should not be overlooked. Although a hard-wired type I mechanism may seem more efficient, it only produces a single product. In contrast, the type II system is not only responsible for producing all the diversity of fatty acids in membranes, the intermediates of the pathway are diverted for the synthesis of other key molecules. These include biotin, lipoic acid, and the quorum-sensing... 

A laser flash photolysis study of the behaviour of the lowest excited triplet state and semiquinone radical anion of hypocrellin A (HA") suggests that, in the presence of substrates such as ascorbic acid and cysteine, formation and decay of (HA") occurs by electron transfer. The production of superoxide radical anion (O2") was also confirmed, and the conclusion is drawn from the experimental results that an electron transfer (Type I) mechanism may be important in the photodynamic interaction between HA and some biological substrates. Photoelectron transfer and hydrogen abstraction in the phenothiazine/p-benzoquinone system proceeds competitively, and a series of porphyrin quinones (5 = H,... 

To avoid any teraiinological confusion, the reported amadoriase I and amadoriase II icom Aspergillus sp both operate by the type I mechanism. 

With ketones, various photoinduced processes can occur. In what is known as the Norrish type I mechanism, a free radical reaction occurs ... 

If the monomer itself cannot form free radicals by this process, then a photoinitiator must be added. Bisulfides form two free radicals RS on irradiation. The azo group of azobisisobutyronitrile absorbs light at 350 nm and then forms free radicals [reaction (20-3)]. Certain aliphatic ketones decompose according to a Norrish type I mechanism into two free radicals ... 

However, the subsequent steps in the mechanism, in which active oxygen species are formed, are less well understood. T vo mechanisms have been proposed these are termed the Type I and Type II mechanisms. In the Type I mechanism the excited triplet sensitizer is involved in redox reactions leading to electron transfer and the formation of superoxide... 

For type I mechanism, when strong interactions exist between the monomer and the template, i rei vs [T] shows a maximum for [T] = [M]q. 

Type I mechanism, which does not involve the singlet oxygen reaction, is the another mechanism for the photosensitized oxidation of flavor compounds. Benzophenone-sensitized photooxidation of tetrahydrofuran leads to formation of the a-hydroperoxide (Scheme 27) (Schench et al, 1963), which is the same product formed in the unsensitized photooxidation of tetrahydrofuran as demonstrated in Scheme 11. 

Although reactions with an immediate type I mechanism are the most important chlor-hexidine-induced allergic reactions and receive most attmtion, delayed hypersoisitivity reactions to the drag do occur and inunediate and delayed reactions can occur in the same patient. 

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