Resonance Raman spectroscopy formation

The yellow disulfide radical anion and the briUiant blue trisulfide radical anion often occur together for what reason some authors of the older Hterature (prior to 1975) got mixed up with their identification. Today, both species are well known by their E8R, infrared, resonance Raman, UV-Vis, and photoelectron spectra, some of which have been recorded both in solutions and in solid matrices. In solution these radical species are formed by the ho-molytic dissociation of polysulfide dianions according to Eqs. (7) and (8). 8ince these dissociation reactions are of course endothermic the radical formation is promoted by heating as well as by dilution. Furthermore, solvents of lower polarity than that of water also favor the homolytic dissociation. However, in solutions at 20 °C the equilibria at Eqs. (7) and (8) are usually on the left side (excepting extremely dilute systems) and only the very high sensitivity of E8R, UV-Vis and resonance Raman spectroscopy made it possible to detect the radical anions in liquid and solid solutions see above. 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

Measurements of the proximal histidine-iron stretching frequency by Resonance Raman spectroscopy revealed that this bond is very weak in relation to other heme protein systems (vFe.His = 204 cm-1) (130). Formation of the sGC-NO complex labilizes this ligand resulting in the formation of a 5-coordinate high spin iron(II) complex, and the conformational change responsible for the several hundred-fold increase in catalytic activity (126,129,130). 

In the case of NOS 12, different kinetics were observed at 436 nm. In cyclohexane, there was a rapid rise, with a lifetime of 6.6 psec followed by a decay with a 100-psec lifetime. In 1-butanol, there was a rapid rise (lifetime=4.3 psec), a decay (43-psec lifetime), and a second longer decay within a 1.4-nsec lifetime. These findings were confirmed by picosecond time-resolved resonance Raman spectroscopy. In these Raman studies in cyclohexane, a single rate constant was observed, whereas in 1-butanol, three spectral components grew with different time constants. The data were said to be consistent with the photo-formation of two or three isomers trans about the central methine bond however, other transient species could be responsible for the observed kinetics because the absorption envelope obviously shifts and this would affect the resonance Raman bands. 

Extensive studies of enzyme-substrate complexes by resonance Raman spectroscopy (RR) have prompted the synthesis of new peptide bond modifications such as thionoesters and dithioesters (Scheme l7)t82-83l within simple model substrates. The resulting acyl-enzyme complexes are especially amenable to RR analysis with cysteine proteases such as papain due to formation of the transient dithioester intermediates. 

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Schiffs base, but the spectra for the M-412 intermediate indicate that this proton is lost. The deprotonation of the Schiffs base is apparently after the K intermediate [262], and proposed to be during the L to M transition [209,263,264], Reprotonation of the nitrogen is suggested to occur during the M-412 to 0-640 conversion [265], Part of the blue-shift in the formation of M-412 is, of course, explained by the fact that, in model retinal compounds, loss of the proton leads to a 440-380 nm shift [266], but other effects must also be present. Circumstantial evidence, which includes the finding of 13-cis retinal in M-like intermediates stabilized under somewhat denaturing conditions [198,267], favors the idea that the retinal is isomerized in the M intermediate, as do the more direct resonance Raman data [268,269], In fact, the K and L intermediates seem already to contain the 13-c/i isomer of retinal, as indicated by extraction of 13-c/i retinal from the L intermediate [270] and spectroscopic data on the K and L intermediates [271-274], The resonance Raman spectroscopy of bacteriorhodopsin photointermediates has been recently reviewed [275],... 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers coiresponding to disrotatory 4jr-electrocyclization (185) and bicyclo[3.1.0]pentene (186) fonnation along with small amounts of the reduced product 187 (equation 69)2 -. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms) -, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184 82 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy . 

Mixed-valence linear-chain complexes of palladium display very similar features to those of platinum (97). Indeed, the great intensity of the t j progression in all these cases means that resonance Raman spectroscopy provides a sensitive and rapid means for detecting the formation of new linear-chain complexes of this sort. 

Metal/Organic Interface Formation Studied In Situ by Resonant Raman Spectroscopy... 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

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