Raman spectroscopy, at low temperature

The reaction sequence of Eq. 23-37 can be slowed by lowering the temperature. Thus, at 70K illumination of rhodopsin leads to a photostationary state in which only rhodopsin, bathorhodopsin, and a third form, isorhodopsin, are present in a constant ratio.510 Isorhodopsin (maximum absorption at 483 nm)513 contains 9-ds-retinal and is not on the pathway of Eq. 23-37. Resonance Raman spectroscopy at low temperature supports a distorted all-frans structure for the retinal Schiff base in bathorhodopsin.510 The same technique suggests the trans geometry of the C = N bond shown in Eqs. 23-38 and 23-39. Simple Schiff bases of 11-cz s-retinal undergo isomerization just as rapidly as does rhodopsin.514... 

Section I) and nuclear derivatives, which treat globally all the excited electronic transitions. In this subsection, we consider only Raman spectroscopy at low temperatures,47-79 as illustrated in Fig. 2.16. 

Model heme complexes containing the ferryl unit and a nonoxidized porphyrin ring have been prepared and investigated by H NMR spectroscopy at low temperatures by Balch and La Mar and coworkers and by resonance Raman spectroscopy by Nakamoto and coworkers. The ferryl species have been produced by several means, including... 

The bis-hydroxylamine adduct [Fe (tpp)(NH20H)2] is stable at low temperatures, but decomposes to [Fe(tpp)(NO)] at room temperature. [Fe(porphyrin)(NO)] complexes can undergo one-and two-electron reduction the nature of the one-electron reduction product has been established by visible and resonance Raman spectroscopy. Reduction of [Fe(porphyrin)(NO)] complexes in the presence of phenols provides model systems for nitrite reductase conversion of coordinated nitrosyl to ammonia (assimilatory nitrite reduction), while further relevant information is available from the chemistry of [Fe (porphyrin)(N03)]. Iron porphyrin complexes with up to eight nitro substituents have been prepared and shown to catalyze oxidation of hydrocarbons by hydrogen peroxide and the hydroxylation of alkoxybenzenes. ... 

Minkwitz et al.250 have obtained tris(methylthio)sulfonium hexafluoroantimonate in a solventless reaction at low temperature [Eq. (4.69)] and characterized the salt by Raman, IR, H and 13C NMR to spectroscopy. The salt is stable below 10°C but decomposes in S02 solution at —45°C with rapid elimination of sulfur. It possesses C3 symmetry with a pyramidal trithiosulfonium unit and the methyl groups point to the central sulfur atom (ab initio calculations). 

A more serious objection is that the evidence for an anhydride intermediate is based upon observation of a transient phase during the hydrolysis of an ester intermediate under cryoenzymological conditions, i.e. at low temperature and in the presence of an organic cosolvent (Makinen et al., 1979). No evidence was obtained by these workers as to the molecular nature of the transient, but subsequently Hoffman et al. (1983) examined the system by multichannel resonance Raman spectroscopy. The characteristic carbonyl stretching frequency of an anhydride should have been detected by their experiments, but was not. Kuo and Makinen (1985), however, re-... 

Among a variety of spectroscopic methods, vibrational spectroscopy is most commonly used in structural chemistry. IR/Raman spectroscopy provides information about molecular symmetry of relatively small molecules and functional groups in large and complex molecules. Furthermore, Raman spectroscopy enables us to study the structures of electronically excited molecules and unstable species produced by laser photolysis at low temperatures. Several other applications that are important in structural chemistry are also discussed in this section. 

While the internal vibrational modes of molecules can display sharp spectral features, the vibrational spectra of modes of bulk matter are broad and relatively featureless. Nonetheless, Raman and infrared methods can be used to study the bulk, the intermolecular degrees of freedom of condensed matter systems. A great deal of information on bulk degrees of freedom has been extracted from electronic spectroscopy, particularly at low temperatures. Such experiments, however, rely on the influence of the medium on an electronic transition. Using ultrafast Raman techniques, including multidimensional methods, and emerging far-IR methods, it is possible to examine the bulk properties of matter directly. 

At low temperatures and in dry samples, protein macromolecules exhibit high frequency low amplitude harmonic nuclear vibrations withuc = 1012 - 1014 s and amplitude A = 0.01 - 0.05 A. This type of motion, directly detected by the methods of IR, Raman, Mossbauer, NMR and ESR spectroscopy, takes place in all proteins, at all temperatures and degrees of humidity, and apparently is not directly related to their functions and stability. 

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