Optical properties, spectroscopy dichroism

The heme moiety provides de novo designed heme proteins with an intrinsic and spectroscopically rich probe. The interaction of the amide bonds of the peptide or protein with the heme macrocycle provides for an induced circular dichroism spectrum indicative of protein-cofactor interactions. The strong optical properties of the heme macrocycle also make it suitable for resonance Raman spectroscopy. Aside from the heme macrocycle, the encapsulated metal ion itself provides a spectroscopic probe into its electronic structure via EPR spectroscopy and electrochemistry. These spectroscopic and electrochemical tools provide a strong quantitative base for the detailed evaluation of the relative successes of de novo heme proteins. 

The highly interesting and now classical monograph of Born contains the theory of the optical properties of matter covering atomic and molecular structural considerations. The fundamentals of molecular optics, presented by Bom, have been developed by Volkenshteyn in tensorial form. Various monographs have been written on the laws of classical optics and the fundamentals of physical optics, including natural optical activity," optical rotatory dispersion and circular dichroism," " i.r. spectroscopy, and other related topics. 

The definitions given above for An and Ca are in terms of the anisotropies in the optical properties over the wavelength range of visible light. Similar anisotropies can also exist or can be induced in other wavelength ranges. For example, infrared dichroism (the absorption of different amounts of polarized infrared radiation in the directions parallel and perpendicular to the direction of orientation) is useful in characterizing polymers by vibrational spectroscopy. 

A third type of measurement is aimed at characterizing the optical properties of the nanomaterials, and for this both conventional optical microscopy and transmission UV-visible spectroscopy are applicable. Another method-circular dichroism-can be used to study the heUcity of silica nanomaterials, while Raman spectroscopy has also been used for their characterization. 

See also Biomacromolecular Applications of Circular Dichroism and ORD Chiroptical Spectroscopy, Emission Theory Chiroptical Spectroscopy, General Theory Chiroptical Spectroscopy, Oriented Molecules and Anisotropic Systems Circularly Polarized Luminescence and Fluorescence Detected Circular Dichroism Light Sources and Optics Luminescence, Theory Nonlinear Optical Properties Vibrational CD Spectrometers Vibrational CD, Applications Vibrational CD, Theory. 

The results of studying the optical properties (birefringence, IR and UV dichroism), x-ray, spectroscopy, NMR, and EPR (with specially introduced paramagnetic probes) methods are usually used for estimating the order parameter [40]. The so-called guest-host effect is a widely used method (cf. Chapter 8). 

To summarize, from ouf data, it can be concluded that in both LMH and LM complexes, all the protein secondary structures are oriented almost to the same extent. This observation suggests a model of the RC in which all three subunits L, M, H, have transmembrane a-helices. The observed differences between LMH and LM in the absence of nujol could be due to differences i) in the state of hydration in LMH and LM, ii) in the optical properties of the multilayers, iii) in oriented secondary structures (ex g-structure) which are removed with nujol. Similar findings were observed on other oriented air-dried membranes such as chro-matophore, thylakoid and purple membrane. We find that distortions of the amide I and amide II absorptions brought about by dehydration and/ or optical reflection effects are a general feature of protein infrared spectra. These effects are more evident in samples with small extents of dichroism (eg. air-dried samples of soluble proteins such as cytochrome c or ribonuclease) since these effects are present to the same extent, but largely hidden in samples with large dichroic signals (eg. oriented purple membrane). These results indicate that a more precise determination of membrane protein orientation by polarized IR spectroscopy requires these effects to be taken into account. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

Chiroptical properties related to molecular vibrations can be studied not only by vibrational circular dichroism, but also by using the chiral variant of the Raman spectroscopy - Raman optical activity (ROA) [14-16]. This method has been developed into practical use only recently, but it is very promising and similarly to the... 

A review of rheo-optical techniques by Sherman et al. (1996) notes that there has been an increase in the use of rheo-optic set-ups both for FT-IR dichroism and for dynamic IR dichroism spectroscopies for polymer melts and polymer blends. Skytt et al. (1996) highlight the use of simultaneous measurement of the transient or steady-state rheological properties and IR dichroism to characterize orientation in polymer melts. However, there is little reference to dual spectroscopic-rheological techniques for reactive polymer systems in the literature. 

Every property of an interface that can be optically probed can, in principle, be measured with the SFA. This may include information obtainable from absorption spectroscopy [55], fluorescence, dichroism, birefringence, or nonlinear optics [43], some of which have already been realized. 

Fluorescence spectroscopy in combination with circular dichroism (CD), optical rotatory dispersion. X-ray crystallography, UV and NMR spectroscopy of the main chain is a powerful probe for identifying helical conformation, uniformity, and rigidity in polymers. In recent years, these techniques have been applied extensively to investigate the structures of polysilanes in both the solid state and in solution and it is now clear that after electronic structure main chain helicity is the principal determinant of the properties of polysilanes. In... 

We have seen that the enantiomers of a molecule have the same properties (melting and boiling points, refractive index, etc.) and that spectroscopy that does not involve polarized light is incapable of distinguishing between them, so that their infrared, NMR, ultraviolet-visible and Raman spectra are identical. On the other hand, the optical rotation, optical dispersion and circular dichroism give results that are opposite in sign for the two enantiomers. We have also seen that in certain cases it is possible to determine the absolute configuration of a molecule in the crystal by X-ray diffraction. 

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