Anisotropic chromophore

Note that according to Eqs. (77) and (78) the index change induced by the reorientation of anisotropic chromophores is quadratic in the poling held Ep and is therefore an orientational Kerr effect. The index change has also a quadratic dependence on the dipole moment. Equation (78) shows that the refractive index increases for an optical held polarized along the direction of the poling held and decreases for a polarization in a direction perpendicular to it. 

A second approach with respect to anisotropic flavin (photo-)chemistry has been described by Trissl 18°) and Frehland and Trissl61). These authors anchored flavins in artificial lipid bilayers by means of C18-hydrocarbon chains at various positions of the chromophore. From fluorescence polarization analysis and model calculations they conclude, that the rotational relaxation time of the chromophore within the membrane is small compared to the fluorescence lifetime (about 2 ns74)). They further obtain the surprising result that the chromophore is localized within the water/lipid interface, with a tilt angle of about 30° (long axis of the chromophore against the normal of the membrane), irrespective of the position where the hydrocarbon chain is bound to the flavin nucleus. They estimate an upper limit of the microviscosity of the membrane of 1 Poise. 

It should be noted that for C6F13NC8 a remarkably anisotropic orientation of the chromophore in the film plane was observed in the polarized UV-vis. spectra at the normal incidence using the rays with electric vectors perpendicular and parallel to... 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

We have shown that redox chromophores organized in LB films with resped to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level, photoinduced anisotropic eledrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future. 

H-NMR spectra show lowfield shifts of H-8, H-9, and H-11 caused by anisotropic effects of both C=O and C=N groups and also by the positively charged pyridinium moiety. C-NMR data reveal analogous effects, pointing to a charge separation between the pyridinium part and the extended negative chromophore (349). 

One aspect of the research will examine equilibrium aspects of solvation at hydro-phobic and hydrophilic interfaces. In these experiments, solvent dependent shifts in chromophore absorption spectra will be used to infer interfacial polarity. Preliminary results from these studies are presented. The polarity of solid-liquid interfaces arises from a complicated balance of anisotropic, intermolecular forces. It is hoped that results from these studies can aid in developing a general, predictive understanding of dielectric properties in inhomogeneous environments. 

Second, the absorption characteristic of each Rhodonine chromophore is highly directional (15R). This anisotropic absorption is only observed for radiation applied perpendicular to the surface of the film, i.e., parallel to the axis of the Outer Segment. The peak absorption wavelength for resonant absorption by these chromophores is nominally either 342,437, 532 or 625 nm. The chromophore is not polarization sensitive for excitation along this axis. For radiation applied along other axes, such as transverse to the axis of the OS, only the intrinsic absorption characteristic due to conjugate absorption and shared by all retinoids of the Vitamin A Group will be observed. This intrinsic spectrum has a nominal spectral peak at 502 nm at 37C. 

A primary reason for the above disparities is the critically important structural organization of the chromophores when found in-vivo. These relationships make a major (several orders of magnitude) difference in the absorbance of the material and also lead to anisotropic absorption. These relationships have not been maintained by the chemists. A second reason relates particularly to the L-channel. The chromophore of that channel exhibits a more intimate relationship with the electronic portion of the photoreceptor neuron than do the S- and M-channel chromophores. As a result, the L-channel exhibits an effective absorption characteristic very different from that observed by the chemist. This characteristic also accounts for the loss of red response in the mesopic and scotopic regions. These relationships have not been emulated in the environment of the chemist. Failure to emulate these conditions leads to extraneous absorption spectra for the L-channel chromophore. A third reason is due to the frequent chemical reactions occurring in the chemists solutions that he may not be aware of. It has been rare in the past for the chemist to document the pH of the solutions he has measured. This is a particular problem as mentioned in a later section [Section 5.5.12], The chromophores of vision are members of the "indicator class of chemicals. Their spectral characteristics are intimately related to the pH of their environment. They are also complex organics. Their spectral characteristics are a function of the organic solvent used. They are also subject to chemical attack. This mechanism has been documented by Wald, et. al. and more recently by Ma, et. al. 

Wald, et. al97,98. performed a set of experiments during the 1940 s that purported to demonstrate the formation of rhodopsin from either retinene, (now known as retinal) or Vitamin A, and a native protein. While their work involved materials showing a peak absorption at 500 nm, this is the wavelength of peak isotropic absorption of a large number of dipolar retinoids. Such a peak is not exclusive to the chromophoies of vision. Neither is it relevant to the anisotropic absorption spectrum of the chromophores of vision. 

Figure 5.5.4-1 Putative arrangement of a liquid crystalline chromophore on the surface of the opsin substrate. The individual molecules are arranged with their long axis nearly perpendicular to the surface of the substrate. The angle of tilt of the array is estimated. It is not documented in both directions and may differ slightly from a straight line drawn between the two auxochromes of the molecules. The pitch and dimensions of the substrate molecules are from Corliss and from Nilsson. The hydrogen bonds between the chromophores and the substrate are shown as dots. The anisotropic absorption profile of the chromophore is illustrative due to the many quantum-mechanical factors in determining it precisely.

As discussed in previous paragraphs, the spectral absorption of the chromophores of vision is much more comphcated than that described by Beer s Law for true solutions of low molarity. The absorption of the chromophores of vision is a function of the environment, the chemical state, the spatial relationship, and the orientation of the molecules. They actually exhibit additional, and generally dominant, absorption spectra in the liquid crystalline state that are not found for the same material in low molarity solution. These additional spectra are highly anisotropic. They also exhibit a high absorption coefficient along the preferred axis. Because of these properties, it is extremely difficult to make a comparison of the properties of the chromophores when they are in-vivo with their properties when in-vitro. Because the chromophores are not in solution, when in-vivo, it is not appropriate to use Lambert-Beer s Law to evaluate them (See Section 5.3.5.3), The results of using this law are generally spurious. 

For experiments where the radiation is applied parallel to the axis of the disk stack, entirely different results are obtained. This is the case of some of the later Baylor experiments and most noninvasive photo-micro-spectrometry studies. These studies, if performed so as to only illuminate one photoreceptor at a time, produce peak spectral absorptions that conform to the actual anisotropic absorption spectra of the chromophores of vision. These peaks are at 437, 532, 625 nm 2 nm at mammalian temperatures. 

If the molarity is further increased, and the material is allowed to precipitate on a substrate, the liquid crystalline structure of the precipitate will exhibit a highly anisotropic absorption spectrum at the resonant wavelength of its chromophore as well as its normal isotropic molecular absorption spectrum. The relaxation characteristic will either remain unchanged or be impacted by the electronic characteristics of the substrate. 

Ma, Znoiko, et. al. also provide data on the effect of adding hydroxylamine to one of their samples at 4° C. The change of the absorption spectrum with time is reminiscent of other tests involving the change in pH of a chromophore. It once again demonstrates that the chromophores of vision involve a retinoid with the complexity and form of an indicator. Within the context of this work, the chromophore, rhodonine(9) shifted its spectral peak from its functional (anisotropic) peak at 432 nm to its intrinsic (isotropic) peak at 350-360 nm. 

Spatial Profile of resonant (anisotropic) absorption by resonant liquid crystalline structure of chromophore... 

Molecular (but anisotropic) absorption, at 500 nm, reflecting the physical arrangement of the chromophores... 

Figure 5.5.11-1 Absorption characteristics of a complete disk showing the variation in absorption properties as a function of spatial angle and absorbing species. Top 3-D isometric view. Bottom 2-D projection, plane contains vertical axis perpendicular to disk surface. The shared quantum-mechanical structure of the liquid crystalline chromophore(s) creates a highly focused (anisotropic) absorption profile. This structure is in quantum-mechanical contact with the microtubules surrounding the disk. The retinoids within the opsin proteins are not in quantum-mechanical contact with each other or the microtubules.

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