Coupled chromophore model

The explicit form of the interaction Hamiltonian // ,(() consists of a series of multipolar terms, but for most purposes the electric-dipole (El) approximation is sufficient. Although the results are calculated within this approximation for each molecular center detailed analysis of the coupling provides results equivalent to the inclusion of higher-order multipole terms for the pair. The same assumption underlies the well-known coupled-chromophore model of optical rotation (Kuhn 1930 Boys 1934 Kirkwood 1937). The Hamiltonian for the system may thus be written as... 

Hansen AE, Ballhausen CJ (1965) Electronic structure of copper acetate monohydrate based on a coupled chromophore model. Trans Far Soc 61 631... 

Figure 12. A representation of the coupled chromophore model. The orbitals designated (a) represent the frontier four-otbitals of one monomer chlorophyll unit, those labeled (b), the orbitals of the second monomer unit. Upon mixing, symmetry determines combinations of the monomer orbitals, yielding those of the dimer, labeled with (d).

By resolution of both light-absorption and CD bands of ARN and FAD and by the use of both experimental and theoretical values for transition moments, conformations of the relevant molecules were deduced. However, because of difficulties in accurate assignment and determination of the various transition moments and because of the limitations inherent in calculations using the coupled oscillator model and dipole-dipole interaction potentials, only a qualitative picture of the conformations involved could be expected (12). Generally, reciprocal relations between known transitions were taken as evidence for juxtaposition of the corresponding chromophores (12). The optical data obtained for coenzymes or their model compounds may be utilized in favorable cases for estimating structural features in the bound state, in which the interactions with the protein environment will also have to be evaluated. 

Gozem S, Melaccio F, Lindh R, Krylov AI, Granovsky AA, Angeli C, Olivucci M (2013) Mapping the excited state potential energy surface of a retinal chromophore model with multireference and equation-of-motion coupled-cluster method. J Chem Theor Comput 9 4495 506... 

For quantitative work, it is necessary to estimate the concentration of 5-amino-l-(P-D-ribofuranosyl)imidazole in aqueous solution. It seems that the only available method is the Bratton-Marshall assay, which was originally developed for the estimation of arylamines in biological fluids. The principle of the method is the spectrometric estimation of a salmon-pink colored dyestuff obtained by diazotation in situ, followed by coupling with /V-( 1 -naphthyl)ethyl-enediamine.65 The only remaining problem then is to know the molar extinction of this dye because pure samples of AIRs are not available. A value of 16800 at 520 nM was obtained for the dyes prepared from a model compound, 5-amino-l-cyclohexylimidazole-4-carboxylic acid (54), which is crystalline. A comparable molar extinction can be expected for the dye prepared from imidazole 55, if the carboxyl group does not exert too much influence on the chromophore. Actually, its influence is perceptible even with the naked eye, the dyestuff prepared from 53 having a somewhat different, wine-red color, with max>520 nM. The molar extinction for 55 is 17400 at 500 nM. When the decarboxylation of 54 was conducted under mild acidic conditions (pH 4.8, 50°C, 1 hour), estimation of 5-aminoimidazole 55 by the Bratton-Marshall method led to the conclusion that the reaction was almost quantitative.66 Similar conditions for the final decarboxylation were adopted in the preparation of samples of AIRs labeled with stable isotopes.58... 

The identification and quantification of potentially cytotoxic carbonyl compounds (e.g. aldehydes such as pentanal, hexanal, traw-2-octenal and 4-hydroxy-/mAW-2-nonenal, and ketones such as propan- and hexan-2-ones) also serves as a useful marker of the oxidative deterioration of PUFAs in isolated biological samples and chemical model systems. One method developed utilizes HPLC coupled with spectrophotometric detection and involves precolumn derivatization of peroxidized PUFA-derived aldehydes and alternative carbonyl compounds with 2,4-DNPH followed by separation of the resulting chromophoric 2,4-dinitrophenylhydrazones on a reversed-phase column and spectrophotometric detection at a wavelength of378 nm. This method has a relatively high level of sensitivity, and has been successfully applied to the analysis of such products in rat hepatocytes and rat liver microsomal suspensions stimulated with carbon tetrachloride or ADP-iron complexes (Poli etui., 1985). 

We shall consider here in more detail two models first a dynamic coupling approach, due to Weigang33, who considered optical activity deriving from the coupling of electric dipoles (the diene chromophore and the polarizable bonds around it) and second, a localized orbital investigation, which permits one to separate the contributions from the intrinsic diene optical activity and from the axial substituents. 

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