Chromophore models, separable

In many circumstances, the spectroscopic properties of a biological specimen are a composite of the properties of several different chromo-phores within the specimen, and the investigator would like to know the properties of the individual chromophores. Physical separation of the chromophores is often difficult, and it may alter important properties of the system being studied. Thus, one looks to mathematical methods for separation of the contributions of the different chromophores. This chapter focuses on one class of mathematical methods, the application of multilinear models. A major advantage of this class of methods is that component resolution may often be achieved with no prior information about the properties of the components. 

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. 

The results of theoretical calculation using both general rate and transport-dispersive models were in good agreement with the overloaded band profiles determined experimentally, therefore, the method has been found to be suitable for the prediction of band profiles [88], Natural pigments were generally used as a complicated mixture of various compounds with chromophore substructure. Their separation by preparative RP-HPLC is not necessary, and the application of preparative RP-HPLC for the purification of one or more pigment fractions is not expected in the near future. 

Some early kinetic studies on the enzymic reaction indicated that LADH exhibits pre-steady state half-of-the-sites reactivity. Bernard et al. reported that two distinct kinetic processes, well separated in rate, were observed for the conversion of reactants into products under conditions of excess enzyme.1367 They also reported that each of the two phases corresponded to conversion of exactly one half of the limiting concentration of substrate being converted to products. On the basis of this they proposed two possible models, the favoured one based on catalytically non-equivalent but interconvertible states of the two binding sites, with the possibility that the asymmetry of the sites may be induced by coenzyme binding. Further evidence for this non-equivalence of the subunits was obtained in similar subsequent studies using a chromophoric nitroso substrate, p-nitroso-A,JV-dimethylaniline with limiting NADH concentrations.1368... 

In this case both e.t. processes are intramolecular, the distance between the porphyrin and the quinone being fixed by a short space. The highest rate constants of the photo-induced charge separation reach about 4 x 1011 s J and these would not be observable in a diffusion controlled intermolecular reaction. In these measurements, two solvents were used — nonpolar toluene and polar butyronitrile — and the AG values are therefore subject to the uncertainties discussed in Sect. 2.1. In particular, the Coulomb term for butyronitrile assumes the point charge model with full solvent screening, which may well underestimate the free energy values, especially in such an intramolecular system where it is clear that the solvent cannot be inserted between the chromophores. 

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