Chromophore selection

The rational approach to chromophore design detailed in this work has been demonstrated to facilitate chromophore selection. The synthesis of two dyes selected... 

First, it is possible to excite a chromophore corresponding to the active site, and detennine which modes interact with it. Second, by using UV excitation, the amino acids with phenyl rings (tryptophan and tyrosine, and a small contribution from phenylalanine) can be selectively excited [4], The frequency shifts in the resonance Raman spectrum associated with them provide infomiation on their enviromnent. 

The development of tunable, narrow-bandwidtli dye laser sources in tire early 1970s gave spectroscopists a new tool for selectively exciting small subsets of molecules witliin inhomogeneously broadened ensembles in tire solid state. The teclmique of fluorescence line-narrowing [1, 2 and 3] takes advantage of tire fact tliat relatively rigid chromophoric... 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

Dyestuff organic chemistry is concerned with designing molecules that can selectively absorb visible electromagnetic radiation and have affinity for the specified fiber, and balancing these requirements to achieve optimum performance. To be colored the dyestuff molecule must contain unsaturated chromophore groups, such as a2o, nitro, nitroso, carbonyl, etc. In addition, the molecule can contain auxochromes, groups that supplement the chromophore. Typical auxochromes are amino, substituted amino, hydroxyl, sulfonic, and carboxyl groups. 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

When the enone chromophore of the diketone (148) is excited selectively using 2537 A-light, a smooth conversion to the two stereoisomeric cyclopropyl diketones (149) and (150) takes place exclusively. Experiments... 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

It is clear that, having the same chain length, these macromolecules are in reality experimentally indistinguishable. One could however think of a labelling technique to make Nf different from Nj, for example by using a chromophore-bound radical scavenger which added selectively to the macroradicals issued from the chain scission. Equation (87) can be split into a system of two equations (100) and (101)... 

Fig. 42. A series of boronic acids 159-166 with chromophoric groups has been evaluated with respect to its capacity to selectively bind d-glucose or other saccharides...

In 159 and 163-166 the tertiary amine function is coordinated to the boron atom and transmits the electronic change due to the ester formation to the chromophore. In 160-162 the boron atom is directly connected to the chromophore. After the complexation of the saccharide, the change of the charge transfer, e.g., for 159 [249-251], or the fluorescence bands, e.g., for 160-166 [252-255], can be measured and interpreted. The most selective binding of n-glucose has been achieved with host 164 that forms a 1 1 complex with a macrocyclic structure (Scheme 1). 

Chiral crown ethers possessing two chiral cw-l-phenylcyclohexane-l,2-diol moieties as well as a p-(2,4-dinitrophenylazo)phenol chromophore were prepared and with chiral alkylamines were observed to show enantiomeric selectivity <96JCS(P1)383>. 

Resonance Raman (RR) spectroscopy provides information about the vibrational characteristics of a chromophore, for example, a metal center, within the complex environment of a protein. In RR spectra, those vibrational transitions are observed selectively that are coupled to electronic transitions. In iron sulfur proteins, this technique has been used to resolve the complex electronic absorption spectra and to identify both vibrational and electronic transitions. 

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

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