Aromatic chromophores and

D. A. Chignell and W. B. Gratzer, Solvent effects on aromatic chromophores and their relation to ultraviolet difference spectra of proteins, J. Phys. Chem. 72, 2934-2941 (1968). 

Practical Considerations. Typical absorption assay methods utilize ultraviolet (UV) or visible (vis) wavelengths. With most spectrophotometers, the measured absorbance should be less than 1.2 to obtain a strictly linear relationship (/.c., to obey the Beer-Lambert Law). Nonlinear A versus c plots can result from micelle formation, sample turbidity, the presence of stray light (see below), bubble formation, stacking of aromatic chromophores, and even the presence of fine cotton strands from tissue used to clean the faces of cuvettes. One is well advised to confirm the linearity of absorbance with respect to product (or substrate) concentration under the exact assay conditions to be employed in... 

The three aromatic amino acids (Phe, Tyr, Trp) have side-chain groups corresponding to the benzene, phenol, and indole chromophores, respectively. The spectroscopic properties of the rat transitions in these chromophores have been reviewed.136-381 Coupling of aromatic transitions among themselves and with peptide transitions can give rise to CD bands in the near and far (k < 250 nm) UV. Near-UV CD bands are useful indicators of the environment of the aromatic chromophores and can frequently be assigned to specific types of side chain, based upon band position, presence of vibrational fine structure, etc. Far-UV CD bands due to aromatic side chains, except for the La band of Tyr (-230 nm) and the Bb band of Trp (-225 nm), are generally difficult to resolve from peptide CD bands and can complicate the conformational analysis of peptides. 

Interesting antenna systems based on polymers have also been designed by Fox and co-workers [26], They prepared a series of well-defined block copolymers labeled with aromatic chromophores and demonstrated that directional singlet energy migration across the block interface takes place, whereas extensive exciplex formation, a decay channel which dissipates electronic energy in flexible polymers, is strongly inhibited in these special polymers. 

The stereochemistry at the C-3 position was established by a circular dichroism study in comparison with compounds having analogous aromatic chromophores and known absolute stereochemistry, viz. hydrastine, narcotine, and oxindole alkaloids. 

Model Photochromic Compounds and Dyes Aromatic Chromophores and Related Compounds Polysilanes... 

An important application of the PFR in polymers is photostabilization [217]. This is associated with light absorption by the aromatic chromophores and by the internal filter effect of the ortho-rearranged products. 

The UV spectrum between 300 and 185 nm of a styrene/(/ )-3,7-dimethyl-l-octene copolymer (20 80) shows three major absorption bands centered at about 260,210, and 190 nm (Figure 2). These bands, as suggested by the fine structure and by the intensity pattern, are related to the 7r-electron system of the aromatic chromophores and can be associated with the electronic transitions in benzene. 

Chemical degradation studies carried out on streptovaricias A and C, which are the primary components of the cmde complex, yielded substances shown ia Figure 1. Streptovaricia A (4), consumes two moles of sodium periodate to yield variciaal A [21913-68-8] (1), 0 2 200, which accounts for the ahphatic portion of the molecule, and prestreptovarone [58074-37-6] (2), C2C)H2C)N02, which accounts for the aromatic chromophore of the streptovaricias (Fig. 2). Streptovaricia G (9) is the only other streptovaricia that yields prestreptovaroae upoa treatmeat with sodium periodate. Treatmeat of streptovaricias A (4), B (5), C (6), E (8), and G (9) with sodium periodate and osmium tetroxide yields streptovarone [36108-44-8] (3), C24H23NO2, which is also produced by the reaction of prestreptovarone with sodium periodate and osmium tetroxide (4,65). A number of aliphatic products were isolated from the oxidation of streptovaricia C and its derivatives (66). 

Since highly basic heterocycles accommodate the positive charge readily and maintain the aromatic nature of the dimethylarnino benzene group, the resulting bond alternation induces a polyene character to the dye chromophore, and the absorption is shifted accordingly to a shorter wavelength. 

Morishima et al. [30, 50-54, 73-76] have made extensive investigations on photoinduced ET, using polycyclic aromatic chromophores covalently attached to polyelectrolytes. They were the first to show that the polyeletrolyte molecular surface provides an unusual microenvironment which greatly changes the rate of photoinduced ET and the fate of the charged photoproducts. 

Among the spectroscopic methods applicable to polysaccharides, u.v. spectrophotometry is of little value for characterizing heparin, whose main, electronic chromophore (the C02 group) displays a band at 220 nm, that is, in a region where all glycosaminoglycans absorb (also through their N-acetyl chromophores), and where minor proportions of unsaturated or aromatic contaminants cause serious interference.77 With pure heparin preparations, the carboxylate chromophore is most useful for chiroptical measurements, and a semi-quantitative evaluation of the extent of N-acetylation of 2-amino-2-deoxy-D-glucose residues is also possible.78... 

We have also examined the behavior of copolymers of o-tolyl vinyl ketone and methyl vinyl ketone (CoMT). In this case the light is absorbed exclusively at the aromatic carbonyl chromophore and the reaction proceeds from this site, while the methyl vinyl ketone moieties provide a relatively constant environment but prevent energy migration along the chain. The values of Tg and Tip in benzene have been included in Table II. These copolymers axe also soluble in some polar solvents for example, we have used a mixture of acetonitrile acetone methanol (30 30 Uo, referred to as AAM). This mixture is also a good solvent for the electron acceptor paraquat (PQ++) which has been shown to be good biradical trap in a number of other systems (9.). 

Figure 1 Jablonskii diagram illustrating the energy levels and photophysical processes for an aromatic chromophore (ISC = inter-system crossing IC = internal conversion).

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