Chromophores aromatic

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). 

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. 

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

The pyrene-like aromatic chromophore of BaPDE is characterized by a prominent and characteristic absorption spectrum in the A 310-360 nm spectral region, and a fluorescence emission in the X 370-460 nm range. These properties are sensitive to the local microenvironment of the pyrenyl chromophore, and spectroscopic techniques are thus useful in studies of the structures of the DNA adducts and in monitoring the reaction pathways of BaPDE and its hydrolysis products in DNA solutions. 

Figure 1.2. Jablonski energy level diagram showing the singlet state and the triplet state with its zero-field splittings for a planar aromatic chromophore.

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. 

DeAngelis, G.G. and Wildman, W.C., Circular dichroism studies. 1. A quadrant rule for the optically active aromatic chromophore in rigid polycyclic systems. Tetrahedron, 25, 5099, 1969. 

Dimethylaminobenzoates have also been used to establish the absolute configuration of diols in the presence of an aromatic chromophore, including metabolites of benzo[a]pyrene163 and precocene I164, as well as the cis-dihydrodiol derived from 12-methylbenz[a]anthracene165 and diastereomeric 2,2 -spirobiindane-l,l -diols166. 

From the optical and chiroptical properties of [2.2]metacyclophanes, compared with those of [10]paracyclophanes40), it was concluded that in the former an inherently achiral aromatic chromophore perturbed by vibrations is presumably responsible... 

In addition to red-shifted spectra and low rotational amplitude, Ehrlich ascites plasma membranes exhibit Cotton effects in the region of aromatic chromophores, 250-300 m/ (89). Similar behavior has been observed in other proteins, such as carbonic anhydrase (34) and lysozyme (58), and disappear upon denaturation. Ehrlich ascites plasma mem-... 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

The UV and CD curves of optically active (S)-9-hydroxy-6-methyl-3,4-dihydro-2//-pyrido[2,1 -6] [1,3]thiazinium-4-carboxylate were measured in 1 N hydrochloric acid and in 1 N sodium hydroxide solutions (73ACS1059). The CD spectrum in acid solution exhibits positive bands at 212 and 233 nm and negative bands at 257 and 345 nm, and UV maxima were found at 211, 239, and 342 nm. The two latter bands are associated with the aromatic chromophore. In alkaline solution a bathochromic shift occurred, and the... 

Because methylene-interrupted polyunsaturates do not have strongly absorbing chromo-phores in the UV region, detection by refractive index or far-UV detection (205-214 nm) has been utilized in RP-HPLC separations of free fatty acids and their aliphatic esters. Refractive index detection is less sensitive than UV detection. However, with far-UV detection, solvents absorbing UV below 210 cannot be used. The RP-HPLC separations reported to date have generally involved derivatization designed to incorporate aromatic chromophores allowing detection by fluorescence or UV detection. 

The CD of a series of eight differently-substituted analogues of 9,10-ethano-9,10-dihydroanthracen-ll-one (142 and 143) was studied experimentally and theoretically329. Alteration of the substituent(s) on the benzene ring(s) affects the transition dipole magnitude and the transition energy of the aromatic chromophore without much change in the polarization direction. 

An extended study of the optical behavior of this class of products has been performed in connection with investigations on the conformations of lycorine (1) and related compounds. An empirical rule, similar to the octant rule, which allows the prediction of both the sign and magnitude of the Cotton effect at 290 nm from the steric orientation of the atoms about the aromatic chromophore, has been deduced from considerations on the ORD and CD curves of lycorine (1) and derivatives. When the benzene ring is viewed along the — z —+ z axis (the coordinates are fixed on the aromatic ring as depicted in Fig. 1), four back octants are defined which contribute, with the signs indicated, to the Cotton effect. 

Subsequently, the application of a quadrant rule developed for compounds bearing an asymmetric center adjacent to the aromatic chromophore by means of considerations of the CD and ORD spectra of products of established absolute configuration to the alkaloids of the 5,10b-ethanophenanthridine series led to conclusions on their absolute stereochemistry at variance with those mentioned above 42). 

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