Chromophores perturbation

MP >260°C, [a]D —40°), which is also a natural product from T. coffeoides (67). Its UV spectrum [ max 227(19200), 260(17700), and 335(13200) in acid solution] indicated a 2-acylindole chromophore perturbed by the presence of a carbonyl group in position 6. Its IR spectrum exhibited two carbonyl absorptions at 1670 and 1615 cm-1 for the carbonyl functions located at positions 3 and 6, respectively, while in the H-NMR spectrum (pyridine-d5) the aromatic proton H-9 was shifted downfield by the neighboring C-6 carbonyl. 

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... 

Herskovits and Laskowski (1962) have critically discussed the necessary assumptions of the solvent perturbation technique, and the experimental limitations imposed thereby. In summary, these are (a) a quantitative measure of the chromophore-perturbing ability of each perturbant must be in hand. This requires specific determinations with adequate model compounds. (5) The perturbant must not change the protein conformation, i.e., it must be inert in all respects except in perturbing exposed chromo-phores. It would clearly be of no value if it changed the degree of chromo-phore exposure, (c) It is an oversimplification to class protein chromo-phores as either completely exposed or completely inaccessible. Gradations between these extremes certainly do exist. 

In practical terms, transition (a) is observed as a 190 nm absorption band with a shoulder at 208 nm. Transition (b) is observed only at lower wavelengths with good optical equipment. UV-visible tt tt transitions involving either /i-sheets or less regular structures (known broadly as random coil) are not subject to extensive chromophore perturbation effects and hence UV-visible spectroscopy is unable to identify the presence of either secondary structural elements directly. 

FRET is a nonradiative process that is, the transfer takes place without the emission or absorption of a photon. And yet, the transition dipoles, which are central to the mechanism by which the ground and excited states are coupled, are conspicuously present in the expression for the rate of transfer. For instance, the fluorescence quantum yield and fluorescence spectrum of the donor and the absorption spectrum of the acceptor are part of the overlap integral in the Forster rate expression, Eq. (1.2). These spectroscopic transitions are usually associated with the emission and absorption of a photon. These dipole matrix elements in the quantum mechanical expression for the rate of FRET are the same matrix elements as found for the interaction of a propagating EM field with the chromophores. However, the origin of the EM perturbation driving the energy transfer and the spectroscopic transitions are quite different. The source of this interaction term... 

Magnetic circular dichroism (MCD) has now become a tool with valuable applications to analytical and structural studies. The MCD spectrum of thiophene is only slightly perturbed by substituents, and this is also expected to be true of the quite similar MCD spectra of selenophene.18 These molecules can therefore be classified as hard chromophores. 

A distorted conjugated pair of double bonds is an intrinsically chiral chromophoric system, and its overall chiroptical properties depend on the reduced symmetry of the chromophore itself as well as on the perturbing action of a dissymmetric environment. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

Here ei and e2 are the unit vectors of oscillator 1 (the diene chromophore) and 2 (the C-CH3 dipole), respectively R12 is the vector joining their midpoints, G12 is the point-dipole interaction term and N is the Avogadro number all quantities are expressed in c.g.s. units. In the absorption region of the chromophore, where the perturber does not show a significant absorption, the above formula reduces to the simpler equation410... 

The term angular overlap model was first used in 1965 to describe a more general treatment, which may also be called the model (12,13). The relative energies of the orbitals in a partly-filled /-shell are expressed in terms of the parameters ex (X = a, rr, etc.), whose coefficients can be calculated from the geometry of the system. The eK model was developed from perturbation theory, but is equivalent to the E2 model when only a-overlap is considered, and to the Yamatera-McClure model for orthoaxial chromophores with linear ligators. The notation ex is often used, where ... 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

In colour vision there are three specific types of cone cell corresponding to red, green and blue receptors. The chromophore is the same for all three colours, being 11-cis-retinal bound to a protein which is structurally similar to opsin. Colour selectivity is achieved by positioning specific amino acid side chains along the chromophore so as to perturb the absorption spectrum of the chromophore. 

Dynamic In this case, the distribution of lifetimes is the result of the electronically excited tryptophan chromophore being perturbed by and colliding with the surrounding groups of atoms in the protein molecule and with solvent molecules. 

The ORD and CD curves of optically active polymers containing chromo-phoric groups show that the chromophores can be asymmetrically perturbed by the chirality of the substituents and of the main chain conformation. This is the case with poly( ec-butyl vinyl ketone) (377), which presents a Cotton effect at 292 nm, its intensity being greater in the prevalently isotactic polymer than in the atactic polymer. 

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