Chromophores intrinsic

Metallochromic dyes have been used to study the kinetics of aurothionein due to the lack of intrinsic and distinctive chromophoric changes during reaction [105]. 

The values of ks/kp for partitioning of carbocations are most conveniently determined as the ratio of the yields of products from the competing nucleophile addition and proton transfer reactions (equation 1 derived for Scheme 2). The determination of these product yields has been simplified in recent years by the application of high-pressure liquid chromatography (HPLC). Typically, the product peaks from an HPLC analysis are detected and quantified by UV-vis spectroscopy. In cases where the absorbance of reactants and products is small, substrates may be prepared with a chromophore placed at a sufficient distance so that its effects on the intrinsic reactivity of the carbocationic center are negligible. For example, the aliphatic substrates [1]-Y have proved to be very useful in studies of the reactions of the model tertiary carbocation [1+].21,23... 

Thus, in the present approach, the major focus is on the question of how we can influence the external parameters like solvent and counterion and the intrinsic structural parameters within the systems A-l-A to force the electron-hopping process into the timescale of the experiment, or at least to establish clearly the borderline cases. That we are still looking at an electron-hopping process in the case of effective charge delocalization over the entire molecule and not at a pure resonance phenomenon may be reassured by VIS/NIR spectroscopy of the neutral and charged species the absorption of a single chromophore should be detected unless a very fast process > 1012 Hz is taking place. 

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. 

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

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 planar conformations s-cis and s-trans cannot be appreciably populated owing to the repulsive steric interaction between the Cg—H and C5-methyl (in the s-cis) or between the same Cg—II and the methyl group on (Q in the s-trans). This repulsion is minimized by introducing a twist around the Q,—( 7 bond. Limiting conformations, with skew angles of about 40° and 140°, can be assumed, as shown in Scheme 8. In this way, an intrinsically dissymmetric chromophore is created. 

In the skewed form, instead, the transition is allowed both electrically and magnetically, with parallel transition moments. The product in equation 1 is hence non-vanishing, implying that this transition has finite rotational strength. This observation leads to the conclusion that skewed 1,3-butadiene is an intrinsically dissymmetric chromophore. 

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

For alkyl-substituted A-nitrosazetidines, it was shown that the CD spectra can be interpreted on the basis of conformational diastereoisomerism, taking into account the nonplanarity of the nitrosazetidine chromophore137. For a particular configuration, the CE sign of the n - tt transition in the 350-400 nm region is determined by the intrinsic chirality of the chromophore and obeys a spiral rule, the same as that for nonplanar nitrosaziridines where the absence of the local plane of symmetry in the nitrosamine chromophore does not permit the use of the planar sector rule 148. 

The indole chromophore of tryptophan is the most important tool in studies of intrinsic protein fluorescence. The position of the maximum in the tryptophan fluorescence spectra recorded for proteins varies widely, from 308 nm for azurin to 350-353 nm for peptides lacking an ordered structure and for denatured proteins. (1) This is because of an important property of the fluorescence spectra of tryptophan residues, namely, their high sensitivity to interactions with the environment. Among extrinsic fluorescence probes, aminonaphthalene sulfonates are the most similar to tryptophan in this respect, which accounts for their wide application in protein research.(5)... 

The steps leading to the formation of the intrinsic chro-mophore have recently been investigated kinetically with S65T-GFP. The process of chromophore formation is an ordered sequence of three distinct steps (1) slow protein folding (kf = 2.44 X 10 s ) that precedes chromophore modification (2) an intermediate step occurs that includes, but may not be necessarily limited to, cycli-zation of the tripeptide chromophore motif (kc = 3.8 X 10 s ) and (3) rate-limiting oxidation of the cyclized chromophore (kox = 1 51 X s ). Reid and Flynn also reasoned that because chromophore forms de novo from purified denatured protein and is a first-order process, GFP chromophore formation is likely to be an autocatalytic process. 

Hao W, Fong HK 1999 The endogenous chromophore of retinal G protein-coupled receptor opsin from the pigment epithehum. J Biol Chem 274 6085—6090 Hattar S, Liao HW, Takao M, Berson DM, Yau KW 2002 Melanopsin-containing retinal ganglion cells architecture, projections, and intrinsic photosensitivity. Science 295 1065— 1070... 

In TPM, fluorescence emission of a fluorophore (either intrinsic to the sample or an exogenously applied chromophore) is detected. A fluorophore is used to obtain specific chemical information in the skin, as discussed in more detail below. To excite the fluorophore, two-photon excitation is used, which is achieved by the simultaneous... 

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