What is a Chromophore

Alcohols, Ethers, Amines, and Sulfur Compounds. In saturated molecules that contain atoms bearing nonbonding pairs of electrons, transitions of the n — r type become important. They are also 

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

Alkenes and Alkynes. With unsatuiated molecules, transitions become possible. These 

Other substituents may have any of four kinds of effects on the absorption  

Carbonyl Compounds. Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n— -n transitions. These are perhaps the most interesting and most studied transitions, particularly among carbonyl compounds. These transitions are also rather sensitive to substitution on the chromophoric structure. The typical carbonyl compound undergoes an n — r transition around 280 to 290 nm ( = 15). Most transitions are forbidden and hence are of 

Bathochromic shift (red shift)—a shift to lower energy or longer wavelength. 

Hypsochromic shift (blue shift)—a shift to higher energy or shorter wavelength. 

Carbonyl Compounds. Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo transitions. These are perhaps the most interesting and most studied transi- 

---

The concept of the chromophore is of eminent importance for probing chiral structures of molecules and their surroundings. But what is a chromophore In chemical textbooks, a chromophore is a molecule or a group in a molecule that is the carrier of color in the visible spectral region. More generally, one chromophore does belong to every absorption band and vice versa. But besides this, the notion is only intuitively defined. From the spectroscopic point of view a chromophore can be defined as an area of a molecule in which some properties have changed after the molecule became excited. In this context measurable and computable properties are the... 

Humic substances represent a large fraction of what is termed chromophoric dissolved organic matter (CDOM) in aquatic systems around the world. Aquatic humic substances can be further categorized as fulvic acids, humic acids, and humin based on theis solubility in acid and base solutions. 

The absorption spectra of several cystine-containing peptides as obtained by Otey and Greenstein (1954) are shown in Fig. 3. Some nonadditivity of absorptivities is evident for example, the spectrum of l-cystinyl-L-cystine is obviously not simply the sum of 2 cystine spectra, even at wavelengths greater than 2500 A, where the disulfide group is the only significant chromophore. The question, What is a suitable model for cystine absorptivity in a protein is apparently farther from a satisfactory answer than is the parallel question for aromatic amino acid residues. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

From a theoretical perspective, since the designation of the lab-fixed axes is arbitrary, what is relevant is the relative orientation of the polarizations of the excitation and scattered light. Thus the line shape for excitation light polarized along axis p, and scattered light polarized along axis q (p or q denote X, Y, or Z axes in the lab frame) is called Ipq(co). When p = q this is lyy, and when p q this is IVH. Mixed quantum/classical formulae for Ipq(co) are identical to those for the IR spectmm, except mPi is replaced by apqP which is the pq tensor element of the transition polarizability for chromophore i. Thus we have, for example [6],... 

We wanted to extend this approach to include dynamical effects on line shapes. As discussed earlier, for this approach one needs a trajectory co t) for the transition frequency for a single chromophore. One could extract a water cluster around the HOD molecule at every time step in an MD simulation and then perform an ab initio calculation, but this would entail millions of such calculations, which is not feasible. Within the Born Oppenheimer approximation the OH stretch potential is a functional of the nuclear coordinates of all the bath atoms, as is the OH transition frequency. Of course we do not know the functional. Suppose that the transition frequency is (approximately) a function of a one or more collective coordinates of these nuclear positions. A priori we do not know which collective coordinates to choose, or what the function is. We explored several such possibilities, and one collective coordinate that worked reasonably well was simply the electric field from all the bath atoms (assuming the point charges as assigned in the simulation potential) on the H atom of the HOD molecule, in the direction of the OH bond. 

What mechanisms can be used to create a lifetime-based glucose sensor In our opinion, the mechanism should be fluorescence resonance energy transfer (FRET). The phenomenon of FRET results in transfer of the excitation from a donor fluorophore to an acceptor chromophore, which need not itself be fluorescent. FRET is a through-space interactor which occurs over distances of 20-60 A. 

Foster Yes I agree, there has been much discussion about the absorption spectrum of the cr3rptochromes because of the two potential chromophores, but what is fascinating is that the normalized flavoprotein and CRY 1 action spectrum I showed in my presentation (see Fig. 4) shows a striking similarity with that action spectrum iot Arabidopsis CRYl as published by Margaret Ahmad and colleagues (Ahmad et al 2002). 

Young What is the half-hfe of these proteins If this is taking place over several months I have a hard time imagining melanopsin as a sponge that holds on to chromophore for this long. 

What is the advantage of a time-resolved emission measurement with Eu3+ versus measurement of fluorescence from organic chromophores ... 

---