Molecule chromophore

The first step in XJVR-induced skin cancer is UVR-initiated DNA mutation, which causes the transformation of the normal cells to malignant cells. For UVR to initiate a biological reaction, it has to be absorbed by endogenous molecules (chromophores). UVB is absorbed directly by the DNA, and therefore can directly induce DNA mutation (224), in the form of thymine dimer formation (289). Some protein components may also ad as chromophores for UVB (224). UVA is absorbed by the reduced forms of the co-enzymes nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH), tryptophan, riboflavin, and melanin (224,290). UVA-induced DNA damage is believed to be mediated by oxygen reactive species that are released after the absorption of UVA by those endogenous chromophores and results in photooxidation of selected bases... 

Two types of groups can affect the absorption spectrum of a molecule chromophores and auxochromes. 

Generally, only limited excitations are considered — primarily singly-excited exciton configurations in which a single molecule (chromophore) i is excited, e.g.. 

The fluorescence properties of a given molecular system, namely the intensity (or eflEciency) and the spectral shape, recorded under continuous-wave excitation, are sensitive to the medium in which they are embedded (namely solution versus solid state) and the neighboring molecules (chromophores and/or fluorophores). It is this sensitivity that allows information to he obtained on the sample s morpholep. 

To separate small molecule chromophores from the chromophores in the polymer backbone, Allan et al. [67] utilized GPC-UV/vis [79-81]. Five bulk... 

AH the properties dealt with up until now involve linear interactions between light and polymer. Interaction of li t with polymers in the nonlinear region involves second- and third-order effects as well as the phenomenon of photo refrac-tivity (56,57). An optical nonlinear optical (NIX)) polymer is one that, in response to an externally applied electric field, can either vary the speed of incoming light or alter its fi uency. Var3dng the speed of light involves a change in the reflective index of the material. An optically nonlinear polymer has two components the polymer itself and an optically nonlinear molecule (chromophore), which is either chemically attached to the polymer or dissolved in it. 

Dubai H-R, Ha T-K, Lewerenz M and Quack M 1989 Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore In CHXg molecules J. Chem. Phys. 91 6698-713... 

The development of tunable, narrow-bandwidtli dye laser sources in tire early 1970s gave spectroscopists a new tool for selectively exciting small subsets of molecules witliin inhomogeneously broadened ensembles in tire solid state. The teclmique of fluorescence line-narrowing [1, 2 and 3] takes advantage of tire fact tliat relatively rigid chromophoric... 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

When two or more chromophores are present in the same molecule, their absorption is usually additive as long as they are separated by two or more single bonds. Two chromophores in conjugation (i.e., separated by only one single bond) give rise to a new tjrpe of absorption with increased Some examples are given in Table IX. 

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

UV-VIS Unless the molecule has other chromophores alcohols are transparent above about 200 nm for methanol for example is 177 nm... 

Molecules with two or more isolated chromophores (absorbing groups) absorb light of nearly the same wavelength as does a molecule containing only a single chromophore of a particular type. The intensity of the absorption is proportional to the number of that type of chromophore present in the molecule. Representative chromophores are given in Table 7.9. 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

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