Fluorescence electron donating groups

Fluorescence is greatly affected by the structure of a molecule. Usually only aromatic compounds fluoresce although some aliphatic and alicyclic molecules are known to fluoresce. Electron-donating groups such as -OH and -OCH3, that can increase the electron flow of an aromatic system usually increase the fluorescence while other groups that contain hetero atoms with n-electrons that can absorb the emitted energy, will usually quench the fluorescence. However, it is always difficult to predict whether or not, or to what extent, a compound will fluoresce. 

Substituent groups have a marked effect on the fluorescence quantum yield of many compounds. Electron-donating groups such as -OH, -NH2 and -NR.2 enhance the fluorescence efficiency, whereas electron-withdrawing groups such as -CHO, -C02H and -N02 reduce the fluorescence quantum yield, as shown by naphthalene and its derivatives in Table 4.3. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

The addition of electrons to an aromatic system can be accomplished by adding electron-donating groups or by changing the pH of the solution. The effect of pH on the fluorescence of 1-naphthol is well known. In acidic or neutral media the molecule exists as the form 1 ... 

Small molecule or peptide substrates can also be used to profile protease and other enzyme activities. Salisbury et al. described a protease-substrate microarray in which the carboxyl end of the peptide substrates was conjugated to 7-amino-4-car-bamoylmethyl coumarin, a fluorogenic compound. The conjugate was non-fluores-cent when the electron-donating group on the coumarin was attached to the peptide. Upon proteolysis, the peptide was released and the microarray spot fluoresced. Zhu et al. demonstrated that small molecule microarrays could be used to detect enzyme activities of epoxide hydrolases and phosphatases. 

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