Aromatic group fluorescence

If microstates lead to the existence of a distribution of energies of interaction between aromatic groups and neighboring groups of atoms, then the individual spectra of these groups in different microstates shift differently, which results in an inhomogeneous contour of the absorption band. The application of selective photoexcitation permits specific effects of the distribution of microstates on spectral, temporal, and polarization fluorescence properties to be observed. 221 Such effects have been observed in studies of proteins, 1,8) and, as we show below, they may be used to obtain important information on dynamics. 

Small molecules that act as collisional quenchers may penetrate into the internal structure of proteins, diffuse, and cause quenching upon collision with the aromatic groups. Lakowicz and Weber(53) have shown that the interaction of oxygen molecules with buried tryptophan residues in proteins leads to quenching with unexpectedly high rate constants—from 2 x 109 to 7 x 109 M l s 1. Acrylamide is also capable of quenching the fluorescence of buried tryptophan residues, as was shown for aldolase and ribonuclease 7V(54) A more hydrophobic quencher, trichloroethanol, is a considerably more efficient quencher of internal chromophore groups in proteins.(55)... 

If the aromatic group is bound tightly within the protein molecule, then one may obtain information on the rotational diffusion of the whole molecule from fluorescence polarization studies. Such investigations, which were started by Weber,(68) were widely popular in the 1960s and 1970s. Correlation times D of macromolecule rotations were determined according to the Perrin equation ... 

Polystyrene has been examined as 1/16" thick plates of commercial samples and also as specially prepared isotactic powder. Both spectra were similar. The commercial sample like its more pure specimen gave little trouble from fluorescence. The most striking feature of the spectra is the fact that almost all of the spectral features originate from the monosubstituted aromatic groups. This is contrary to the normally held view that the Raman spectrum consists principally of bands due to... 

Some biomolecules are intrinsic fluors that is, they are fluorescent themselves. The amino acids with aromatic groups (phenylalanine, tyrosine,... 

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. 

It is the aromatic amino acid side-chains that give proteins their fluorescent characteristics. If these aromatic groups are damaged, the fluorescence is modified. However, phenylalanine fluorescence is not observed in the presence of other aromatic groups and tyrosine fluorescence is detected only in the absence of tryptophan (Teale, 1960). Even in proteins containing high relative proportions of tyrosine to tryptophan, the fluorescence of the former is masked. 

Conformations of oligo(pyridine-aA-pyrimidine)s 119 have been studied. On the basis of NMR analysis and the fluorescence spectrum in solution, the oligomers were found to take a helical conformation.203 The conformation was characterized by distinct chemical shifts (upheld shift), NOE effects, and excimer emission arising from the overlap of aromatic groups. The helical structure was confirmed for 120 in the... 

Energy transfer in polymers has been studied in the pure solid state, in heterogeneous systems (e.g. polymer blends), in liquid solutions and in solid solutions. The last case, which will be considered here, provides relatively simple and clear experimental conditions since interactions between the macromolecules can be excluded by dilution and molecular movement is severely restricted by low temperature and rigid environment. Thus, excitonic energy transfer can be studied without competing molecular movement. The luminescence of dilute, solid solutions of aromatic polymers is not dominated by excimers - in sharp contrast to the other modes of observation - so that side group fluorescence and phosphorescence can be observed. This does not mean, however, that exciton trapping processes are absent in these systems. 

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