Search

Molecular dynamics (MD), coupled with fluorescence data for charged dipeptides of tryptophanyl glutamic acid (Trp-Glu), reveal a detailed picture of how specific conformation affects fluorescence. Fluorescence emission spectra and time-resolved emission measurements have been collected for all four charged species. MD simulations 20 to 30 ns in length have also been carried out for the Trp-Glu species, as simulation provides aqueous phase conformational data that can be correlated with the fluorescence data. The calculations show that each dipeptide species is characterized by a similar set of six, discrete Chi 1, Chi 2 dihedral angle pairs. The preferred Chi 1 angles-60°, 180°, and 300°-play the significant role in positioning the terminal amine relative to the indole ring. A Chi 1 angle of 60° results in the arching of the backbone over the indole ring and no interaction of the ring with the terminal amine. Chi 1 values of 180° and 300° result in an extension of the backbone away from the indole ring and a NH3 cation-π interaction with indole. This interaction is believed responsible for charge transfer quenching. Two fluorescence lifetimes and their corresponding amplitudes correlate with the Chi 1 angle probability distribution for all four charged Trp-Glu dipeptides. Fluorescence emission band maxima are also consistent with the proposed pattern of terminal amine cation quenching of fluorescence. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Although tryptophan is a natural probe of protein structure, interpretation of its fluorescence emission spectrum is complicated by the presence of two electronic transitions, 1 La and 1 Lb. Theoretical calculations show that a point charge adjacent to either ring of the indole can shift the emission maximum. This study explores the effect of pyrrole and benzyl ring substitutions on the transitions' energy via absorption and fluorescence spectroscopy, and anisotropy and lifetime measurements. The survey of indole derivatives shows that methyl substitutions on the pyrrole ring effect 1 La and 1 Lb energies in tandem, whereas benzyl ring substitutions with electrophilic groups lift the 1 La/1 Lb degeneracy. For 5- and 6-hydroxyindole in cyclohexane, 1 La and 1 Lb transitions are resolved. This finding provides for 1 La origin assignment in the absorption and excitation spectra for indole vapor. The 5- and 6-hydroxyindole excitation spectra show that despite a blue-shifted emission spectrum, both the 1 La and 1 Lb transitions contribute to emission. Fluorescence lifetimes of 10 ns for 5-hydroxyindole are consistent with a charge acceptor-induced increase in the nonradiative rate (1). [ABSTRACT FROM AUTHOR]