title: Relating Trp-Glu dipeptide fluorescence to molecular conformation: the role of the discrete Chi 1 and Chi 2 angles.
abstract: 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.
authors.affiliation: Department of Chemistry, Brooklyn College of The City University of New York, 2900 Bedford Avenue, Brooklyn, New York 11210, USA.
authors.forename: Azaria Solomon
authors.initials: AS
authors.lastname: Eisenberg
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Juszczak
mesh_terms: D004151:Dipeptides / Q000737:chemistry*; D005453:Fluorescence; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory
chemical_list: D004151:Dipeptides; C057081:alpha-glutamyltryptophan
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