Excited State Electron Distribution and Role of the Terminal Amine in Acidic and Basic Tryptophan Dipeptide Fluorescence

Abstract

The results of quantum yield (QY) study of tryptophanyl glutamate (Trp-Glu), tryptophanyl lysine (Trp-Lys) and lysinyl tryptophan (Lys-Trp) dipeptides over the pH range, 1.5 - 13, show that the charge state of the N-terminal amine, and not the nominal molecular charge determines the QY. When the terminal amine is protonated, QY is low (10^-2) for all three dipeptides. As the terminal amine cation is found proximal to the indole ring in Trp-Glu and Trp-Lys conformers but not in those for Lys-Trp, its effect may lie only in the partitioning of energy between nonradiative processes, not on QY reduction. QY is also low when both the N-terminal amine and indole amine are deprotonated. These two low QY states can be distinguished by fluorescence lifetime measurement. Molecular dynamics simulation shows that the Chi 1 conformers persist for tens of nanoseconds such that 10⁰ - 10¹ nanosecond lifetimes may be associated with individual Chi 1 conformers. The ground state electron density or isosurface of high QY (0.30) 3-methyindole has a uniform electron density over the indole ring as do the higher QY Trp dipeptide conformers. This validates the association of ground state isosurfaces with QY. Excited state orbitals from calculated high intensity, low energy absorption transitions are typically centered over the indole ring for higher QY dipeptide species and off the ring in lower QY species. Thus excited state orbitals substantiate the earlier finding that the ground state isosurface charge density pattern on the indole ring can be predictive of QY.

Keywords: Tryptophan fluorescence; excited state; fluorescence lifetime; molecular dynamics simulation; quantum mechanics calculations; quantum yield.

Fig. 1

Molecular structure of tryptophan with carbon atom numbering used in the text.

Fig. 2

Isosurfaces at the 0.020 level for Trp-Glu dipeptide species in different charge states and for aqueous 3-methylindole. Representative Chi 1 conformations are shown for each of the three dominant Chi 1 angles of 60°, 180° and 300° of Trp-Glu. Probabilities (%) for each conformation are given. The color bar at the top of each isosurface indicates the charge scale.

Fig. 3

Isosurfaces at the 0.020 level for Trp-Lys dipeptide species in different charge states. Representative Chi 1 conformations are shown for each of the three dominant Chi 1 angles of 60°, 180° and 300°. Probabilities (%) for each conformation are given. The color bar at the top of each isosurface indicates the charge scale.

Fig. 4

Orbitals for Trp-Glu dipeptide Chi 1 conformations. a. HOMO, Trp-Glu 1+, Chi 1=60° b. LUMO, Trp-Glu 1+, Chi 1= 60° c. LUMO, Trp-Glu 1+, Chi 1=300° d. LUMO, Trp-Glu 0, Chi 1=300° e. LUMO, Trp-Glu 3−, Chi 1=60°.

Fig. 5

Orbitals for Trp-Lys dipeptide Chi 1 conformations. a. LUMO+1, Trp-Lys 2+, Chi 1= 60° b. LUMO+1, Trp-Lys 1+, Chi 1= 300° c. LUMO+2, Trp-Lys 2−, Chi 1=60° d. LUMO, Trp-Lys 2−, Chi 1=60°.