The BrokenRing: Reduced Aromaticity in Lys-Trp Cationsand High pH Tautomer Correlates with Lower Quantum Yield and ShorterLifetimes

摘要

Several nonradiative processes compete with tryptophan fluorescence emission. The difficulty in spectral interpretation lies in associating specific molecular environmental features with these processes and thereby utilizing the fluorescence spectral data to identify the local environment of tryptophan. Here, spectroscopic and molecular modeling study of Lys-Trp dipeptide charged species shows that backbone-ring interactions are undistinguished. Instead, quantum mechanical ground state isosurfaces reveal variations in indole π electron distribution and density that parallel charge (as a function of pK1, pK2, and pKR) on the backbone and residues. A pattern of aromaticity-associated quantum yield and fluorescence lifetime changes emerges. Where quantum yield is high, isosurfaces have a charge distribution similar to the highest occupied molecular orbital (HOMO) of indole, which is the dominant fluorescent ground state of the ¹La transition dipole moment. Where quantum yield is low, isosurface charge distribution over the ring is uneven, diminished, and even found off ring. At pH 13, the indole amine is deprotonated, and Lys-Trp quantum yield is extremely low due to tautomer structure that concentratescharge on the indole amine; the isosurface charge distribution bearsscant resemblance to the indole HOMO. Such greatly diminished fluorescencehas been observed for proteins where the indole nitrogen is hydrogenbonded, lending credence to the association of aromaticity changeswith diminished quantum yield in proteins as well. Thus tryptophanground state isosurfaces are an indicator of indole aromaticity, signalingthe partition of excitation energy between radiative and nonradiativeprocesses.