4 research outputs found
Correlation of TrpGly and GlyTrp Rotamer Structure with W7 and W10 UV Resonance Raman Modes and Fluorescence Emission Shifts
Tryptophyl glycine (TrpGly) and glycyl tryptophan (GlyTrp) dipeptides at pH 5.5 and pH 9.3 show a pattern of fluorescence emission shifts with the TrpGly zwitterion emission solely blue shifted. This pattern is matched by shifts in the UV resonance Raman (UVRR) W10 band position and the W7 Fermi doublet band ratio. Ab initio calculations show that the 1340 cm−1 band of the W7 doublet is composed of three modes, two of which determine the W7 band ratios for the dipeptides. Molecular dynamics simulations show that the dipeptides take on two conformations: one with the peptide backbone extended; one with the backbone curled over the indole. The dihedral angle critical to these conformations is χ1 and takes on three discrete values. Only the TrpGly zwitterion spends an appreciable amount of time in the extended backbone conformation as this is stabilized by two hydrogen bonds with the terminal amine cation. According to a Stark effect model, a positive charge near the pyrrole keeps the 1La transition at high energy, limiting fluorescence emission red shift, as observed for the TrpGly zwitterion. The hydrogen bond stabilized backbone provides a rationale for the Cmethylene-Cα-Ccarbonyl W10 symmetric stretch that is unique to the TrpGly zwitterion
The Broken Ring: Reduced Aromaticity in Lys-Trp Cations and High pH Tautomer Correlates with Lower Quantum Yield and Shorter Lifetimes
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 p<i>K</i><sub>1</sub>, p<i>K</i><sub>2</sub>, and p<i>K</i><sub>R</sub>) 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 <sup>1</sup>L<sub>a</sub> 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 concentrates
charge on the indole amine; the isosurface charge distribution bears
scant resemblance to the indole HOMO. Such greatly diminished fluorescence
has been observed for proteins where the indole nitrogen is hydrogen
bonded, lending credence to the association of aromaticity changes
with diminished quantum yield in proteins as well. Thus tryptophan
ground state isosurfaces are an indicator of indole aromaticity, signaling
the partition of excitation energy between radiative and nonradiative
processes