Session: Physical Chemistry Poster:

We report photophysical studies of 2-cyanophenylalanine used as both a spectroscopic reporter of the local solvent environment and a site-specific probe within peptides. Absorption of an ultraviolet photon by 2-cyanophenylalanine results in a π* ← π transition with a molar absorptivity similar to common biological probes, such as tyrosine and tryptophan, but a factor four larger than phenylalanine. Due to intermolecular interactions with the nitrile group on the chromophore, the local solvent environment can be monitored by changes in the quantum yield of the S₁ ← S₀ transition. As a site-specific probe, 2-cyanophenylalanine can be paired with tryptophan to perform Förster resonance energy transfer (FRET) experiments with an R₀ of 15 Å and only slight perturbation of the native geometries of peptides. Incorporation of 2-cyanophenylalanine at two locations in olfactory peptides where tryptophan occupies the N-terminus provides a model system for spectroscopic reporting of structural dynamics. Measurement of the FRET efficiency within various solvent environments that either promote or inhibit secondary structures of these olfactory peptides results in distances of 6 to 13 Å between the intrinsic probes. Computational calculations of the electronic structure of 2-cyanophenylalanine are compared with those of the 3- and 4-cyanophenylalanine derivatives illustrating the dependence of the electron density on the location of the nitrile group within the chromophore. Both the experimental and computational results further demonstrate the utility of cyanophenylalanine derivatives as spectroscopic reporters of peptide structure and dynamics in various solvent environments.

Austin Haider

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