Microsolvation Effects on the Excited-State Dynamics of Protonated Tryptophan

• Published in: Journal of the American Chemical Society Vol. 128; no. 51; pp. 16938 - 16943
• Main Authors: Mercier, Sébastien R, Boyarkin, Oleg V, Kamariotis, Anthi, Guglielmi, Matteo, Tavernelli, Ivano, Cascella, Michele, Rothlisberger, Ursula, Rizzo, Thomas R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27.12.2006
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Models, Molecular; Protons; Thermodynamics; Tryptophan - chemistry; Water; Quantum chemistry; Molecular structure; Nitrogen heterocycle; Theoretical study; Tryptophan; Molecular interaction; Protein; α-Aminoacid; Solvation; Simulation; Aminoacid; Gas phase; Spectrophotometry; Excited state
• ISSN: 0002-7863; 1943-2984; 1520-5126; 1520-5126
• DOI: 10.1021/ja065980n

To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure and dynamics, we have measured electronic spectra of protonated, gas-phase tryptophan solvated with a controlled number of water molecules and cooled to ∼10 K. We observe that, even at this temperature, the bare molecule exhibits a broad electronic spectrum, implying ultrafast, nonradiative decay of the excited state. Surprisingly, the addition of two water molecules sufficiently lengthens the excited-state lifetime that we obtain a fully vibrationally resolved electronic spectrum. Quantum chemical calculations at the RI-CC2/aug-cc-pVDZ level, together with TDDFT/pw based first-principles MD simulations of the excited-state dynamics, clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited-state lifetime.