An ab initio analysis of the structure of l-tryptophan tautomers in microhydrated environments, in water and in hydrophobic solvents

Transient 19F photo-CIDNP: A practical tool to distinguish intermediate radical species and determine isotropic hyperfine coupling constants of 19F nuclei

Fluorine-containing flavin derivatives can be used as probes in flavin-binding proteins forming radical pairs to exploit the photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect. Knowledge of the hyperfine structure is crucial for studying the mechanism of intramolecular radical-pair formation in proteins. Transient 19F photo-CIDNP NMR has so far not been used to determine the isotropic hyperfine coupling constants of 19F nuclei. Here, we show that this method provides reliable results by studying three monofluorinated flavin mononucleotide (FMN) derivatives in conjunction with 6-fluoro-tryptophan. Combining this method with transient 1H photo-CIDNP spectroscopy leads to a more accurate interpretation of the intermediate radical species forming a radical pair. The gathered information can be used to identify the most promising FMN derivative for usage as a probe for formation of radical pairs in proteins.

Solvent promoted tautomerism in thione-containing tetraazatricyclics: evidence from 1H NMR spectroscopy and transition state studies

Tautomerism in the nitro substituted thione-containing traazatricyclics has been investigated. Evidence from ¹H NMR indicating the existence of the tautomers has been augmented with computational studies providing evidence of the stability or otherwise of these tautomers. The role of water and DMSO in the formation of the tautomers has been explained. The role of the nitro group in assisting in the formation of the tautomers has been discussed.

A study of the effects of the polarity of the solvents acetone and cyclohexane on the luminescent properties of tryptophan

The luminescent properties of tryptophan in solvents less polar than water, such as acetone, and non-polar ones, such as cyclohexane, are experimentally studied and compared with theoretical calculations using time-dependent density functional theory (TD-DFT) methods. Since tryptophan may present different configurations and charge distributions, the most stable conformer is analyzed for both solvents, including its neutral and zwitterionic forms. To perform the simulation two clusters are proposed with the Zpt conformer in acetone: [Formula: see text] and [Formula: see text] , and four clusters with the Nag⁺ conformer in cyclohexane: (Trp)₁-(C₆H₁₂), (Trp)₂-(C₆H₁₂), (Trp)₃-(C₆H₁₂) and (Trp)₄-(C₆H₁₂), in order to conveniently emulate the concentration in each solvent by reducing the distance between adjacent tryptophan molecules as the concentration increases, since there is no control over the volume parameter. In each case, the UV-vis absorption is computed and compared with the experimental excitation spectra; the results show a good agreement. This calculation allows a more detailed analysis of the experimental results based on the properties of the molecular orbitals involved in electronic transitions. In the present work, a strong effect of the solvent acetone on tryptophan is observed; for this solvent, a charge transfer from the solute to solvent happens. This behavior does not occur with water (polar solvent) or cyclohexane (non-polar solvent). Finally, experimental spectroscopic data of Trp in cyclohexane are explained through the hydrogen bonds between amino acid molecules present in the fluorescent states. In this case, the theoretical and experimental results are compared and also show good agreement.