Infrared and Electronic Spectra of Radicals Produced from 2‑Naphthol and Carbazole by UV-Induced Hydrogen-Atom Eliminations

The photoreaction mechanisms of 2-naphthol and carbazole in low-temperature argon matrices have been investigated by infrared and electronic absorption spectroscopy with aids of density-functional-theory (DFT) and time-dependent DFT (TD-DFT) calculations. When the matrix samples were irradiated upon UV light, 2-naphthoxyl and N-carbazolyl radicals were produced by the elimination of the H atom in the O–H group of 2-naphthol and in the N–H group of carbazole, respectively. The observed IR and electronic absorption spectra of the radicals were reproduced satisfactorily by the quantum chemical calculations. To understand a role of the radicals in the excited-state proton transfer (ESPT), the fluorescence and excitation spectra of 2-naphthol and carbazole were measured in aqueous solution at room temperature as well as in the low-temperature argon matrices. It was found that the intensity of the fluorescence emitted from carbazole anion in aqueous solution decreased when oxygen gas was blown into the solution

Macromolecular Crowding Modifies the Impact of Specific Hofmeister Ions on the Coil–Globule Transition of PNIPAM

Macromolecular crowding alters many biological processes ranging from protein folding and enzyme reactions in vivo to the precipitation and crystallization of proteins in vitro. Herein, we have investigated the effect of specific monovalent Hofmeister salts (NaH₂PO₄, NaF, NaCl, NaClO₄, and NaSCN) on the coil–globule transition of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) in a crowded macromolecular environment as a model for understanding the specific-ion effect on the solubility and stability of proteins in a crowded macromolecular environment. It was found that although the salts (NaH₂PO₄, NaF, and NaCl) and the macromolecular crowder (polyethylene glycol) lowered the transition temperature almost independently, the macromolecular crowder had a great impact on the transition temperature in the case of the chaotropes (NaClO₄ and NaSCN). The electrostatic repulsion between the chaotropic anions (ClO₄^– or SCN^–) adsorbed on PNIPAM may reduce the entropic gain of water associated with the excluded volume effect, leading to an increase in the transition temperature, especially in the crowded environment. Furthermore, the affinity of the chaotropic anions for PNIPAM becomes small in the crowded environment, leading to further modification of the transition temperature. Thus, we have revealed that macromolecular crowding alters the effect of specific Hofmeister ions on the coil–globule transition of PNIPAM

Simultaneous Interaction of Hydrophilic and Hydrophobic Solvents with Ethylamino Neurotransmitter Radical Cations: Infrared Spectra of Tryptamine⁺‑(H₂O)_<i>m</i>‑(N₂)_<i>n</i> Clusters (<i>m</i>,<i>n</i> ≤ 3)

Solvation of biomolecules by a hydrophilic and hydrophobic environment strongly affects their structure and function. Here, the structural, vibrational, and energetic properties of size-selected clusters of the microhydrated tryptamine cation with N₂ ligands, TRA⁺-(H₂O)_<i>m</i>-(N₂)_<i>n</i> (<i>m</i>,<i>n</i> ≤ 3), are characterized by infrared photodissociation spectroscopy in the 2800–3800 cm^–1 range and dispersion-corrected density functional theory calculations at the ωB97X-D/cc-pVTZ level to investigate the simultaneous solvation of this prototypical neurotransmitter by dipolar water and quadrupolar N₂ ligands. In the global minimum structure of TRA⁺-H₂O generated by electron ionization, H₂O is strongly hydrogen-bonded (H-bonded) as proton acceptor to the acidic indolic NH group. In the TRA⁺-H₂O-(N₂)_<i>n</i> clusters, the weakly bonded N₂ ligands do not affect the H-bonding motif of TRA⁺-H₂O and are preferentially H-bonded to the OH groups of the H₂O ligand, whereas stacking to the aromatic π electron system of the pyrrole ring of TRA⁺ is less favorable. The natural bond orbital analysis reveals that the H-bond between the N₂ ligand and the OH group of H₂O cooperatively strengthens the adjacent H-bond between the indolic NH group of TRA⁺ and H₂O, while π stacking is slightly noncooperative. In the larger TRA⁺-(H₂O)_<i>m</i> clusters, the H₂O ligands form a H-bonded solvent network attached to the indolic NH proton, again stabilized by strong cooperative effects arising from the nearby positive charge. Comparison with the corresponding neutral TRA-(H₂O)_<i>m</i> clusters illustrates the strong impact of the excess positive charge on the structure of the microhydration network

External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′‑<i>N</i>,<i>N</i>‑Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films

The external electric field effects on the steady-state electronic spectra and excited-state dynamics were investigated for 4′-<i>N</i>,<i>N</i>-(dimethylamino)-3-hydroxyflavone (DMHF) in a poly­(methyl methacrylate) (PMMA) film. In the steady-state spectrum, dual emission was observed from the excited states of the normal (N*) and tautomer (T*) forms. Application of an external electric field of 1.0 MV·cm^–1 enhanced the N* emission and reduced the T* emission, indicating that the external electric field suppressed the excited-state intramolecular proton transfer (ESIPT). The fluorescence decay profiles were measured for the N* and T* forms. The change in the emission intensity ratio N*/T* induced by the external electric field is dominated by ESIPT from the Franck–Condon excited state of the N* form and vibrational cooling in potential wells of the N* and T* forms occurring within tens of picoseconds. Three manifolds of fluorescent states were identified for both the N* and T* forms. The excited-state dynamics of DMHF in PMMA films has been found to be very different from that in solution due to intermolecular interactions in a rigid environment