Tautomeric and conformational equilibria of tyramine and dopamine in aqueous solution

Dopamine Photochemical Behaviour under UV Irradiation

To understand the photochemical behaviour of the polydopamine polymer in detail, one would also need to know the behaviour of its building blocks. The electronic absorption, as well as the fluorescence emission and excitation spectra of the dopamine were experimentally and theoretically investigated considering time-resolved fluorescence spectroscopy and first-principles quantum theory methods. The shape of the experimental absorption spectra obtained for different dopamine species with standard, zwitterionic, protonated, and deprotonated geometries was interpreted by considering the advanced equation-of-motion coupled-cluster theory of DLPNO-STEOM. Dynamical properties such as fluorescence lifetimes or quantum yield were also experimentally investigated and compared with theoretically predicted transition rates based on Fermi's Golden Rule-like equation. The results show that the photochemical behaviour of dopamine is strongly dependent on the concentration of dopamine, whereas in the case of a high concentration, the zwitterionic form significantly affects the shape of the spectrum. On the other hand, the solvent pH is also a determining factor for the absorption, but especially for the fluorescence spectrum, where at lower pH (5.5), the protonated and, at higher pH (8.0), the deprotonated forms influence the shape of the spectra. Quantum yield measurements showed that, besides the radiative deactivation mechanism characterized by a relatively small QY value, non-radiative deactivation channels are very important in the relaxation process of the electronic excited states of different dopamine species.

Competing intramolecular vs. intermolecular hydrogen bonds in solution

A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

Theoretical study of the gauche-trans equilibrium with and without an intramolecular hydrogen bond for +H3NCH2CH2X systems (X = OH, NH2, COO-) in solution

The gauche-trans conformational equilibrium has been studied for (+)H(3)NCH(2)CH(2)X systems (X = OH, NH(2), COO(-)) and for the neutral model of the simplest β-amino acid in aqueous solution and chloroform. Each structure exhibits an intramolecular hydrogen bond in the NCCX gauche conformation, which is necessarily disrupted in the local energy minimum trans form. Geometries were optimized at the IEF-PCM/B97D/aug-cc-pvtz level of theory and indicated that the solvent effect of chloroform vs. water is moderate when the geometries for the corresponding gauche and trans conformers are compared. The only remarkable difference was found for β-alanine, which can exist in gauche, zwitterionic form in aqueous solution but not in chloroform. Total relative free energies were obtained as the sum of the relative IEF-PCM/B97D/aug-cc-pvtz internal free energy and ΔG(solv). The latter was calculated both by means of IEF-PCM and the explicit solvent FEP/Monte Carlo methods. The resulting ΔG(tot) values could differ by 1-2 kcal mol(-1) depending on the accepted ΔG(solv) value, but any calculation indicated that the internally bound gauche conformers are far more populated than the corresponding trans species. A difference of 2.6 kcal mol(-1) for ΔG(solv) by the two methods resulted in the preference of the zwitterionic gauche β-alanine structure by FEP/MC in contrast to IEF-PCM favoring the neutral form in aqueous solution. Monte Carlo characterization of the solution structure in the first solvation shell of polar groups indicates that solvation of a N-H(+) bond is sensitive to its involvement in an intramolecular hydrogen bond. For the zwitterionic, gauche β-alanine, almost no water oxygen can reach that N-H(+) bond, which is in strong interaction with the -COO(-) group by forming a six-membered ring and favorable local geometry for the hydrogen bond.

Salts of aromatic amines: crystal structures, spectroscopic and non-linear optical properties

Crystal structures, spectroscopic and NLO properties of 8 novel organic salts with aromatic amines, i.e. tyrammonium iodide (1), tyrammonium benzene-1,2,4-tricarboxilate (2), octopammonium 5-sulfosalicylate monohydrate (3), octopammonium hydrogen squarate (4), dopammonium 5-sulfosalicylate dihydrate (5), ephedrinium mandelate (6), dopammonium hydrogen squarate (7) and bis(ephedrinium) squarate (8) are reported and discussed with a view of their application in optical technologies. The conformations of the cations of the aromatic amines are discussed comparing the experimental and theoretical data.

Coupling of complex aromatic ring vibrations to solvent through hydrogen bonds: effect of varied on-ring and off-ring hydrogen-bonding substitutions

In this study, we examine the coupling of a complex ring vibration to solvent through hydrogen-bonding interactions. We compare phenylalanine, tyrosine, l-dopa, dopamine, norepinephrine, epinephrine, and hydroxyl-dl-dopa, a group of physiologically important small molecules that vary by single differences in H-bonding substitution. By examination of the temperature dependence of infrared absorptions of these molecules, we show that complex, many-atom vibrations can be coupled to solvent through hydrogen bonds and that the extent of that coupling is dependent on the degree of both on- and off-ring H-bonding substitution. The coupling is seen as a temperature-dependent frequency shift in infrared spectra, but the determination of the physical origin of that shift is based on additional data from temperature-dependent optical experiments and ab initio calculations. The optical experiments show that these small molecules are most sensitive to their immediate H-bonding environment rather than to bulk solvent properties. Ab initio calculations demonstrate H-bond-mediated vibrational coupling for the system of interest and also show that the overall small molecule solvent dependence is determined by a complex interplay of specific interactions and bulk solvation characteristics. Our findings indicate that a full understanding of biomolecule vibrational properties must include consideration of explicit hydrogen-bonding interactions with the surrounding microenvironment.

Theoretical Studies of the Tautomeric Equilibria for Five-Member N-Heterocycles in the Gas Phase and in Solution

Tautomeric equilibria have been studied for five-member N-heterocycles and their methyl derivatives in the gas phase and in different solvents with dielectric constants of epsilon = 4.7-78.4. The free energy changes differently for tautomers upon solvation as compared to the gas phase, resulting in a shift of the equilibrium constant in solution. Solvents with increasing dielectric constant produce more negative solute-solvent interaction energies and increasing internal energies. The methyl-substituted imidazole and pyrrazole form delicate equilibria between two tautomeric forms. Depending on the solvent, the methyl-substituted triazoles and tetrazole have one or two major tautomers in solution. When estimating the relative solvation free energies by means of an explicit solvent model and using the FEP/MC method, one observes that the preferred tautomers differ in several cases from those predicted by the continuum solvent model. The 1,2-prototropic shift, as an intramolecular tautomerization path, requires about 50 kcal/mol activation energy for imidazole in the gas phase, and this route is also disfavored in a solution. The calculated activation free energy along the intramolecular path is 48-50 kcal/mol in chloroform and water as compared to a literature value of 13.6 kcal/mol for pyrrazole in DMSO. A molecular dynamics computer experiment favors the formation of an imidazole chain in chloroform, making the 1,3-tautomerization feasible along an intermolecular path in nonprotic solvents. In aqueous solution, one strong N-H...Ow hydrogen bond is formed for each species, whereas all other nitrogens in the ring form weaker, N...HwOw type hydrogen bonds. The tetrahydrofuran solvent acts as a hydrogen bond acceptor and forms N-H... Oether bonds. Molecules of the dichloromethane solvent are in favorable dipole-dipole interactions with the solute. The results obtained are useful in the design of N-heterocyclic ligands forming specified hydrogen bonds with protein side chains.