Atomic and molecular analysis highlights the biophysics of unprotonated and protonated retinal in UV and scotopic vision.

During the photoreaction of rhodopsin, retinal isomerizes, rotating the C11[double bond, length as m-dash]C12 π-bond from cis to an all-trans configuration. Unprotonated (UR) or protonated (PR) retinal in the Schiff's base (SB) is related to UV and light vision. Because the UR and PR have important differences in their physicochemical reactivities, we compared the atomic and molecular properties of these molecules using DFT calculations. The C10-C11[double bond, length as m-dash]C12-C13 dihedral angle was rotated from 0° to 180° in 45° steps, giving five conformers, and the following were calculated from them: atomic orbital (AO) contributions to the HOMO and LUMO, atomic charges, bond length, bond order, HOMO, LUMO, hardness, electronegativity, polarizability, electrostatic potential, UV-vis spectra and dipole moment (DM). Similarly, the following were analyzed: the energy profile, hybridization, pyramidalization and the hydrogen-out-of-plane (HOOP) wagging from the H11-C11[double bond, length as m-dash]C12-H12 dihedral angle. In addition, retinal with a water H-bond (HR) in the SB was included for comparison. Interestingly, in the PR, C11 and C12 are totally the LUMO and the HOMO, respectively, and have a large electronegativity difference, which predicts an electron jump in these atoms during photoexcitation. At the same time, the PR showed a longer bond length and lower bond order, with a larger DM, lower HOMO-LUMO gap, lower hardness and higher electronegativity. In addition, the AOs of -45° and -90° conformers changed significantly, from pz to py, during the rotation concomitantly with marked hybridization, smooth pyramidalization and lower HOOP activity. Clearly, the atomic and molecular differences between the UR and PR are overwhelming, including the rotational energy profile and light absorption spectra, which indicates that light absorption of UR and PR is already determined by the retinal characteristics of the SB protonation. The HR-model compared with UR shows a lower energy barrier and a discreet bathochromic effect in the UV region.

Potential amoebicidal activity of hydrazone derivatives: synthesis, characterization, electrochemical behavior, theoretical study and evaluation of the biological activity.

Four new hydrazones were synthesized by the condensation of the selected hydrazine and the appropriate nitrobenzaldehyde. A complete characterization was done employing 1H- and 13C-NMR, electrochemical techniques and theoretical studies. After the characterization and electrochemical analysis of each compound, amoebicidal activity was tested in vitro against the HM1:IMSS strain of Entamoeba histolytica. The results showed the influence of the nitrobenzene group and the hydrazone linkage on the amoebicidal activity. meta-Nitro substituted compound 2 presents a promising amoebicidal activity with an IC50 = 0.84 µM, which represents a 7-fold increase in cell growth inhibition potency with respect to metronidazole (IC50 = 6.3 µM). Compounds 1, 3, and 4 show decreased amoebicidal activity, with IC50 values of 7, 75 and 23 µM, respectively, as a function of the nitro group position on the aromatic ring. The observed differences in the biological activity could be explained not only by the redox potential of the molecules, but also by their capacity to participate in the formation of intra- and intermolecular hydrogen bonds. Redox potentials as well as the amoebicidal activity can be described with parameters obtained from the DFT analysis.

Crystal structure of (E)-1-(2-nitro-benzyl-idene)-2,2-di-phenyl-hydrazine.

The title compound, C19H15N3O2, shows an E conformation of the imine bond. The dihedral angle between the planes of the phenyl rings in the di-phenyl-hydrazine groups is 88.52 (4)°. The 2-nitro-benzene ring shows a torsion angle of 10.17 (8)° with the C=N-N plane. A short intra-molecular C-H⋯O contact occurs. In the crystal, only van der Waals contacts occur between the mol-ecules.

(E)-1,1-Diphenyl-2-(thio-phen-2-yl-methyl-idene)hydrazine.

The asymmetric unit of the title compound, C17H14N2S, consists of two crystallographically independent mol-ecules with similar conformations. The dihedral angles between the phenyl rings are 89.32 (5) and 82.80 (5)° in the two mol-ecules. In the crystal, mol-ecules are linked by C-H⋯π inter-actions, forming a three-dimensional network.

(E)-1-(2,4-Di-nitro-benzyl-idene)-2,2-di-phenyl-hydrazine.

In the crystal of the title compound, C19H14N4O4, the asymmetric unit consists of two discrete mol-ecules. The C=N bonds in both mol-ecules show an E conformation. The dihedral angles between the C atoms of the 2,4-dinitrobenzene rings and the C=N-N planes are 13.52 (9) and 13.82 (9)° for the two mol-ecules. In the crystal, C-H⋯O hydrogen bonds, mainly between the phenyl ring and the nitro group along the b axis.

1-{(E)-[5-(2-Nitro-phen-yl)furan-2-yl]methyl-idene}-2,2-diphenyl-hydrazine.

In the title compound, C23H17N3O3, the terminal benzene rings are oriented at dihedral angles of 3.67 (7), 76.02 (7) and 16.37 (7)° with respect to the central furan ring. In the crystal, mol-ecules are connected via weak C-H⋯O hydrogen bonds, resulting in a three-dimensional supra-molecular array.

(E)-1-(4-Nitro-benzyl-idene)-2,2-diphenyl-hydrazine.

The asymmetric unit of the title compound, C(19)H(15)N(3)O(2), contains two mol-ecules, both of which show an E conformation of the imine bond. The dihedral angles between the phenyl rings in the phenyl-hydrazine groups are 86.09 (6) and 83.41 (5)° in the two mol-ecules. The 4-nitrobenzene rings show torsion angles of 4.4 (2) and 10.9 (2)° from the two C=N-N planes. In the crystal, C-H⋯π inter-actions and C-H⋯O hydrogen bonds are observed growing along the a, b and c axes, resulting in a complex supramolecular array.

(E)-1-Benzyl-idene-2,2-diphenyl-hydrazine.

The asymmetric unit of the title compound, C(19)H(16)N(2), contains two independent mol-ecules, both of which show an E configuration with respect to the C=N bond. The dihedral angles between the phenyl rings bonded to the hydrazine group are 81.00 (10) and 88.34 (8)° in the two mol-ecules. Inter-molecular C-H⋯π inter-actions are observed in the crystal structure.

(E)-1-(3,4-Dimethyl-benzyl-idene)-2,2-diphenyl-hydrazine.

The asymmetric unit of the title compound, C(21)H(20)N(2), contain two mol-ecules, both of them showing an E configuration of the C=N bond. The dihedral angles between the phenyl rings in the phenyl-hydrazone groups are 86.84 (10) and 84.85 (8)° for the two mol-ecules. Inter-molecular C-H⋯π inter-actions are observed in the crystal structure.

N'-[5-(4-Nitro-phen-yl)furan-2-yl-methyl-idene]-N,N-diphenyl-hydrazine.

The title compound, C(23)H(17)N(3)O(3), has an E configuration with respect to the C=N bond. The dihedral angle between the two phenyl rings bonded to the hydrazine group is 86.45 (13)°. The furan ring makes dihedral angles of 3.4 (2) and 7.06 (13)°, respectively, with the methyl-idenehydrazine C=N-N plane and the benzene ring.

(1E,2E)-1,2-Bis(2,2-diphenyl-hydrazin-1-yl-idene)ethane.

In the crystal structure of the title compound, C(26)H(22)N(4), the mol-ecule is located on an inversion centre and shows an E configuration with respect to each C=N bond. The dihedral angle between the phenyl rings in the diphenyl-hydrazone group is 83.69 (11)°. These two rings make dihedral angles of 30.53 (15) and 84.53 (16)° with the central N-N=C-C=N-N dihydrazonoethane plane. Inter-molecular C-H⋯π inter-actions are observed.