Imaging the state-specific vibrational predissociation of the C2H2-NH3 hydrogen-bonded dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions.     

Temperature effect on the complex formation between tricyclic antidepressant drugs (amitriptyline or imipramine) and hydroxypropyl-β-cyclodextrin in water

The molecular encapsulation of two tricyclic antidepressants (TCA) drugs, amitriptyline and imipramine, by a glycosidic receptor, 6-hydroxypropyl-β-cyclodextrin (HPBCD), has been carried out in water solution by means of conductometric studies at different temperatures ranging from 15 °C to 45 °C. Conductivity measurements of aqueous solutions of the drug were performed: (i) in the absence of HPBCD, as a function of drug concentration; and (ii) in the presence of HPBCD, as a function of HPBCD concentration. Both drugs, amitriptyline and imipramine, form inclusion complexes characterized by a 1:1 stoichiometry and an association constant () in the range of 500–1200 M⁻¹. The ionic molar conductivities at infinite dilution of the free () and complexed () drugs have been calculated from these conductivity data as well. From the dependency of the association constant on temperature, changes on the enthalpy, ΔH ⁰, entropy, ΔS ⁰, and heat capacity at constant pressure, , have been determined. This thermodynamic information, which reveals that the complexes formed by HPBCD and the antidepressant drugs, AMYTPH⁺ and IPRH⁺, are enthalpy driven at T ≥ 25 °C but entropy driven at T < 25 °C, points to the contribution of van der Waals interactions, hydrophobic effect and solvent reorganization, as the main driven forces promoting the interaction. The analysis of these association processes was also used to elucidate the potential viability of using HPBCD as a vector of these antidepressant drugs.     

Dinuclear zinc(II) complexes of symmetric Schiff-base ligands with extended quinoline sidearms

The synthesis of two 2-formylquinolines is reported via the Skraup method followed by SeO(2) oxidation. Each aldehyde is condensed with (1R,2R)-diaminocyclohexane and (R)-BINAM, yielding four enantiomerically-pure bis(imine-quinoline) ligands. The neutral ligands are reacted with ZnCl(2) to give complexes with bis(bidentate) coordination of ZnCl(2) units. X-Ray structural characterization of three complexes shows them to have a single-stranded helical motif, with M helicity, except in one case where a 1:1 mixture of M and P helices is seen. The ligands and complexes are further characterized spectroscopically by solution (1)H and (13)C NMR, UV-vis and ECD.     

Macrocyclic diketopiperazine receptors: effect of macrocyclization on the binding properties of two-armed receptors

The synthesis of macrocyclic two-armed diketopiperazine receptors and their binding properties toward peptides is described. Macrocyclization with short linkers led to receptors with significantly modified binding properties compared to their flexible open chain parent receptors, whereas with long linkers the original binding selectivities were largely retained.     

Synthesis of the pyrrolidinone core of KSM-2690 B

[reaction: see text] The first synthesis of the pyrrolidinone core of the polyene beta-lactone antibiotic KSM-2690 B is described. An ammonia-free Birch reductive aldol reaction utilizing acetaldehyde is one of the key steps, together with a ruthenium-catalyzed alkene isomerization reaction.     

Efficient gene transfection with functionalised multicalixarenes

Novel amino-functionalised multicalixarenes have been synthesised which show low cellular toxicity, effective DNA binding and, when featuring aliphatic amines, are efficient gene transfection agents.     

Proton solvation and transport in aqueous and biomolecular systems: insights from computer simulations

The excess proton in aqueous media plays a pivotal role in many fundamental chemical (e.g., acid-base chemistry) and biological (e.g., bioenergetics and enzyme catalysis) processes. Understanding the hydrated proton is, therefore, crucial for chemistry, biology, and materials sciences. Although well studied for over 200 years, excess proton solvation and transport remains to this day mysterious, surprising, and perhaps even misunderstood. In this feature article, various efforts to address this problem through computer modeling and simulation will be described. Applications of computer simulations to a number of important and interesting systems will be presented, highlighting the roles of charge delocalization and Grotthuss shuttling, a phenomenon unique in many ways to the excess proton in water.