Unimolecular chemistry of oxazole and isoxazole radical cations in the gas phase: Combined experimental and molecular orbital study

Molecular radical cations of oxazole (1) and isoxazole (2) dissociate by losing carbon monoxide or a hydrogen atom, respectively. These fragmentations were examined by use of tandem mass spectrometry, flash vacuum pyrolysis and ab initiomolecular orbital calculations. A multi-step mechanism is proposed which incorporates these new experimental and theoretical data. The case of methylated homologues of 1 and 2 is also considered.     

Fine Tuning of β‐Peptide Foldamers: a Single Atom Replacement Holds Back the Switch from an 8‐Helix to a 12‐Helix

Cyclic homologated amino acids are important building blocks for the construction of helical foldamers. N‐aminoazetidine‐2‐carboxylic acid (AAzC), an aza analogue of trans‐2‐aminocyclobutanecarboxylic acid (tACBC), displays a strong hydrazino turn conformational feature, which is proposed to act as an 8‐helix primer. tACBC oligomers bearing a single N‐terminal AAzC residue were studied to evaluate the ability of AAzC to induce and support an 8‐helix along the oligopeptide length. While tACBC homooligomers assume a dominant 12‐helix conformation, the aza‐primed oligomers preferentially adopt a stabilized 8‐helix conformation for an oligomer length up to 6 residues. The (formal) single‐atom exchange at the N terminus of a tACBC oligomer thus contributes to the sustainability of the 8‐helix, which resists the switch to a 12‐helix. This effect illustrates atomic‐level programmable design for fine tuning of peptide foldamer architectures.     

Structure and Dynamics of Oligonucleotides in the Gas Phase

By combining ion‐mobility mass spectrometry experiments with sub‐millisecond classical and ab initio molecular dynamics we fully characterized, for the first time, the dynamic ensemble of a model nucleic acid in the gas phase under electrospray ionization conditions. The studied oligonucleotide unfolds upon vaporization, loses memory of the solution structure, and explores true gas‐phase conformational space. Contrary to our original expectations, the oligonucleotide shows very rich dynamics in three different timescales (multi‐picosecond, nanosecond, and sub‐millisecond). The shorter timescale dynamics has a quantum mechanical nature and leads to changes in the covalent structure, whereas the other two are of classical origin. Overall, this study suggests that a re‐evaluation on our view of the physics of nucleic acids upon vaporization is needed.     

Diketopyrrolopyrrole‐Porphyrin Conjugates with High Two‐Photon Absorption and Singlet Oxygen Generation for Two‐Photon Photodynamic Therapy

Two‐photon photodynamic therapy is a promising therapeutic method which requires the development of sensitizers with efficient two‐photon absorption and singlet‐oxygen generation. Reported here are two new diketopyrrolopyrrole‐porphyrin conjugates as robust two‐photon absorbing dyes with high two‐photon absorption cross‐sections within the therapeutic window. Furthermore, for the first time the singlet‐oxygen generation efficiency of diketopyrrolopyrrole‐containing systems is investigated. A preliminary study on cell culture showed efficient two‐photon induced phototoxicity.     

Isoguanine and 5‐Methyl‐Isocytosine Bases,In Vitro and In Vivo

The synthesis, base‐pairing properties and in vitro and in vivo characteristics of 5‐methyl‐isocytosine (isoC^Me) and isoguanine (isoG) nucleosides, incorporated in an HNA(h) (hexitol nucleic acid)–DNA(d) mosaic backbone, are described. The required h‐isoG phosphoramidite was prepared by a selective deamination as a key step. As demonstrated by T_m measurements the hexitol sugar showed slightly better mismatch discrimination against dT. The d‐isoG base mispairing follows the order T>G>C while the h‐isoG base mispairing follows the order G>C>T. The h‐ and d‐isoC^Me bases mainly mispair with G. Enzymatic incorporation experiments show that the hexitol backbone has a variable effect on selectivity. In the enzymatic assays, isoG misincorporates mainly with T, and isoC^Me misincorporates mainly with A. Further analysis in vivo confirmed the patterns of base‐pair interpretation for the deoxyribose and hexitol isoC^Me/isoG bases in a cellular context, through incorporation of the bases into plasmidic DNA. Results in vivo demonstrated that mispairing and misincorporation was dependent on the backbone scaffold of the base, which indicates rational advances towards orthogonality.     

EPR Studies of the Binding Properties,Guest Dynamics,and Inner‐Space Dimensions of a Water‐Soluble Resorcinarene Capsule

Nitroxide free radicals have been used to study the inner space of one of Rebek’s water‐soluble capsules. EPR and ¹H NMR spectroscopy, ESI‐MS, and DFT calculations showed a preference for the formation of 1:2 complexes. EPR titrations allowed us to determine binding constants (K_a) in the order of 10⁷ M ^?2. EPR spectral‐shape analysis provided information on the guest rotational dynamics within the capsule. The interplay between optimum hydrogen bonding upon capsule formation and steric strain for guest accommodation highlights some degree of flexibility for guest inclusion, particularly at the center of the capsule where the hydrogen bond seam can be barely distorted or slightly disturbed.     

Linear and non linear rheology of a wormlike micellar system in presence of sodium tosylate

In this experimental work, we investigate the influence of an organic counterion, sodium tosylate, on the rheological properties of an aqueous solution of CTAB at the concentration of 0.05M. With this system we can clearly see shear thickening for small salt concentrations C _s and only shear thinning behavior at higher C _s characterized by a linear evolution of η=f(γ) in a log-log representation. In these evolutions it is only in a very small domain of concentrations of the salt (near C _s =0.035M) that we can observe a nearly constant plateau of the shear stress against shear rate. The values of σ₀ (characterizing the stress plateau), G ₀ (the plateau modulus) and τ_R (the relaxation time) obtained by dynamical rheological measurements, allow to compare experimental results obtained to predicted values of the theory of Cates corresponding to the occurrence of shear induced banding structures. Received: 22 July 1997 Accepted: 3 February 1998     

Coordination Chemistry‐Assembled Multicomponent Systems Built from a Gable‐Like Bis‐Porphyrin: Synthesis and Photophysical Properties

Multiporphyrinic assemblies were quantitatively formed, in one step, from a gable‐like zinc(II) bis‐porphyrin ZnP₂ and free‐base porphyrins bearing pyridyl groups. The different fragments are held together by axial 4′‐N(pyridyl)–Zn interactions. Formation of a macrocycle ZnP₂?(4′‐cisDPyP) and a bis‐macrocycle (ZnP₂)₂?(TPyP) is discussed. The macrocycle and the bis‐macrocycle were crystallized and studied by X‐ray diffraction, which confirmed the excellent complementarity between the various components. Spectrophotometric and spectrofluorimetric titrations and studies reveal high association constants for both multiporphyrinic assemblies due to the almost perfect geometrical match between the interacting units. As expected, energy transfer from the zinc porphyrin component to the free‐base porphyrin quenches the fluorescence of the zinc porphyrin components in both compounds. But while in ZnP₂?(4′‐cis DPyP) sensitization of the emission of the free‐base porphyrin was observed, in (ZnP₂)₂?(TPyP) excitation of the peripheral Zn porphyrin units does not lead to quantitative sensitization of the luminescence of the free‐base porphyrin acceptor. An unusual HOMO–HOMO electron transfer reaction from ZnP₂ to the excited TPyP unit was detected and studied.     

Growth mechanisms involved in the synthesis of smooth and microtextured films by acetylene magnetron discharges

The growth of hydrogenated amorphous carbons (a-C:H) produced by continuous or pulsed discharges of acetylene (C(2)H(2)) in an unbalanced magnetron setup was investigated. At 5 × 10(-3) Torr, only smooth films are obtained, whereas at 5 × 10(-1) Torr using a pulsed discharge some microtextured films are formed if the duty cycle is low. The morphology of these microtextured films consists of nanoparticles, filamentary particles, and particular agglomerates ("microflowers"). This paper presents a study of acetylene gas phase polymerization by mass spectrometry, and a detailed analysis of bulk structure of films by combining three techniques which include IR spectroscopy, Raman spectroscopy, and laser desorption/ionization Fourier transform mass spectrometry (LDI-FTMS). Finally, based on the study of gas phase and film structure, we propose a model for the growth of both smooth and microtextured films.