Computational Screening of Weak Hydrogen Bond Networks: Predicting Stable Structures for Difluoromethane Oligomers.

The stability and the structure of small difluoromethane oligomers are studied by combining classical Monte Carlo and quantum mechanical calculations. A hierarchical procedure was adopted to validate the accuracy of the whole protocol: the force field used in Monte Carlo simulations is parametrized on the basis of dimer intermolecular energies computed with density functional theory. The density functional is similarly chosen by comparing the interaction energies with reference values, purposely computed at a coupled cluster level, extrapolated at the complete basis set. The structures of dimers, trimers, and tetramers identified by the screening as local minima are first characterized by some geometrical parameters and by their dipole moment and eventually validated by comparison with results of microwave spectroscopy. The results are found in very good agreement with the experiment for all considered structures.

Oligomers based on weak hydrogen bond networks: a rotational study of the tetramer of difluoromethane.

We report the rotational spectrum of one conformer of the tetramer of difluoromethane (CH2F2)4 (the first pure rotational spectrum of a tetramer of an asymmetric rotor), and describe the network of weak hydrogen bonds which connect the four subunits.

Biochemical fluorometric method for the determination of riboflavin in milk.

UNLABELLED: Methods of analysis of vitamin B2 in foods generally consist of the extraction of the sample, followed by enzymatic hydrolysis and quantitative measurement of the analyte, typically through RP-HPLC. The scope of our work here is to present a soft method to measure the free riboflavin content of a nontransparent and nonhomogeneous matrix such as milk, avoiding any extraction and separation of phases that are required in any published method for determination of the free RBF content in foods. We combine the front-face (FF) measurement of the light emission of milk with the ability of the apo-form of the riboflavin-binding protein (RBP) from chicken egg white to quench the riboflavin fluorescence. Thus, we titrate the RBF present in milk by gradually adding a solution of RBP to the milk sample and measuring, upon each addition, the FF residual emission due to uncomplexed RBF. The RBP binding capability has been measured in the same matrix of the analyte. Our results indicate a concentration of free RBF practically co-incident with the certified value for total B2 vitamin content in reference milk CRM 421. KEYWORDS: Front-face fluorescence; riboflavin; apo-riboflavin-binding protein; milk fluorescence.

Fluorescence of vegetable oils: olive oils.

Fluorescence spectra of undiluted extra virgin olive oil obtained with the traditional setup (right-angle fluorescence) show considerable artifacts and deformations due to self-absorption phenomena, even when the spectra are corrected for inner filter effects. On the other side, front-face fluorescence spectra are much less affected by self-absorption. Front-face fluorescence of native olive oil reveals the presence of different fluorophores and can provide information about their amount. From the intense emission at ca. 315-330 nm, it is possible to detect fluorescent polyphenols and pherols and to evaluate their overall content. Low-intensity emission bands at 350-600 nm are correlated to vitamins and other important molecules. Among them, the fluorescence of the riboflavin fluorophore can be used to evaluate its concentration. The intense emission of chlorophyll derivatives, measured in the 640-800 nm spectral region, can provide information on their concentration.

New N-arylsulfonyl-N-alkoxyaminoacetohydroxamic acids as selective inhibitors of gelatinase A (MMP-2).

New N-arylsulfonyl-substituted alkoxyaminoaceto hydroxamic acid derivatives of types 8 and 10 designed as oxa-analogues of known sulfonamide-based MMPi of types 2 and 7 were synthesized and tested for their inhibitory activities on some matrix metalloproteinases. The combination of a biphenylsulfonamide group with oxyamino oxygen in the pharmacophoric central skeleton of sulfonamide-based MMPi obtained in the new sulfonamides 10 seems to be able to give selectivity for MMP-2 over MMP-1. The most potent derivative of this type, 10a, shows similar anti-invasive properties to the analogue reference drug CGS27023A, 2, in an in vitro model of invasion on matrigel, carried out on cellular lines of fibrosarcoma HT1080 (tumoural cells over-expressing MMP-2 and MMP-9).

Direct fluorometric determination of fluorescent substances in powders: the case of riboflavin in cereal flours.

Front-face emission spectra of powders can be recorded with a commercial spectrofluorometer. By combining the emissions of a scatterer powder and of a wheat flour sample, the scattering contribution to the front-face emission spectra of flour is removed, and the fluorescence of the flour is isolated. The fluorescence depends on the concentration of the fluorophores. By choosing convenient measurement parameters and by measuring the emission spectra of flour samples suitably enriched with riboflavin, the fluorescence of riboflavin could be isolated from that of other substances present in flours and the concentration of vitamin B(2) in native substrates could be determined. This method is particularly apt for the measurement of vitamin B(2) in low riboflavin-containing powders such as wheat flours, which are usually analyzed through complex chemical and microbiological methods. The method is essentially phenomenological, in view of the interpretation difficulties connected to the origin of the fluorescence resulting from the absorption of multiply scattered photons.

Spectrophotometric Study of the Equilibria between Nickel(II) Schiff-Base Complexes and Alkaline Earth or Nickel(II) Cations in Acetonitrile Solution.

Adduct formation between M(ClO(4))(2) and nickel(II) complexes of tetradentate Schiff-base ligands ([NiL]) have been investigated in acetonitrile by means of UV-vis spectrophotometric analysis (M(2+)= Mg(2+), Ca(2+), Sr(2+), Ba(2+), Ni(2+); [NiL] = [Ni(salen)] = [N,N'-ethylenebis(salicylideneiminato)]nickel(II), [Ni(salphen)] = [N,N'-o-phenylenebis(salicylideneiminato)]nickel(II), [Ni(salpren)] = [N,N'-propylenebis(salicylideneiminato)]nickel(II)). [NiL] complexes act as bidentate oxygen-donor ligands forming adducts of the type [(NiL)M](2+) and [(NiL)(2)M](2+) whose general order of increasing stability was correlated to the dimensions of the M(2+) cations. Complexes of general composition 2[Ni(salen)].M(ClO(4))(2) were isolated as powders in the solid state (M = Sr, Ca, Mg). X-ray structures of the crystalline complexes [{Ni(salen)}(2)Ba(ClO(4))(2)(thf)] (1) (thf = tetrahydrofuran) and [{Ni(salen)}(2)Ni(H(2)O)(2)](ClO(4))(2) (2) are reported. 1 crystallizes in the space group P2(1)/c (a = 10.995 Å; b = 20.830 Å; c = 19.567 Å, beta = 116.94 degrees ) and 2 in C2/c (a = 10.3020 Å; b = 21.818 Å; c = 17.796 Å; beta = 118.98 degrees ). Two [Ni(salen)] units act as oxygen bidentate ligands in both the compounds. The barium is coordinated to eight oxygen atoms in a distorted square antiprismatic geometry. An 8-fold coordination is completed by a molecule of tetrahydrofuran and two perchlorate anions acting as bi- and monodentate O-ligands, respectively. In 2, the nickel atom, coordinated to two [Ni(salen)] complexes, has a distorted octahedral geometry and completes its coordination by linking two molecules of water.