The preparation of monitors and comparators for k0-INAA using standard solutions

Journal of Radioanalytical and Nuclear Chemistry - The methodology adopted at the Laboratory of Analytical Techniques of Instituto Peruano de Energía Nuclear for preparation of monitors and...     

Formation of novel rare-gas-containing molecules by molecular photodissociation in clusters

Recent work by R?s?nen and coworkers showed that photolysis of hydrides in rare-gas matrices results in part in formation of novel, rare-gas-containing molecules. Thus, photolysis of HCl in Xe and of H2O in Xe result respectively in formation of HXeCl and HXeOH in the Xe matrices. Ab initio calculations show that the compounds HRgY so formed are stable in isolation, and that by the strength and nature of the bonding these are molecules, very different from the corresponding weakly bound clusters Rg...HY. This paper presents a study of the formation mechanism of HRgY following the photolysis of HY in clusters Rgn(HY). Calculations are described for HXeCl, as a representative example. Potential energy surfaces that govern the formation of HXeCl in the photolysis of HCl in xenon clusters are obtained, and the dynamics on these surfaces is analyzed, partly with insight from trajectories of molecular dynamics simulations. The potential surfaces are obtained by a new variant of the DIM (diatomics in molecules) and DIIS (diatomics in ionic systems) models. Non-adiabatic couplings are also obtained. The main results are: (1) Properties of HXeCl predicted by the DIM-DIIS model are in reasonable accord with results of ab initio calculations. (2) The potential along the isomerization path HXeCl-->Xe...HCl predicted by DIM is in semiquantitative accord with the ab initio results. (3) Surface-hopping molecular dynamics simulations of the process in clusters, with "on the fly" calculations of the DIM-DIIS potentials and non-adiabatic couplings are computationally feasible. (4) Formation of HXeCl, following photolysis of HCl in Xe54(HCl), requires cage-exit of the H atom as a precondition. The H atom and the Cl can then attack the same Xe atom on opposite sides, leading to charge transfer and production of the ionic HXeCl. (5) Non-adiabatic processes play an important role, both in the reagent configurations, and at the charge-transfer stage. The results open the way to predictions of the formation of new HRgY species.     

Homogeneous fluorescent derivatization of large proteins

A method of homogeneously derivatizing large proteins for highly sensitive analysis is described. Homogeneity of the derivative was realized by tagging all the free amino groups of proteins. With this method, alpha-chymotrypsinogen A, ovalbumin and bovine serum albumin were derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). Prior to the derivatization, all the proteins were reduced and alkylated. After reacting the resulting unfolded proteins with excessive amounts of AQC, the samples were analyzed with matrix assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) to determine the derivatization degree. The results indicated that all three proteins had been, or had almost been, fully derivatized. HPLC and CE were used for characterizing these protein derivatives. Under the optimized fluorescence detection conditions, the detectability of the tagged proteins was 2400-6200 times better than that detected at UV 280 nm, 170-300 times better than detected at UV 214 nm, and 150-420 times better than measured with their native fluorescence.     

Stereoselection in the Prins-pinacol synthesis of acyltetrahydrofurans

[reaction: see text]. Depending upon the nature of the alkene and allylic substituents, acid-promoted rearrangements of acetals derived from anti allylic diols give 12 or stereoisomeric acyltetrahydrofurans 13. Stereoelectronic effects of the allylic substituents and the extent of bonding in the Prins cyclization transition state are central features of a proposed new model for predicting stereoselection in the Prins-pinacol synthesis of acyltetrahydrofurans.     

THERMODYNAMICS OF PORPHYRIN BINDING TO SERUM ALBUMIN: EFFECTS OF TEMPERATURE, OF PORPHYRIN SPECIES and OF ALBUMIN-CARRIED FATTY ACIDS

Abstract Porphyrin binding to serum albumin was studied at the molecular level probing the effects of: porphyrin self-aggregation, porphyrin species, temperature and protein-bound fatty acids. Human serum albumin was found to have a single high-affinity site for porphyrin monomers, with binding constants of 2 x 10⁶, 5 x 10⁷ and 3 x 10⁸ (37^o C, neutral pH, M ⁻¹), for hemato-, deutero- and protoporphyrins, respectively. Three equilibria models for the dimer binding were developed and tested. The data were found to fit best with a model proposing a single high-affinity binding site for the dimer, independent of and different than the monomer site. The binding constants of the hematoporphyrin and deuteroporphyrin dimers to human serum albumin (37^o C, neutral pH, M^−l) being 4 x 10* and 5 x 10⁸ respectively. The temperature dependence (Dp and HSA, 22-37^o C) of the monomer binding showed the process to be entropy-driven (δG^o= -45 kJ mol⁻¹; δS^o=+146 kJ mol⁻¹; δH^o= 0 kJ mol⁻¹). For the dimer binding, the enthalpy change was found to be highly temperature-dependent implying continuous changes in the heat capacity of the system over the entire temperature range, the trend at the 37^o C region fitting an entropy-driven process. The monomer vs dimer differences in temperature dependence strongly support separate and independent binding sites for these species. Similar thermodynamics were determined for fatty-acid carrying as well as for fatty-acid free HSA, with mild quantitative (but not qualitative) shifts.     

Chemical and biological caging effects on the relaxation of a proton-transfer dye

We report studies of the interaction between a proton-transfer dye (1'-hydroxy,2'-acetonaphthone, HAN), with the human serum albumin (HSA) protein and a beta-cyclodextrin derivative (DM-beta-CD) in neutral water solutions. We used steady-state and picosecond time-resolved emission spectroscopy to follow the structural changes of HAN due to the hydrophobicity and confinement effect of these nanocavities. Upon encapsulation, the fluorescence intensity of the 1:1 inclusion complex in both cavities increases, and the emission lifetimes become longer. For the DM-beta-CD complexes, we obtained 430 and 920 ps, whereas for the HSA complexes we obtained 630 ps and 2 ns. Picosecond anisotropy measurements show strong confinement due to protein docking. The rotational time for the CD complex is 660 ps, whereas for the protein complex we find 6 ns. The process of energy transfer from the excited triptophan 214 (Trp214) of HSA to the trapped HAN occurs with high efficiency (71%), and the calculated distance between both chromophores is 17 A. We believe that the results are important for a better understanding of the processes occurring in inclusion complexes such as those in nanopharmacodynamics.