Showing papers in "Chemical Reviews in 1987"

Reaction mechanisms in the radiolysis of peptides, polypeptides, and proteins

Abstract: The purpose of this review is to bring together and to correlate the wide variety of experimental studies that provide information on the reaction products and reaction mechanisms involved in the radiolysis of peptides, polypeptides and proteins (including chromosomal proteins) in both aqueous and solid-state systems. The comparative radiation chemistry of these systems is developed in terms of specific reactions of the peptide main-chain and the aliphatic, aromatic-unsaturated and sulfur-containing side-chains. Information obtained with the various experimental techniques of product analysis, competition kinetics, spin-trapping, pulse radiolysis and ESR spectroscopy is included. 147 refs.

Interlocking of molecular threads: from the statistical approach to the templated synthesis of catenands

Abstract: The search for aesthetically attractive molecules has been a concern going back to the origin of chemistry. The criteria for beauty have obviously changed with time, being connected to analytical and synthetic tools. The ability of synthetic chemists to build more and more complicated structures has been developing spectacularly; recently highly sophisticated molecular architectures have been achieved. Transition metals can form molecular clusters whose arrangements are remarkably beautiful. In particular, platinum carbonyl clusters lead to geometrical figures whose regularity and harmony were completely unexpected not so long ago.' Dodecahedrane2 is another example whose importance stems both from the amount of work involved and from the cultural significance of the geometrical shape reached. This smart molecule represents the achievement of considerable efforts, and it remains today one of the mast difficult patterns to be synthesized. Clearly, the imagination of the chemist is dependent on the power of the synthetic tool. As the latter becomes more and more powerful, the synthetic chemist will be able to conceive and then to construct molecules that were not envisaged previously. Concurrently, the analytical chemist becomes able to recognize molecular structures mi(cycbpropanes. cyclopropenes; syime~ts and r e a m ) and total synthesii of chlysanhmic acids. h. Dmmchauchecker has been a member of the Centre Natlonal de la Recherche Scientitique since 1970. and she did postdoctwal work with Prof. K. 6. Sharpless (MIT. CambrkJge. MA, 1973-1974) on the study of new organoselenium compounds. She joined Prof. J. P. Sauvage's group in 1981. and her present research field is the synthesis of topologically novel molecules. 121and [3]-catenanes.

Temporary Anion States of Polyatomic Hydrocarbons

Abstract: The experimental methods used to characterize molecular anions differ substantially depending on the stability of the species. The application of laser spectroscopic methods, discussed elsewhere in this issue, is generally limited to anions possessing hound electronic ground states or to those with lifetimes against autodetachment of the electron in excess of a few microseconds. For a great many molecules, including such important prototypes as ethylene, butadiene, benzene, naphthalene, formaldehyde, and acetylene, the ground states of the anions are known to be unstable in the gas phase with lifetimes less than lo-'* s, typically in the neighborhood of s. Even for those molecules with hound ground-state anions, nearly all of the excited anion states lie in the continuum and hence decay by electron detachment. Thus from consideration of numbers alone, the manifold of temporary anion states

Ab initio electronic structure of anions

Abstract: In characterizing anion states either experimentally or theoretically different techniques are required for bound and temporary or metastable anions. Anions that lie energetically above the ground state of the neutral molecule (or atom) are called temporary anions since they are unstable with respect to electron detachment. Temporary anions generally have lifetimes in the range of 10-13-10-16 s and thus are difficult to study by using optical spectroscopic methods. Various types of electron-impact spectroscopy have proven particularly valuable for characterizing temporary anions.’I2 Some of these experimental methods are discussed in the accompanying paper3 of Jordan and Burrow. Because temporary anion states lie in the continuum of the neutral species plus free electron, they cannot be treated in general by means of a straightforward variational calculation. Techniques designed for their study are discussed later in this paper. Bound or stable anions lie energetically below the ground state of their parent neutral and can be studied by a variety of experimental methods including photodetachment spectroscopy* and laser photoelectron spectro~copy.~ The contribution by Wetzel and Brauman in this issue covers many of these experimental techniques. Theoretically they can be characterized by using traditional quantum chemical methods such as the self-consistent field (SCF), configuration interaction (CI), multiconfiguration self-consistent field (MCSCF), and manybody techniques, although extra care must be exercised for reasons outlined below. The ab initio theoretical study of stable atomic and molecular negative ions involves complications not encountered in analogous studies of neutral or cationic species. In particular, the diffuse spatial extent of the outermost orbitals of anions places additional requirements on any quantum chemical calculation. Within the conventional LCAO-MO finite atomic basis approach, adequate description of diffuse charge densities requires that diffuse atomic orbitals be added to the core-and-valence atomic basis sets. Because computational expense varies as a high power of the size of the atomic orbital basis set, this greatly increases the computer-time requirements for negative ion calculations. Diffuse charge densities correspond to weak binding energies and low average values of kinetic energy. The weak binding energies imply that any method used to compute the electron affinities (EA) must be very reliable. The low classical kinetic energies suggest that energy transfer from vibrational and rotational degrees of freedom to the electronic degrees of freedom may be more facile than in neutral or cationic species where the more rapidly moving electrons readily “track” the motion of the underlying nuclei. Thus, the neglect of nonadiabatic interactions, which is the cornerstone upon which the concept of the potential energy surface is predicated, is less likely to be reasonable for anions. For most classes of molecules, the calculation of accurate EA’S requires that the molecular orbital picture be corrected by including so-called electron correlation effeds. In the self-consistent field (SCF) Hartree-Fock molecular orbital model of electronic structure, each electron is allowed to “feel” the other electrons only via an average interaction potential. This potential consists of the averaged Coulomb and exchange interaction

Chemical aspects of fast atom bombardment

Abstract: A. Chemistry in solution E. Chemistry in the Sehre& C. Ionization of Gaseous Samples IV. Bringing the Sample to the Surface A. Surface Activity 8. Mixture Analysis V. Choosing the Matrix A. Volatility 8. Fluidity C. Solvency D. Acidity and Basicity E. Surfactant Properties F. Addnives 0. Background Contributbns VI. Reactions in the Matrix A. Ion Fcfmation E. Directed Reactions In the Matrix C. Sampling Solution Equilibria D. Reactions wim the Matrix VII. Chemical Background and Noise VIII. Dlrecting Fragmentation IX. Optimizing Sensltivity A. Instrumental Setup E. Betler Sensltivity ltwcugh Chemlsby C. Temporal Variatlons X. Conclusions