Synthesis of Amine Derivatives of the Polyhedral Borane Anion [B(20)H(18)](4)(-)

Ethylenediamine (en) reacts with the polyhedral borane anion [n-B(20)H(18)](2)(-), in the presence or absence of strong nonnucleophilic auxiliary bases, to produce the [ae-B(20)H(17)(en)](3)(-) anion. In either case, substitution is accompanied, to an approximately equal extent, by reduction of the starting material to form [a(2)()-B(20)H(18)](4)(-). As found with similar substituted systems derived from this 20-boron structure, [ae-B(20)H(17)(en)](3)(-) may be conveniently rearranged to the [a(2)()-B(20)H(17)(en)](3)(-) anion. A more convenient synthesis of the known [ae-B(20)H(17)NH(3)](3)(-) ion, which employs acetonitrile as the source of the NH(3) ligand instead of liquid ammonia, is also described.     

Intramolecular Hydrogen Site Exchange in an HRu(SiHPh(2)) Moiety

RuH(Ph)(CO)L(2) (L = P(t)Bu(2)Me) reacts with SiH(2)Ph(2) to give first benzene and RuH(SiHPh(2))(CO)L(2), and then RuH(3)(SiHPh(2))(CO)L(2) and Ru(H)(2)(CO)L(2), the trihydride being formed by a dehydrogenative silane coupling reaction when excess SiH(2)Ph(2) is present. Variable-temperature spin saturation transfer experiments reveal exchange between H(a) and H(b) in RuH(a)(SiH(b)Ph(2))(CO)L(2); this occurs both by an intramolecular mechanism and (when SiH(2)Ph(2) is present) by a mechanism dependent on SiH(2)Ph(2) concentration. Spin saturation transfer also reveals exchange between all three of the above complexes via addition/loss of SiH(2)Ph(2) or H(2).     

Combinatorial complexity bounds for arrangements of curves and spheres

We present upper and lower bounds for extremal problems defined for arrangements of lines, circles, spheres, and alike. For example, we prove that the maximum number of edges boundingm cells in an arrangement ofn lines is (m ^2/3 n ^2/3 +n), and that it isO(m ^2/3 n ^2/3 (n) +n) forn unit-circles, where(n) (and later(m, n)) is a function that depends on the inverse of Ackermann's function and grows extremely slowly. If we replace unit-circles by circles of arbitrary radii the upper bound goes up toO(m ^3/5 n ^4/5 (n) +n). The same bounds (without the(n)-terms) hold for the maximum sum of degrees ofm vertices. In the case of vertex degrees in arrangements of lines and of unit-circles our bounds match previous results, but our proofs are considerably simpler than the previous ones. The maximum sum of degrees ofm vertices in an arrangement ofn spheres in three dimensions isO(m ^4/7 n ^9/7 (m, n) +n ²), in general, andO(m ^3/4 n ^3/4 (m, n) +n) if no three spheres intersect in a common circle. The latter bound implies that the maximum number of unit-distances amongm points in three dimensions isO(m ^3/2 (m)) which improves the best previous upper bound on this problem. Applications of our results to other distance problems are also given.The research of the second author was supported by the National Science Foundation under Grant CCR-8714565. Work by the fourth author has been supported by Office of Naval Research Grant N00014-87-K-0129, by National Science Foundation Grant No. NSF-DCR-83-20085, by grants from the Digital Equipment Corporation and the IBM Corporation, and by a research grant from the NCRD, the Israeli National Council for Research and Development. A preliminary version of this paper has appeared in theProceedings of the 29th IEEE Symposium on Foundations of Computer Science, 1988.     

Thermally reversible polymer systems by cyclopentadienylation. II. The synthesis of cyclopentadiene-containing polymers

The synthesis of polymers containing either terminal or pendant cyclopentadiene (CPD) groups has been studied. Using information obtained from model studies, the synthesis of polyisobutylenes containing a CPD terminus was accomplished by the use of the tert-butylchloride–dimethylcyclopentadienylaluminum initiator system. Isobutylene polymerization by this system under carefully chosen conditions is essentially transfer free, and termination occurs by cyciopentadienylation. The presence of CPD end group has been established by UV spectroscopy and reaction with maleic anhydride. Selectivity toward termination by cyclopentadienylation increases with decreasing temperatures; at ?41°C and low conversion (<10%) ca. 72% of the polymer chains terminate in this manner. The synthesis of polymers containing randomly distributed pendant CPD groups along the chain has been accomplished by cyclopentadienylating with dimethylcyclopentadienylaluminum chlorobutyl rubber and chlorinated poly(ethylene-co-propylene) (Cl-EPM). Pendant CPD groups undergo Diels-Alder/retro-Diels-Alder condensations yielding thermally reversible networks; e.g., a cyclopentadienylated Cl-EPM was remolded three times to elastic sheets.     

Influence of structure on binding of chlorophylls to peptide ligands

Four classes of chlorophyll (Chl), a, b, c, and d, are involved in photosynthesis within cyanobacteria, algae, and plants. These classes have different evolutionary origins, chemical properties, and biological functions. Our results demonstrate that peptide-bound ligands provided by the imidazole group of histidine and the charge-compensated glutamate-arginine ion pair readily form coordination bonds with Chls a and d but do not interact significantly with Chls b and c. These ligands are apparently not sufficiently strong Lewis bases to displace strongly coordinated water from Chls b and c. These differences determine specificity of binding of Chls in light-harvesting complexes and play an important role in assembly of stable Chl-protein complexes, which has had a profound impact on the evolution of photosynthetic organisms.     

Gauge-origin-independent magnetizabilities of solvated molecules using the polarizable continuum model

We present an implementation of the polarizable continuum model in its integral equation formulation for the calculation of the magnetizabilities of solvated molecules. The gauge-origin independence of the calculated magnetizabilities and the fast basis set convergence are ensured through the use of London atomic orbitals. Our implementation can use Hartree-Fock and multiconfigurational self-consistent-field (MCSCF) wave functions as well as density-functional theory including hybrid functionals such as B3LYP. We present the results of dielectric continuum effects on water and pyridine using MCSCF wave functions, as well as dielectric medium effects on the magnetizability of the aromatic amino acids as a model for how a surrounding protein environment affects the magnetizability of these molecules. It is demonstrated that the dielectric medium effects on the magnetizability anisotropies of the aromatic amino acids may be substantial, being as large as 25% in the case of tyrosine.     

Solid-phase synthesis and biochemical studies of O-boranophosphopeptides and O-dithiophosphopeptides

Signal transduction cascades maintain control over important cellular processes such as cell growth and differentiation by orchestrating protein phosphorylation and dephosphorylation. Specific control of these processes in vivo and in vitro can be achieved with peptide analogues that mimic the binding properties of phosphoproteins. We present here the solid-phase synthesis of two novel classes of phosphopeptide mimetics, O-boranophosphopeptides and O-dithiophosphopeptides, derivatized on tyrosine, serine, and threonine. The use of H-phosphonate and H-phosphonothioate monoesters containing the base labile 9-fluorenemethyl protecting group was key to the synthesis of both phosphopeptide mimetics. O-Boranophosphopeptides were synthesized by condensing O-(9-fluorenemethyl)-H-phosphonate to the peptide hydroxylic component (tyr, ser, or thr) followed by oxidation with borane complexes. Similarly, the synthesis of O-dithiophosphopeptides used the O-(9-fluorenemethyl)-H-phosphonothioate synthon and oxidation with elemental sulfur. Base elimination of the Fmol protecting group and cleavage from the solid support with concentrated ammonium hydroxide afforded the boranophosphopeptide and dithiophosphopeptide target compounds. Ac-YIIPLPG-NH2, having either dithiophosphoryl tyrosine or boranophosphoryltyrosine but no sequence specificity for Yersinia protein tyrosine phosphatase (PTP), was found to competitively inhibit this enzyme with KI values of 430 +/- 50 and 670 +/- 50 microM, respectively. In addition, both phosphopeptide analogues were resistant toward Yersinia PTP enzymatic hydrolysis. Under conditions (pH 8.0) where the phosphopeptide was rapidly dephosphorylated, the boranophosphopeptide hydrolyzed slowly (t1/2 = 15 h) and the dithiophosphopeptide was completely stable over 24 h.     

Functionalization of the alicyclic skeleton of epibatidine: synthesis and nicotinic acetylcholine receptor binding affinities of epibatidine analogues

A novel method for the epimerization of endo-2-(6-chloro-3-pyridyl)-7-azabicyclo[2.2.1]heptan-3-one (12) on silica gel was developed and used as the key step to synthesize functionalized analogues of epibatidine which were evaluated for their nicotine receptor subtype selectivity in binding studies.