Aggregation via empirical risk minimization

Given a finite set F of estimators, the problem of aggregation is to construct a new estimator whose risk is as close as possible to the risk of the best estimator in F. It was conjectured that empirical minimization performed in the convex hull of F is an optimal aggregation method, but we show that this conjecture is false. Despite that, we prove that empirical minimization in the convex hull of a well chosen, empirically determined subset of F is an optimal aggregation method.     

Electrostatic interactions dominate the catalytic contribution of arg39 in 4-oxalocrotonate tautomerase

The synthesis and kinetic parameters of a comprehensive set of 4-OT analogues with arginine (X = NH2+) to citrulline (X = O) substitutions at positions 11, 39, and 61 are reported. These data suggest that the main contribution of Arg39' ' to catalysis is by electrostatic stabilization of the anionic transition state leading to intermediate 2, and not by hydrogen bonding.     

Experimental evidence for a collective insulating state in two-dimensional superconductors

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field (B) of disordered, superconducting, thin films of amorphous indium oxide. As the B strength is increased superconductivity degrades, until a critical field (B(c)) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.     

A free,tricoordinate stannylium cation

Tris(2,4,6-triisopropylphenyl)stannylium tetrakis(pentafluorophenyl)borate constitutes a free, tricoordinate tin cation according to its X-ray structure. There is no coordination between the cation and either solvent or anion, and there are no atoms at apical positions. DFT calculations confirm the structure and indicate that there is no agostic bonding between the ortho isopropyl methinyl hydrogens and the Sn atom. Calculation of the 119Sn chemical shift is in good agreement with the observed value.     

Catalysis of the kemp elimination by natural coals

Commercially available coals were found to be efficient heterogeneous catalysts of the Kemp elimination reaction in aqueous solutions. A pH-rate profile study suggests that catalysis originates from specific catalytic groups and not simply from the large graphitic surface area. The low-quality lignite coals, which exhibit similar catalytic efficiency per weight to that of molecularly imprinted polymers, are better catalysts for this reaction in comparison with the bituminous coals.     

Completely irreducible sequences of meromorphic functions

Given a sequence {fn } of meromorphic functions on a plane domain D, there exists a (possibly empty) open set U■D and a subsequence {f n k } which converges uniformly (with respect to the spherical metric on ) on compact subsets of U, while no subsequence of {f n k } converges uniformly on compact subsets of any larger open subset of D.     

Diferential Inequalities,Normality and Quasi-normality

We prove that ifD is a domain in C,α 1 and C 0,then the family F of functions f meromorphic in D such that |f′(z)|/1 + |f(z)|α C for every z ∈ D is normal in D.For α = 1,the same assumptions imply quasi-normality but not necessarily normality.     

Origin of selectivity in the antibody 20F10-catalyzed Yang cyclization

The first antibody-catalyzed Yang (Norrish type II) cyclization has been achieved with antibodies that were elicited against cis- and trans-2,3-diaryloxetanes. The photocyclization of 1,4-diarylbutan-1-one produced a single stereoisomer of cis-1,2-diarylcyclobutanol with very high enantioselectivity. The background photochemical reaction in the absence of the antibody yielded only fragmentation products. The antibody 20F10-catalyzed reaction was studied in detail, exploring its selectivity, substituent effects, substrate and hapten binding, kinetic parameters and irradiation wavelength dependence. Quantum mechanical calculations suggest that the activation enthalpy of fragmentation pathway is favored by 7.9 kcal/mol over cyclization pathway. Hapten, substrate, and transition state docking studies on a homology based modeled antibody binding site indicate that the trans hapten, substrate and the cyclization transition state have similar binding modes. By contrast, the fragmentation transition state is bound in a different way, not easily accessible within the lifetime of the bound substrate excited state. Several side chain residues were identified that can act as local sensitizers to enhance the cyclization process.