Structure and properties of TentaGel resin beads: Implications for combinatorial library chemistry

Summary In view of the widespread use of TentaGel resin beads for the synthesis of combinatorial libraries, the properties of TentaGel resin have been examined using a combination of confocal laser microscopy and NMR spectroscopy. Evidence is presented that trypsin, a 23.5-kDa enzyme, can penetrate to the core of 90-m TentaGel beads, and that the matrix of such beads permits molecular motion at a similar rate to that in solution. The beads act as a separate gel phase rather than as a porous solid. These conclusions have important implications for the bioassay of on-bead combinatorial chemical libraries.     

Reduced motion artifacts in medical imaging by adaptive spatio-temporal reconstruction

In this paper we introduce an algorithm for imaging a time varying object , from its projections at different fixed times. This algorithm differs from other algorithms in that we do not need the object to remain stationary during the data acquisition period. We show that the reconstruction of coarse features, corresponding to low spatial-frequency data, can be made nearly instantaneously in time from the evolving data. A temporal sequence of these low spatial-frequency reconstructions can be used to estimate the motion of the object. Once the motion is estimated, we may use the estimate to compensate for some of the motion of fine scale features. This enables accurate reconstructions of the time varying fine structure if the motion is not too extreme.The algorithm is demonstrated for a selection of phantoms and actual MRI studies. In general, this technique shows promise for a wide variety of applications in MRI, as well as for heart imaging using x-ray CT. Clinical applications should include both functional MRI such as dynamic imaging of oxygen usage and blood flow in the brain, and motion imaging of joints, angiography in the lungs, and heart imaging.This work was supported in part by ARPA as administered by the AFOSR under contracts AFOSR-90-0292 and DOD F4960-93-1-0567.     

Photochemical charge separation within aromatic hydrazines and the effect of excited-state intervalence in dihydrazines

Photolysis into the longest wavelength absorption band of 2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl hydrazine (Hy) substituted naphthalenes causes aryl group reduction electron transfer to give (+)Hy-Ar(-). Electrooptical absorption measurements characterize the charge separation properties from these bands. Emission studies demonstrate that the separation between absorption and emission maxima for symmetrically disubstituted compounds is smaller than that for monosubstituted compounds, which is attributed to excited-state intervalence. The excited-state diabatic surfaces may be described as a Hy(+)-NA(- )-Hy(0), Hy(0)-NA(-)-Hy(+) pair, for which electronic interaction produces a double minimum that qualitatively resembles that in the ground state of the disubstituted intervalence radical cations.     

Deficiencies of the ferricinium-ferrocene redox couple for estimating transfer energies of single ions

The difference between the partial molal entropies of ferrocene and ferricinium has been determined in nine solvents from the temperature dependence of the formal potential of the ferricinium-ferrocene redox couple using a nonisothermal electrochemical cell arrangement in order to probe possible structural reasons for the limitations of the ferrocene assumption for estimating the transfer thermodynamics of single ions between different solvents. In contrast to the uniformly small positive values of predicted by the Born model, the experimental quantities varied widely from small or even negative values in hydrogen-bonded solvents (–5 to 3 e.u.) to substantially larger values (11–14 e.u.) in dipolar aprotic media. These variations appear to arise chiefly from additional solvent ordering in the vicinity of the ferricinium cation compared to the ferrocene molecule which is enhanced in the aprotic solvents. The variations in between water and a number of nonaqueous solvents provide a predominate contribution to the differences between the free energies of single ion transfer calculated using the ferrocene and alternative extrathermodynamic assumptions.     

Double-layer effects on simple electrode reactions I. The reduction of Eu3+ and Cr3+ in the absence of specific adsorption of the supporting electrolyte

The kinetics of the reduction of Eu³⁺ and Cr³⁺ at mercury electrodes in various sodium and lanthanum perchlorate supporting electrolytes have been studied over as large a potential range as possible by using both chronocoulometry and d.c. polarography. The objective was to determine the effect of the diffuse double layer upon the reduction rates of these members of the simplest class of electrode reactions in the absence of specific adsorption. Possible additional influences from perchlorate specific adsorption, ion-pairing, and ionic strength-dependent formal potentials, and differences between the site of reaction and the o.H.p. were also considered. The effects of specific adsorption of an uncharged molecule were assessed by noting the changes in the reduction kinetics of Eu³⁺ caused by the adsorption of thiourea.The classical Gouy-Chapman-Stern model of the double layer was found to give an adequate account of the behavior in relatively dilute lanthanum perchlorate solutions, but it failed badly in more concentrated sodium perchlorate media. Better agreement resulted if the statistical theory due to Krylov and Levich was employed but some discrepancies remained. The diffuse-layer properties evaluated here represent a useful basis for comparisons with systems where specific ionic adsorption of components of the supporting electrolyte adds a second component to the electrode surface charge density which determines the diffuse-layer potential.