Quantitative results for the time-dependent behaviour of the fractional abundance of HC3N in dense cores are given. In addition to the standard ‘early-time’ HC3N maximum found for many situations in which elements other than hydrogen are initially in atomic form, a secondary maximum occurs for a variety of conditions after depletion has become significant. The secondary maximum can be sufficiently...

A catalytic antibody-based approach toward the synthesis of an optically active pyrrolidine ring synthon potentially useful for ptilomycalin A is described. Enantiomerically pure hapten 37 was designed and constructed with the eventual goal of generating antibodies for the enantioselective partial hydrolysis of a meso diester such as 44 into a monoacid 45. This transition state analog possesses a phosphonate group containing the requisite oxyanionic character of the tetrahedral intermediate for ester hydrolysis. A newly developed carbamate-based linker, which was found to be much more hydrolytically stable than the commonly used glutarate ester, was developed for coupling of the hapten to a carrier protein.

Catalysis of the deamidation of asparagine residues may provide a powerful method for the deactivation of proteins. Catalytic antibodies (Gibbs et al. Science 1992, 258, 803) have been induced that catalyze the deamidation of a model dipeptide through an intermediate succinimide. Investigations of the mechanistic characteristics of two such antibodies, RG2-23C7 and RG2-2E4, revealed their ability to accelerate the hydrolysis of either the R- or S-enantiomers of the succinimide by factors of 10-500-fold to yield differing ratios of the aspartate and isoaspartate products. The mixed product ratios imply that two tetrahedral binding sites of unequal effectiveness were induced in response to the tetrahedral mimics (a phosphinate or secondary hydroxyl) within the hapten structure. The antibody RG2-2E4 also catalyzes the deamidation of either the D- or L-asparagine within the dipeptide through the intermediate cyclic imide, resulting in a multistep reaction sequence featuring a series of tetrahedral transition states. pH-rate profiles do not implicate functional groups within the antibodies' combining sites for either the deamidation or hydrolytic reactions. The strategy of bifunctional or higher order transition state mimics should provide a route to developing catalytic antibodies for reactions requiring multistep processing.

Monoclonal antibodies, induced with a phosphonate diester hapten, catalyzed the coupling of p-nitrophenyl esters of N-acetyl valine, leucine, and phenylalanine with tryptophan amide to form the corresponding dipeptides. All possible stereoisomeric combinations of the ester and amide substrates were coupled at comparable rates. The antibodies did not catalyze the hydrolysis of the dipeptide product nor hydrolysis or racemization of the activated esters. The yields of the dipeptides ranged from 44 to 94 percent. The antibodies were capable of multiple turnovers at rates that exceeded the rate of spontaneous ester hydrolysis. This achievement suggests routes toward creating a small number of antibody catalysts for polypeptide syntheses.

Observations of the CO J=1-0 line are commonly used as a tracer for molecular material in clouds. The ratio of the H2 column density to the integrated intensity of this line,X, is often taken to be constant, despite theoretical and observational uncertainty. We have tried to identify how this ratio depends on cloud parameters, testing a simple theoretical argument suggesting its invariance with respect to density. The apparent constancy can be understood if clouds are clumpy on scales of Av ≈ 1-2 mag.