Douglas S. Masterson

Investigation of the Cosolvent Effect on Six Isoenzymes of PLE in the Enantioselective Hydrolysis of Selected α,α‐Disubstituted Malonate Esters

Pig liver esterase (PLE) is among the most widely utilized enzymes in organic synthesis. PLE has demonstrated its versatility in performing enantioselective hydrolysis of a vast variety of esters in moderate to excellent enantiomeric excess. This esterase has proven to be extremely useful for the generation of chiral quaternary carbon centers by hydrolysis of appropriately substituted a,a-disubstituted malonate esters. The acid-ester products obtained from the hydrolysis of prochiral a,a-disubstituted malonate esters have been successfully used for the generation of novel unnatural amino acids as illustrated in Scheme 1. b, g]

A Divergent Approach to the Preparation of Cysteine and Serine Analogs

Malonate diesters containing a prochiral quaternary carbon have been successfully transformed into analogs of cysteine and serine. The chiral half‐esters are obtained in good yield, and enantioselectivity by selective hydrolysis using Pig‐Liver Esterase (PLE) as the catalyst. The resulting half‐ester intermediates are transformed into α2, 2‐, β2, 2‐, and β3, 3‐analogs of cysteine and serine. The methodology described here allows for the preparation of both enantiomers of the amino‐acid analogs by selective manipulation of the ester and acid functionalities. This divergent strategy allows a common synthetic strategy to be used to prepare a variety of unnatural amino‐acid classes from a common intermediate which should prove useful in the design of novel peptide libraries. Copyright

A Stereoselective Cyclization Strategy for the Preparation of γ-Lactams and Their Use in the Synthesis of α-Methyl-β-Proline

A straightforward stereoselective and enantiodivergent cyclization strategy for the construction of γ-lactams is described. The cyclization strategy makes use of chiral malonic esters prepared from enantiomerically enriched monoesters of disubstituted malonic acid. The cyclization occurs with the selective displacement of a substituted benzyl alcohol as the leaving group. A Hammett study illustrates that the cyclization is under electronic control. The resulting γ-lactam can be readily converted into a novel proline analogue.

An Investigation of the Interaction of Co-Solvent with Substrates in the Pig Liver Esterase-Catalyzed Hydrolysis of Malonate Esters

Previously, we have reported the effect of several co‐solvents, including ethanol, on the enantioselective outcome of pig liver esterase (PLE) hydrolysis reactions. The greatest improvements were observed in those substrates that contained an atom capable of forming a hydrogen bond in the sidechain portion of the molecule. To further explore the interaction between substrate and ethanol, a second series of substrates were synthesized, in which the hydrogen bonding atoms of the side chain were modified, and subjected to PLE hydrolysis. Substrates containing atoms capable of forming hydrogen bonds showed the largest equilibrium constants. However, a large equilibrium constant did not always produce significant changes in enantioselectivity as hypothesized. Finally, molecular modeling experiments were performed to obtain a better understanding of the interactions. These experiments revealed that there are substrate–enzyme interactions that can be influenced by the addition of ethanol.

Efficient Esterification of Oxidized l-Glutathione and Other Small Peptides

Oxidized l-glutathione was esterified to the tetra methyl ester using thionyl chloride in methanol solvent. Other alcohols were tested and the reaction progress was monitored via ESI-MS. This procedure proved to be compatible with other small peptides not containing serine and cysteine residues. In contrast to previously reported methods this procedure provided convenient access to esterified peptides requiring no purification, extended reaction times, or complicated reaction setups.

An ESI-MS Method to Determine Yield and Enantioselectivity in a Single Assay

AbstractA mass spectrometry assay is presented here that allows for the simultaneous determination of yield and enantioselectivity in a single analysis. The assay makes use of molecules that are structurally similar to the analytes of interest as standards. The assay predicts the yields of the reactions reasonably well and with little error. For example, in the pig liver esterase catalyzed hydrolysis of one prochiral malonate, the yield predicted by the assay was 72%, while larger scale isolated reaction yields were within 5% of this value. This assay provides a fast method to determine yield and enantioselectivity in one analysis. The strengths and limitations of this method are discussed. Graphical Abstractᅟ

Glutathione reductase activity with an oxidized methylated glutathione analog

Abstract The activity of glutathione reductase with an unnatural analog of oxidized glutathione was explored. The analog, L-γ-glutamyl-2-methyl-L-cysteinyl-glycine disulfide, places an additional methyl group on the alpha position of each of the central cysteine residues, which significantly increases steric bulk near the disulfide bond. Glutathione reductase was completely unable to catalyze the sulfur–sulfur bond reduction of the analog. Additionally, enzyme kinetics experiments indicated that the analog acts as a competitive inhibitor of glutathione reductase. Computational studies confirm that the methylated analog fits within the active site of the enzyme but its disulphide bond geometry is altered, preventing reduction by the enzyme. The substitution of (R)-2-methylcysteine in place of natural (R)-cysteine in peptides constitutes a new strategy for stabilizing disulphide bonds from enzyme-catalyzed degradation.