Zachary D. Aron

Fluorescence Switching of Imidazo[1,5-a]pyridinium Ions: pH-Sensors with Dual Emission Pathways

Imidazo[1,5-a]pyridinium ions are identified as highly emissive and water-soluble fluorophores accessed by an efficient three-component coupling reaction. Synthetic modifications of groups conjugated to the polyheterocyclic core are shown to profoundly impact the emission properties of these molecules. Notably, two structural isomers of functionalized imidazo[1,5-a]pyridinium ions were found to exhibit distinct de-excitation pathways, which are responsible for either a fluorescence turn-on or ratiometric response to pH change.

FenF: Servicing the Mycosubtilin Synthetase Assembly Line in trans

Mycosubtilin biosynthesis is accomplished by a hybrid type I polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) assembly line. During polyketide biosynthesis, the assembly line utilizes an unusual free-standing acyltransferase (AT) enzyme, FenF, to supply the malonate extender unit for chain elongation. The in trans interaction between an AT domain and a type I PKS is unusual, as AT domains typically operate in cis with type I PKS. To better understand the interactions of in trans AT domains with type I PKS, we have completed kinetic and selectivity studies exploring the interactions between FenF and its ACP substrate. Mycosubtilin, an N-acyl cyclic Bacillus subtilis-derived octapeptide with anti-fungal activity, is assembled by a four-protein thiotemplate assembly line (Scheme 1). 2] The first protein in this assembly line is MycA, a 495 kDa polypeptide comprising eleven domains parceled into a two-domain fatty acid activating module, a PKS module, and a NRPS module. The fatty acid activating module contains an acyl-AMP ligase (AL) and an acyl carrier protein (ACP1) ; the PKS module includes a ketosynthase (KS) domain and ACP2, but lacks the third PKS “core” domain, an AT; the NRPS module has two peptidyl carrier proteins (PCP1 and PCP2) even though only a single amino acid (Asn) is thought to be activated by that module. Close inspection of the PKS module of MycA by bioinformatics analysis predicts the presence of a 100-residue stretch, termed a docking (D) domain (see below), between the KS and ACP2 domains. The three modules of MycA are proposed to act in tandem, combining palmitic acid, malonic acid (from malonyl CoA), and Asn to generate b-aminostearoyl-N-Asn1-S-PCP2 prior to transfer to MycB. The b-amino group of b-aminostearoyl-N-Asn1-SPCP2, generated by an aminotransferase (AMT) domain embedded within MycA, serves as the cyclizing nucleophile in heptapeptide macrolactam formation, as mycosubtilin biosynthesis is completed by the NRPS proteins MycB and MycC. The fourth protein in mycosubtilin biosynthesis is FenF, predicted to be a malonyl-selective AT domain acting in trans with MycA to deliver the C2 unit for palmitoyl chain elongation. [2]

Simplifying pyridoxal: practical methods for amino acid dynamic kinetic resolution.

Metal complexes of picolinaldehyde are identified as low-cost and environmentally benign catalysts, providing high reaction rates and turnovers for the racemization of amino acids. These pyridoxal surrogates demonstrate activity toward a variety of amino acid esters. Applications to chemoenzymatic dynamic kinetic resolutions provide access to amino acids in high yields and with excellent enantioselectivities, demonstrating their compatibility with protease-mediated transformations.

Cascade Reactions during Coronafacic Acid Biosynthesis: Elongation, Cyclization, and Functionalization during Cfa7-Catalyzed Condensation

Herein, the biogenesis of the hydrindane ring system within coronafacic acid (CFA) has been investigated. These studies reveal that in addition to the canonical polyketide chain elongation and functionalization encoded by type I polyketide synthase (PKSs), cascade reactions can take place during assembly line-like biosynthesis. Indeed, upon Cfa7-catalyzed Claisen condensation between enzyme-bound malonate and an N-acetylcysteamine (SNAC) thioester, latent reactivity within the elongated enzyme-bound intermediate is unveiled. This reactivity translates into an intramolecular cyclization, which can proceed in a facile manner as observed by the enzyme-independent cyclization of a linear beta-ketothioester intermediate.

Enantioselective synthesis of angularly substituted 1-azabicylic rings: coupled dynamic kinetic epimerization and chirality transfer.

A new strategy for enantioselective synthesis of azacyclic molecules in which dynamic kinetic equilibration of diastereomeric iminium ions precedes a stereochemistry-determining sigmatropic rearrangement is reported. The method is illustrated by the synthesis, in high enantiomeric purity (generally 95-99% ee), of a variety of 1-azabicyclic molecules containing angular allyl or 3-substituted 2-propenyl side chains adjacent to nitrogen and up to three stereogenic centers. In these products, the size of the carbocyclic ring is varied widely (5-12 membered); however, useful yields are obtained in forming 1-azabicyclic products containing only fused pyrrolidine and piperidine rings. Chirality transfer from substituents at carbons 1 and 2 of the 3-butenylamine fragment of the starting material is investigated, with methyl and phenyl substituents at the allylic position shown to provide exquisite stereocontrol (generally 98-99% chirality transfer). An attractive feature of the method is the ability to carry out the key transformation in the absence of solvent. Illustrated also is the high yielding conversion of four such products to a new family of bicyclic β-amino acids of high enantiomeric purity.