Mechanism-guided design and application of palladium catalysts for alcohol oxidation and degradable polymers for RNA delivery
Keywords: Humanities and Sciences.
Chemistry.

Oxidation of alcohols is a fundamental transformation relevant to the synthesis of fine and commodity chemicals. The palladium complex [(neocuproine)Pd(μ-OAc)]2[OTf]2 is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits turnover number and necessitates high Pd loadings. In Chapter 1 we describe the development of more oxidatively robust palladium catalysts supported by 2,2'-biquinoline ligands. Evaluation of [(2,2'-biquinoline)Pd(μ-OAc)]2[OTf]2 as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow observed rates were a consequence of precipitation of an insoluble trinuclear palladium species — [((2,2'-biquinoline)Pd)3(μ-O)2]2+ — formed during catalysis. Density functional theory was used to predict that a sterically modified biquinoline ligand, 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear [(LPd)3(μ-O)2]2+ species. This design strategy was validated, as catalytic aerobic oxidation with [(7,7'-di-tert-butyl-2,2'-biquinoline)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent neocuproine-supported system with the high aerobic turnover numbers of the 2,2'-biquinoline-supported system. Selective and effective delivery of RNA is a key challenge for the development of gene therapies. Charge-Altering Releasable Transporters (CARTs) are a class of amphiphilic polymers that complex RNA, deliver it into cells, and then undergo degradation via structure-dependent mechanisms, thereby enabling RNA release. In Chapter 2 we report three classes of novel CART polymers: one derived from the amino acid ornithine (O-CARTs), one derived from the amino acid tyrosine (Y-CARTs), and one derived via statistical polymerization (statistical CARTs). O-CART/mRNA nanoparticles selectively deliver mRNA to the lung upon intravenous administration. Y-CARTs are also effective for mRNA delivery in vitro and in vivo, and preliminary data suggest that Y-CART/mRNA nanoparticles elicit protein expression in the spleen and the liver. Statistical CARTs with diverse structures are readily synthesized with a streamlined procedure, and statistical copolymers, upon complexation with mRNA, exhibit improved nanoparticle stability and equal or better performance for mRNA delivery compared to traditional block CART copolymers. In Chapter 3 we apply the CART RNA delivery platform in the development of a next-generation RNA vaccine against SARS-CoV-2. We show that CART/RNA formulations can be lyophilized, stored, and reconstituted while retaining efficacy for mRNA delivery, a key step towards the development of stable vaccine formulations. We demonstrate that CART polymers can effectively deliver self-amplifying RNA (saRNA), a promising class of RNA vaccine cargoes that have the potential to enable extended protein expression and dose-sparing effects. We also establish that CARTs are effective for the intradermal delivery of diverse RNA cargoes. In a final demonstration we show that intramuscular and intradermal CART/saRNA vaccines against SARS-CoV-2 elicit robust antibody responses in pigs. These efforts demonstrate the potential of CART amphiphiles as highly effective RNA transporters for research and clinical applications.