Part 1. Synthesis of a cyclic pentapeptide mimic for the active site His-Tyr cofactor of cytochrome c oxidase. Part 2. Photochemically induced Wolff rearrangement towards beta-lactams.

摘要

Cytochrome c oxidase (CcO) belongs to an important class of enzymes, heme-copper oxidases (HCO), which are responsible for generating electrochemical energy via the redox coupling of exergonic reactions. Since the reduction of O2 takes place at the binuclear center, all of the reductive potential should derive from the nearby metal centers and/or cofactors. Two electrons can be derived from the oxidation of Fe(II) to Fe(IV), one electron from the oxidation of Cu(I) to Cu(II). This leaves one electron unaccounted for. It has been proposed to come from a nearby tryptophan residue, oxidation Fe(IV) to Fe(V), oxidation to Cu(III) or, from the phenolic position of Tyr244. Since the high-resolution x-ray structure of the mammalian cytochrome oxidase (2.8 A, further improved to 1.8 A; PDB ID 1OCC) was reported by Yoshikawa in 1998, many facets of research regarding CcO have advanced forward. A startling discovering with respect to mechanistic insights was made in the active site region. A post-translational modification involving an N-arylation linking N1 of the His 240 side chain to the epsilonC of Tyr244 (bovine heart numbering).;An effective and improved method of N-arylation has been described to create the His-tyr dipeptide. This dipeptide was elaborated to a mimic of the complete cyclic pentapeptide cofactor using judicious protecting group strategies and effective coupling procedures. The x-ray structure of an advanced intermediate in the synthesis was solved. Using Macromodel the biaryl dynamics were explored. In addition, NMR restricted minimizations provided highly refined structures of the CcO cofactor mimic. A copper bound system was not obtained in crystalline form suitable for x-ray, however the ligand itself was subjected to spectrophotometric titrations providing valuable data: the pKas of the imidazole and phenol for compound 2.64 are 5.1 and 9.4, respectively. This important piece of data corroborates previous mimics synthesized in this laboratory and strengthens the hypothesis that Tyr244 serves as a proton donor facilitating the cleavage of molecular oxygen.;The exploration of the photochemically induced Wolff rearrangement to afford enantiomerically pure beta-lactams bore many interesting results. Highly scalable routes to both the serine and aspartic acid derived beta-lactams requiring little chromatography were developed. A purely thermal epimerization process was developed which provided access to large amounts of pure trans beta-lactam. The molecular modeling provided highly valuable information that gave key insights into the stability of the trityl and bis-benzyl diazo compounds, implicating a pi-cation interaction as a main stabilizing factor, and also confirmed the s-E,s-Z conformation. This lowest energy conformation is used to explain the high propensity for Wolff rearrangement seen throughout this project.