Synthesis of Aza- and α,α-disubstituted Glycinyl peptides and application of their electronic and steric interactions for controlling peptide folding, and  for biomedical applications

Peptides exhibit pharmaceutical potential as active ingredients and excipients in drug delivery systems.  Their shortcomings, such as low bioavailability, poor selectivity and limited metabolic stability have however necessitated the design and synthesis of peptidomimetics to improve pharmacological properties. The body of this Ph.D. thesis has been targeted towards two objectives: a) the development of azapeptide analogs as modulators of the prostaglandin F2α receptor (FP), and b) the development of methods for the construction, and the conformational analysis of bulky Cα,α-disubstituted glycine amino acids.  Azapeptides are a class of peptidomimetic in which the Cα unit of at least one amino acid residue is replaced by a nitrogen atom. This modification features a semicarbazide as an amino amide surrogate, which reduces conformational flexibility and can induce β -turn conformation.  Preterm birth is an unmet medical need. Despite efforts to counter the onset of preterm labor, the rate of premature birth has increased steadily in developed countries. The prostaglandin F2α receptor (FP) was pursued as target for designing tocolytic (labor delaying) agents. Aza-amino acyl proline analogs had been shown to modulate FP and to inhibit PGF2α-mediated uterine contractions by an allosteric mechanism involving biased G protein-coupled receptor signalling. Moreover, the activity of these analogs was found to be contingent on the nature of their aza-residue side chains.   To better understand of the structure-activity relationships of azapeptide modulators of FP, diversity-oriented approaches were devised to introduce different side chains onto the aza-amino acyl proline residue by alkylation of aza-Gly-Pro analogs, as well as by copper-catalyzed reactions on an aza-propargylglycine (aza-Pra) residue. Analogs were prepared to study the influences of side chain size, hydrophobicity and aromaticity on activity. In addition, a set of proline residue analogs were prepared by the modification of a 4-hydroxyproline peptide linked to a solid support. These analogs were synthesized to study the influence of ring pucker, hydrophobicity and substituents on activity.  Analogs were tested for ability to modulate PGF2α-mediated uterine contractions. Study of the novel modulators demonstrated that activity on myometrial contractions was sensitive to changes in the conformation at the turn region, as well as the size, nature and lipophilicity of the aza-residue side chain. The chemistry of peptides bearing 2-amino adamantane-2-carboxylic acid (Adm) was explored to examine the influence of the bulky residue on conformation.  A set of N-formyl-Adm di- and tripeptides possessing various C-terminal components was synthesized by the development of a multiple-component Ugi reaction sequence using adamantan-2-one as precursor to the bulky residue. The backbone conformation of the hindered peptides was determined using X-ray analysis. The steric bulk of the Adm residue restricted conformational space to a region typical of an intramolecular hydrogen-bonded α- and γ-turn, as well as a distorted type-II (IIβ ) β -turn. The C-terminal residue influenced the backbone conformation. Bulkier C-terminal groups, such as t-butyl, favored regular γ-turns. Smaller C-terminal residues favored β - and α-turn geometry.  Employing an iterative Ugi sequence homo-oligo-Adm tripeptides were prepared possessing i-propyl and t-butyl C-terminal amides.  Crystallization of the Adm trimers and X-ray analysis in the solid state demonstrated that the i-propyl analog adopted the conformation of a single 13-membered hydrogen-bonded α-turn.  On the other hand, the t-butyl analog exhibited two consecutive γ-turns in an incipient γ-helix conformation.  Effective methods have thus been developed for the assembly of azapeptides and sterically bulky Cα,α-disubstituted glycine oligomers. With such methods in hand, the effects of electronic and steric constraint on peptide conformation have been respectively explored in studies of biologically active FP modulators to develop therapy to delay labor, and in conformational analyses of Adm oligomers. This thesis has thus contributed to a better understanding of structure-activity relationships of azapeptide modulators of FP. Moreover, the factors controlling the conformation of the bulky Adm residue have been elucidated in the construction of three different secondary elements (α-,  β   and  γ-turns) contingent on the C-terminal residues. The results offer interesting potential for biomedical and peptide chemistry.

Keyphrases: Peptidomimetic, Azapeptide, Prostaglandin F2alpha, Aza-propargylglycine, alpha-turn, Cα-disubstituted glycine, peptide folding, 2-amino adamantane-2-carboxylic acid, torsion angle, Prostaglandine F2alpha, Tour-alpha, Tour-beta, Tour-gamma