Demonstration of endo-cis-(2S,3R)-bicyclo[2.2.1]hept-5-en-2,3- dicarbonyl unit as a reverse-turn scaffold and nucleator of two-stranded parallel β-sheets: design, synthesis, crystal structure, and self-assembling properties of norborneno peptide analogues

Abstract

Endo-cis-(2S,3R)-bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit with a built-in U-architecture has been demonstrated to be an excellent reverse-turn molecular scaffold. A large variety of endo-cis-(2S,3R)-norborneno bispeptides containing almost all of the coded amino acids were synthesized and examined for conformational preferences by ¹H NMR, FT-IR, CD, and X-ray crystallographic studies. While FT-IR and ¹H NMR variable-temperature studies ruled out the presence of any significant amount of intramolecular hydrogen bonding in simple bispeptides (3a-h) (except in Aib bispeptide), the CD studies were clearly in favor of a β-turn type structure. Single-crystal X-ray studies on Aib, Val and Leu containing norborneno bispeptides (3b-d) provided convincing proof for the presence of reverse-turn conformation. While the interstrand C^α-C^α' distances (5.2-5.7 Å) were well within the range of those for β-turn structures, no interstrand intramolecular hydrogen bonding was seen in Val and Leu bispeptides; the Aib bispeptide forms a seven-membered hydrogen-bonded ring, thus, showing that the norbornene (2S,3R)-dicarbonyl template assembles peptide chains in reverse-turn conformation by virtue of its built-in U-shaped architecture at these positions, and hydrogen bonding may not be necessary to stabilize the turn structure. The endo-cis-(2S,3R) orientation of bispeptide chains is essential for turn structure as shown by the crystal structure of trans-(2R, 3R) and trans-(2S,3S) derivative of Val bispeptide wherein the two peptide chains move away from each other with the C^α-C^α' distance increasing to 7.1-8.2 Å. The norbornene 5,6-double bond was hydrogenated to 5,6-dihydro derivative which showed almost the same CD spectrum as its olefinic analogue. Oxidative cleavage [Ru (VIII)] of the 5,6-double bond in norborneno bispeptides, as demonstrated with Leu bispeptide, afforded novel cyclopentanoid peptide analogues. The promise of norbornene unit as a template for nucleating the formation of two-stranded parallel β-sheets with minimum structural complexity is shown by the preparation of higher members of norborneno bispeptides with the general structure NBE(Pep)₂ [NBE = endo-cis-(2S,3R)-bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit; Pep = peptide strand with two, three, or four (same or different) amino acid residues]. In ¹H NMR, the high ³J_HNα values (7.0-9.3 Hz) observed for the amide protons (Table 3) coupled with the presence of medium to strong intrastrand sequential ROE connectivities d_αN(i,i+1) spanning the entire three- or four-residue sequence in the peptide strands of 9a-e and 10 and tahe exhibition of relatively low-temperature coefficients (dδ/dT = - 0.2 to -3.4 ppb/K) for amide protons in DMSO-d₆ solvent (Table 4) clearly suggested that hydrogen-bonded β-sheet conformers dominate the population. FT-IR and CD studies provided further support for parallel β-sheet structures. A particularly unique feature of the norborneno bispeptides is their strong tendency to self-assemble in the solid state. Thus, while endo-cis-(2S,3R)-Aib bispeptide (3b) forms 16-membered hydrogen bonded centrosymmetric dimers, the half-ester half-acid and the dicarboxylic acid derivatives of 3b self-assemble to form highly ordered hydrogen-bonded molecular ribbons. The Val and Leu cis-(2S,3R)-bispeptides organize into hydrogen-bonded chains and the trans isomer of Val bispeptide self-assembles into hydrogen-bonded β-sheet ribbon.