Isomerization-Locked Alkene Analogues of Xaa–Pro Dipeptides in the Proteins Collagen and Bora
Collagen is one of the most abundant human proteins. It exists as a right-handed superhelix called the triple helix. The triple helix consists of three left-handed polyproline type II (PPII helices) that intertwine around a common axis. Each PPII helix has the repeating peptide sequence (Gly–Xaa–Yaa)n with a high content of (2S)-proline (Pro) in the Xaa position (ca. 28%) and (2S,4R)-hydroxyproline (Hyp) in the Yaa position (ca. 38%). Unique to the prolyl amide is the ease of cis-trans isomerization. Since the triple helix necessitates that all peptide bonds be in the trans conformation, isomerization is the rate-limiting step in collagen folding. However, eliminating isomerization with a trans-locked alkene isostere destabilizes collagen-like peptides. Collagen is stabilized by electronic interactions, namely the n→π* interaction. Halo-alkene isosteres may be used to recapture these electronic interactions and stabilize a collagen-like peptide. An in-depth conformational analysis was conducted at the MP2/6-311+G(2d,p) level of theory to determine the viability of conformationally-locked halo-alkene isosteres. Fluoro-alkenes and chloro-alkenes were modeled at both the Gly–Pro and Pro–Pro (as a Pro–Hyp mimic) amide positions. Compared to the collagen crystal structure PDB ID: 1K6F, we found the fluoro-alkenes were closer geometric matches to both Gly–Pro and Pro–Pro than the corresponding chloro-alkenes. The chloro-alkene was predicted to have stronger n→π* interactions. The trans-locked proteo-alkene was also analyzed to understand why it destabilized the triple helix. We found that these models had other local minima close to the desired PPII geometry, likely leading to enhanced backbone flexibility. This deleterious flexibility was not predicted for either fluoro-alkene or chloro-alkene models. The conformationally-locked halo-alkene isostere Fmoc–Gly–Ψ[(Z)CF=C]-Pro–Hyp(tBu)–OH was designed and synthesized as a (Z)-fluoro-alkene Gly–Pro isostere. We used the chiral catalyst, L-Thr, for asymmetric aldol addition to cyclopentanone, which inadvertently enhanced the yield of the wrong enantiomer, in contrast with aldol addition to cyclohexanone. A Mg2+-promoted Horner-Wadsworth-Emmons reaction afforded the (Z)-fluoro-alkene over the (E)-fluoro-alkene in about a 2:1 ratio. The two diastereomers, Fmoc–Gly–Ψ[(Z)CF=C]-L-Pro–Hyp(tBu)–OH and Fmoc–Gly–Ψ[(Z)CF=C]-D-Pro–Hyp(tBu)–OH were separated by supercritical CO2 chromatography. The collagen-like peptides Ac–(Gly–Pro–Hyp)3–Gly–Ψ[(Z)CF=C]-L-Pro–Hyp–(Gly–Pro–Hyp)4–Gly–Gly–Tyr–NH2, Ac–(Gly–Pro–Hyp)3–Gly–Ψ[(Z)CF=C]-D-Pro–Hyp–(Gly–Pro–Hyp)4–Gly–Gly–Tyr–NH2, and the control peptide Ac–(Gly–Pro–Hyp)8–Gly–Gly–Tyr–NH2 were synthesized on solid-phase resin. The CD spectra of all three peptides showed the characteristic collagen triple-helix signature. The folding stability was determined by thermal melting (Tm). The peptide with the fluoro-alkene guest, Gly–Ψ[(Z)CF=C]-L-Pro–Hyp, was found to have a Tm value of 42.2 °C. The Tm of the control peptide was found to be 49.0 °C, a difference in stability of only ΔTm –6.8. Thus, the (Z)-fluoro-alkene as a Gly–Pro isostere forms a relatively stable triple helix. The peptide with the Gly–Ψ[(Z)CF=C]-D-Pro–Hyp guest was shown to have a linear relationship between ellipticity and temperature, indicating that a stable triple helix did not form. The enhanced stability of the (Z)-fluoro-alkene compared to the (E)-alkene Gly–Pro isostere (Tm = 28.3 °C) may be due to a stabilizing n→π* interaction, as determined by NMR deshielding of the 19F nucleus in the collagen-like peptide. In biological systems, isomerization of the prolyl amide is catalyzed by enzymes called PPIases. The PPIase Pin1 specifically catalyzes isomerization of the pSer–Pro sequence from the cis-conformation to the trans-conformation. Pin1 plays a crucial role in the G2→M transition of the cell cycle, implying the importance of cis-trans isomerization. The dipeptides H–Ser–Ψ[(Z)CH=C]-Pro–OH, H–Ser–Ψ[(E)CH=C]-Pro–OH and native H–Ser–Pro–OH were synthesized by literature methods, and activated for aminoacylation of tRNACUA for in vitro transcription-translation. Aminoacylation by chemical methods required the synthesis of a pdCpA dinucleotide. Formation of the dipeptide-dinucleotide complex was not completed because protection of the Ser side chain was problematic. On the other hand, conversion of the dipeptide into the 3,5-dinitrobenzyl ester conjugate allowed for enzymatic aminoacylation using the dFx flexizyme, an RNA enzyme. The native dipeptide was successfully coupled to tRNACUA and is ready for incorporation into a full-length Bora protein by in vitro transcription-translation. Both cis- and trans-locked alkene mimics have been converted to their respective 3,5-dinitrobenzyl ester conjugates.