Fmoc-Pro(4R-CF3)-OH

Chromatography is a common step in the solution-phase synthesis of typical peptides, as well as peptide fragments for subsequent coupling on a solid support. Combining known reagents that form readily separable byproducts is shown to eliminate this step, which wastes time and other resources. Specifically, activating carboxyl groups with isobutyl chloroformate or as pentafluorophenyl esters and using N-methyl morpholine as a base enable chromatography-free synthetic routes in which peptide products are isolated from byproducts by facile evaporation, extraction, and trituration. This methodology was used to access tripeptides related to collagen, such as Fmoc-Pro-Pro-Gly-OH and Fmoc-Pro-Hyp(tBu)-Gly-OH, in a purity suitable for solid-phase segment condensation to form collagen mimetic peptides.

Synthetic collagen peptides containing larger numbers of Gly-Pro-Hyp repeats are difficult to purify by standard chromatographic procedures. Therefore, efficient strategies are required for the synthesis of higher molecular weight collagen-type peptides. Applying the Fmoc/tBu chemistry, a comparative analysis of the standard stepwise chain elongation procedure on solid support with the procedure based on the use of the synthons Fmoc-Gly-Pro-Hyp(tBu)-OH and Fmoc-Pro-Hyp-Gly-OH was performed. The crude products resulting from the stepwise elongation procedure and from the use of Fmoc-Gly-Pro-Hyp(tBu)-OH clearly revealed large amounts of microheterogeneities that result from incomplete imino acid acylation as well as from diketopiperazine formation with cleavage of Gly-Pro units from the growing peptide chain. Conversely, by the use of the Fmoc-Pro-Hyp-Gly-OH synthon, the quality of the crude products was significantly improved; moreover, protection of the Hyp side chain hydroxyl function is not required using the Fmoc/tBu strategy. With this optimized synthetic procedure, relatively large collagen-type peptides were obtained in satisfactory yields as highly homogeneous compounds.

A new triacid scaffold is described based on the cone-shaped cyclotriveratrylene (CTV) molecule that facilitates the triple helical folding of peptides containing either a unique blood platelet binding collagen sequence or collagen peptides composed of Pro-Hyp-Gly repeats. The latter were synthesized by segment condensation using Fmoc-Pro-Hyp-Gly-OH. Peptides were coupled to this CTV scaffold and also coupled to the Kemp's triacid (KTA) scaffold. After assembly of peptide H-Gly-[Pro-Hyp-Gly]2-Phe-Hyp-Gly-Glu(OAll)-Arg-Gly-Val-Glu (OAll)-Gly-[Pro-Hyp-Gly]2-NH2 (13) by an orthogonal synthesis strategy to both triacid scaffolds, followed by deprotection of the allyl groups, the molecular constructs spontaneously folded into a triple helical structure. In contrast, the non-assembled peptides did not. The melting temperature (Tm) of (+/-) CTV[CH2C(O)N(H)Gly-[Pro-Hyp-Gly]2-Phe-Hyp-Gly-Glu-Arg-Gly-Val-Glu-Gly- [Pro-Hyp-Gly]2-NH2]3 (14) is 19 degrees C, whereas KTA[Gly-Gly-[Pro-Hyp-Gly]2-Phe-Hyp-Gly-Glu-Arg-Gly-Val-Glu-Gly- [Pro-Hyp-Gly]2-NH2]3 (15) has a Tm of 20 degrees C. Thus, it was shown for the first time that scaffolds were also effective in stabilizing the triple helix of native collagen sequences. The different stabilizing properties of the two CTV enantiomers could be measured after coupling of racemic CTV triacid to the collagen peptide, and subsequent chromatographic separation of the diastereomers. After assembly of the two chiral CTV scaffolds to the model peptide H-Gly-Gly-(Pro-Hyp-Gly)5-NH2 (24), the (+)-enantiomer of CTV 28b was found to serve as a better triple helix-inducing scaffold than the (-)-enantiomer 28a. In addition to an effect of the chirality of the CTV scaffold, a certain degree of flexibility between the CTV cone and the folded peptide was also shown to be of importance. Restricting the flexibility from two to one glycine residues resulted in a significant difference between the two collagen mimics 20a and 20b, whereas the difference was only slight when two glycine residues were present between the CTV scaffold and the peptide sequence in collagen mimics 30a and 30b.