ABSTRACT
Collagen is the most abundant human protein, with the canonical sequence (Gly-Pro-Hyp)n in its triple helix region. Cis-trans isomerization of the Xaa-Pro amide has made two of these amide bonds the target of alkene replacement: the Gly-Pro and the Pro-Hyp positions. The conformations of Gly-Pro and Pro-Pro (as a Pro-Hyp model) fluoro-, chloro-, and proteo-alkene mimic models were investigated computationally to determine whether these alkenes can stabilize the polyproline type II (PPII) conformation of collagen. Second-order Møller-Plesset (MP2) calculations with various basis sets were used to perform the conformational analyses and locate stationary points. The calculation results predict that fluoro- and chloro-alkene mimics of Gly-Pro and Pro-Pro can participate in nâπ* donation to stabilize PPII conformations, yet they are poor nâπ* acceptors, shifting the global minima away from PPII conformations. For the proteo-alkene mimics, the lack of significant nâπ* interactions and unstable PPII-like geometries explains their known destabilization of the triple helix in collagen-like peptides.
Subject(s)
Alkenes , Collagen , Dipeptides , Humans , Protein Conformation , Protein Structure, SecondaryABSTRACT
Ab initio calculations of three models of collagen at positions Pro-Pro-Gly (1), Pro-Gly-Pro (2), and Gly-Pro-Pro (3) were performed to assess the conformational variation of nâπ* contributions to the stability of the collagen triple helix. Full conformational analyses by relaxed potential-energy scans of the Ψ dihedral angle of the central residue in models 1, 2, and 3 revealed the presence of several nâπ* interactions. In model 2, with Gly as the central residue, both the Φ and Ψ dihedral angles of Gly were scanned. Most minima of each model contained one or two nâπ* interactions, with pyramidalization at the π* carbon. We also observed pyramidalization at the nâπ* donor amide nitrogens. Minima with hydrogen-bond or non-native nâπ* interactions compete with the collagen stabilizing nâπ* interactions. The collagen-like nâ re-π* conformation was found as the global minimum only in model 3. The global minimum of 1 had a 5-membered ring hydrogen bond with an additional weak nâ si-π* interaction. The global minimum of 2 was in the extended conformation. We predict that the nâπ* interactions found in native collagen, while individually small, cumulatively contribute to the stability of the triple helix conformation of collagen.
Subject(s)
Collagen/chemistry , Hydrogen Bonding , Models, Chemical , Models, Molecular , Protein Conformation, alpha-Helical , Protein Structure, Quaternary , Static Electricity , ThermodynamicsABSTRACT
Three stereoisomeric inhibitors of Pin1: (2R,5S)-, (2S,5R)- and (2S,5S)-Ac-pSer-Ψ[(Z)CH = C]-pipecolyl(Pip)-2-(2-naphthyl)ethylamine 1, that mimic L-pSer-D-Pro, D-pSer-L-Pro, and D-pSer-D-Pro amides respectively, were synthesized by a 13-step route. The newly formed stereogenic centers in the pipecolyl ring were introduced by Luche reduction, followed by stereospecific [2,3]-Still-Wittig rearrangement. The (Z)- to (E)-alkene ratio in the rearrangements were consistently 5.5 to 1. The stereochemistry at the original Ser α-carbon controlled the stereochemistry of the Luche reduction, but it did not affect the stereochemical outcome of the rearrangement, which consistently gave the (Z)-alkene. The epimerized by-product, (2S,5S)-10, resulting from the work-up after Na/NH3 debenzylation of (2S,5R)-9, was carried on to the (2S,5S)-1 isomer. Compound (2S,5S)-10 was resynthesized from the Luche reduction by-product, (2R,3R)-3, and the stereochemistry was confirmed by comparison of the optical rotations. The IC50 values for (2R,5S)-1, (2S,5R)-1 and (2S,5S)-1 Pin1 inhibition were: 52, 85, and 140 µM, respectively.