Collagen is the most abundant protein in animals. This fibrous, structural protein comprises a right-handed bundle of three parallel, left-handed polyproline II-type helices. Much progress has been made in elucidating the structure of collagen triple helices and the physicochemical basis for their stability. New evidence demonstrates that stereoelectronic effects and preorganization play a key role in that stability. The fibrillar structure of type I collagen—the prototypical collagen fibril—has been revealed in detail. Artificial collagen fibrils that display some properties of natural collagen fibrils are now accessible using chemical synthesis and self-assembly. A rapidly emerging understanding of the mechanical and structural properties of native collagen fibrils will guide further development of artificial collagenous materials for biomedicine and nanotechnology.

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Collagen Structure and Stability

The role of fluorine in drug design and development is expanding rapidly as we learn more about the unique properties associated with this unusual element and how to deploy it with greater sophistication. The judicious introduction of fluorine into a molecule can productively influence conformation, pKa, intrinsic potency, membrane permeability, metabolic pathways, and pharmacokinetic properties. In addition, 18F has been established as a useful positron emitting isotope for use with in vivo imaging technology that potentially has extensive application in drug discovery and development, often limited only by convenient synthetic accessibility to labeled compounds. The wide ranging applications of fluorine in drug design are providing a strong stimulus for the development of new synthetic methodologies that allow more facile access to a wide range of fluorinated compounds. In this review, we provide an update on the effects of the strategic incorporation of fluorine in drug molecules and applications in po...

Applications of Fluorine in Medicinal Chemistry

Introduction Chapter 1 "Unique Properties of Fluorine and Their Relevance to Medicinal Chemistry and Chemical Biology" by Takashi Yamazaki, Takeo Taguchi and Iwao Ojima Medicinal Chemistry Chapter 2 "Fluorinated Prostanoids: Development of Tafluprost, a New Anti-glaucoma Agent" by Yasushi Matsumura Chapter 3 "Fluorinated conformationally restricted glutamate analogs for CNS drug discovery and development" by Atsuro Nakazato Chapter 4 "Fluorinated Inhibitors of Matrix Metalloproteinases" by Roberta Sinisi, Monika Jagodzinska, Gabriele Candiani, Florent Huguenot, Monica Sani, Alessandro Volonterio, Raffaella Maffezzoni and Matteo Zanda Chapter 5 "Fluoro-Taxoid Anticancer Agents" by Antonella Pepe, Larisa Kuznetsova, Liang Sun and Iwao Ojima Chapter 6 "Antimalarial Fluoroartemisinins: Increased Metabolic and Chemical Stability"by Jean-Pierre Begue, Daniele Bonnet-Delpon Chapter 7 "Synthesis and Biological Activity of Fluorinated Nucleosides" by Tokumi Maruyama, Masahiro Ikejiri, Kunisuke Izawa and Tomoyuki Onishi Synthetic methods for medicinal chemistry and chemical biology Chapter 8 "Synthesis of gem -Difluoromethylenated Nucleosides via gem -Difluoromethylene-Containing Building Blocks" by Wei-Dong Meng and Feng-Ling Qing Chapter 9 "Recent Advances in the Syntheses of Fluorinated Amino Acids" by Kenji Uneyama Chapter 10 "Fluorinated Moieties for Replacement of Amide and Peptide Bonds" by Takeo Taguchi and Hikaru Yanai Chapter 11 "Perfluorinated Heteroaromatic Systems as Scaffolds for Drug Discovery" by David Armstrong, Matthew W. Cartwright, Emma L. Parks, Graham Pattison, Graham Sandford, Rachel Slater, John A. Christopher, David D. Miller, Paul W. Smith and Antonio Vong Chapter 12 " gem -Difluorocyclopropanes as key building blocks for novel biologically active molecules" by Toshiyuki Itoh Chapter 13 "Fluorous Mixture Synthesis (FMS) of Drug-Like Molecules and Enantiomers, Stereoisomers, and Analogs of Natural Products" by Wei Zhang Chapter 14 "Fluorine-18 Radiopharmaceuticals" by Michael R. Kilbourn and Xia Shao Applications of fluorinated amino acids and peptides to chemical biology and pharmacology Chapter 15 "Application of Artificial Model Systems to Study the Interactions of Fluorinated Amino Acids within the Native Environment of Coiled Coil Proteins" by Mario Salwiczek, Toni Vagt and Beate Koksch Chapter 16 "Fluorinated Amino Acids and Biomolecules in Protein Design and Chemical Biology" by He Meng, Ginevra A. Clark , and Krishna Kumar Chapter 17 "Effects of Fluorination on the Bioorganic Properties of Methionine" by John F. Honek Chapter 18 "Structure analysis of membrane-active peptides using 19 F-labeled amino acids and solid state NMR" by Parvesh Wadhwani and Erik Strandberg Chapter 19 "Metabolism of Fluorine-containing Drugs using in vivo Magnetic Resonance Spectroscopy" by Erika Schneider Appendix "FDA-Approved Active Pharmaceutical Ingredients Containing Fluorine" by Elizabeth Pollina-Cormier, Manisha Das, and Iwao Ojima

Fluorine in medicinal chemistry and chemical biology

The molecular conformation of the collagen triple helix confers strict amino acid sequence constraints, requiring a (Gly-X-Y)n repeating pattern and a high content of imino acids. The increasing family of collagens and proteins with collagenous domains shows the collagen triple helix to be a basic motif adaptable to a range of proteins and functions. Its rodlike domain has the potential for various modes of self-association and the capacity to bind receptors, other proteins, GAGs, and nucleic acids. High-resolution crystal structures obtained for collagen model peptides confirm the supercoiled triple helix conformation, and provide new information on hydrogen bonding patterns, hydration, sidechain interactions, and ligand binding. For several peptides, the helix twist was found to be sequence dependent, and such variation in helix twist may serve as recognition features or to orient the triple helix for binding. Mutations in the collagen triple-helix domain lead to a variety of human disorders. The most common mutations are single-base substitutions that lead to the replacement of one Gly residue, breaking the Gly-X-Y repeating pattern. A single Gly substitution destabilizes the triple helix through a local disruption in hydrogen bonding and produces a discontinuity in the register of the helix. Molecular information about the collagen triple helix and the effect of mutations will lead to a better understanding of function and pathology.

Molecular structure of the collagen triple helix.

Posttranslational modifications can cause profound changes in protein function. Typically, these modifications are reversible, and thus provide a biochemical on-off switch. In contrast, proline residues are the substrates for an irreversible reaction that is the most common posttranslational modification in humans. This reaction, which is catalyzed by prolyl 4-hydroxylase (P4H), yields (2S,4R)-4-hydroxyproline (Hyp). The protein substrates for P4Hs are diverse. Likewise, the biological consequences of prolyl hydroxylation vary widely, and include altering protein conformation and protein-protein interactions, and enabling further modification. The best known role for Hyp is in stabilizing the collagen triple helix. Hyp is also found in proteins with collagen-like domains, as well as elastin, conotoxins, and argonaute 2. A prolyl hydroxylase domain protein acts on the hypoxia inducible factor alpha, which plays a key role in sensing molecular oxygen, and could act on inhibitory kappaB kinase and RNA polymerase II. P4Hs are not unique to animals, being found in plants and microbes as well. Here, we review the enzymic catalysts of prolyl hydroxylation, along with the chemical and biochemical consequences of this subtle but abundant posttranslational modification.

Prolyl 4-hydroxylase

Collagen-like peptides of the type (Pro-Pro-Gly)10 fold into stable triple helices. An electron-withdrawing substituent at the Hγ3 ring position of the second proline residue stabilizes these triple helices. The aim of this study was to reveal the structural and energetic origins of this effect. The approach was to obtain experimental NMR data on model systems and to use these results to validate computational chemical analyses of these systems. The most striking effects of an electron-withdrawing substituent are on the ring pucker of the substituted proline (Proi) and on the trans/cis ratio of the Xaai-1−Proi peptide bond. NMR experiments demonstrated that N-acetylproline methyl ester (AcProOMe) exists in both the Cγ-endo and Cγ-exo conformations (with the endo conformation slightly preferred), N-acetyl-4(R)-fluoroproline methyl ester (Ac-4R-FlpOMe) exists almost exclusively in the Cγ-exo conformation, and N-acetyl-4(S)-fluoroproline methyl ester (Ac-4S-FlpOMe) exists almost exclusively in the Cγ-endo co...

Collagen stability: insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations

Michael J. Waddell

Papers

Collagen stability: insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations