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Biondi B, Formaggio F, Toniolo C, Peggion C, Crisma M. Isolated α-turns in peptides: a selected literature survey. J Pept Sci 2023:e3476. [PMID: 36603599 DOI: 10.1002/psc.3476] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2022] [Revised: 12/21/2022] [Accepted: 12/26/2022] [Indexed: 01/07/2023]
Abstract
The results of classifying into various types the 68 examples of isolated α-turns in the X-ray diffraction crystal structures of peptides documented in the literature are presented and discussed in this review article. α-Turns characterized by the trans disposition of all ω torsion angles are common for the backbone linear peptides investigated. In contrast, the cis arrangement of the N-terminal (ωi + 1 ) torsion angle, among those generated by the three residues internal to the α-turn, is a peculiar feature of 65% of the cyclic peptides. Among linear and cyclic peptides featuring the all-trans disposition of the ω torsion angles, only one third of the α-turns display φ,ψ values not too far from those characterizing regular α-helices. In general, our findings, taken together, suggest that a significant conformational diversity is compatible with the formation of an intramolecularly H-bonded C13 -member pseudocycle (α-turn) in linear and cyclic peptides.
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2
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Santi S, Biondi B, Cardena R, Bisello A, Schiesari R, Tomelleri S, Crisma M, Formaggio F. Helical versus Flat Bis-Ferrocenyl End-Capped Peptides: The Influence of the Molecular Skeleton on Redox Properties. Molecules 2022; 27:6128. [PMID: 36144860 PMCID: PMC9503075 DOI: 10.3390/molecules27186128] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2022] [Revised: 09/07/2022] [Accepted: 09/15/2022] [Indexed: 11/30/2022] Open
Abstract
Despite the fact that peptide conjugates with a pendant ferrocenyl (Fc) have been widely investigated, bis-ferrocenyl end-capped peptides are rarely synthetized. In this paper, in addition to the full characterization of the Fc-CO-[L-Dap(Boc)]n-NH-Fc series, we report a comparison of the three series of bis-ferrocenyl homopeptides synthesized to date, to gain insights into the influence of α-amino isobutyric (Aib), 2,3-diamino propionic (Dap) and Cα,β-didehydroalanine (ΔAla) amino acids on the peptide secondary structure and on the ferrocene redox properties. The results obtained by 2D NMR analysis and X-ray crystal structures, and further supported by electrochemical data, evidence different behaviors depending on the nature of the amino acid; that is, the formation of 310-helices or fully extended (2.05-helix) structures. In these foldamers, the orientation of the carbonyl groups in the peptide helix yields a macrodipole with the positive pole on the N-terminal amino acid and the negative pole on the C-terminal amino acid, so that oxidation of the Fc moieties takes place more or less easily depending on the orientation of the macrodipole moment as the peptide chain grows. Conversely, the fully extended conformation adopted by ΔAla flat peptides neither generates a macrodipole nor affects Fc oxidation. The utilization as electrochemical and optical (Circular Dichroism) probes of the two terminal Fc groups, bound to the same peptide chain, makes it possible to study the end-to-end effects of the positive charges produced by single and double oxidations, and to evidence the presence "exciton-coupled" CD among the two intramolecularly interacting Fc groups of the L-Dap(Boc) series.
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Santi S, Bisello A, Cardena R, Tomelleri S, Schiesari R, Biondi B, Crisma M, Formaggio F. Flat, C α,β -Didehydroalanine Foldamers with Ferrocene Pendants: Assessing the Role of α-Peptide Dipolar Moments. Chempluschem 2021; 86:723-730. [PMID: 33825347 DOI: 10.1002/cplu.202100072] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2021] [Revised: 03/22/2021] [Indexed: 12/28/2022]
Abstract
The foldamer field is continuously expanding as it allows to produce molecules endowed with 3D-structures and functions never observed in nature. We synthesized flat foldamers based on the natural, but non-coded, Cα,β -didehydroalanine α-amino acid, and covalently linked to them two ferrocene (Fc) moieties, as redox probes. These conjugates retain the flat and extended conformation of the 2.05 -helix, both in solution and in the crystal state (X-ray diffraction). Cyclic voltammetry measurements agree with the adoption of the 2.05 -helix, characterized by a negligible dipole moment. Thus, elongated α-peptide stretches of this type are insulators rather than charge conductors, the latter being constituted by peptide α-helices. Also, our homo-tetrapeptide has a N-to-C length of about 18.2 Å, almost double than that (9.7 Å) of an α-helical α-tetrapeptide.
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Marafon G, Crisma M, Masato A, Plotegher N, Bubacco L, Moretto A. Photoresponsive Prion‐Mimic Foldamer to Induce Controlled Protein Aggregation. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202012995] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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5
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Marafon G, Crisma M, Masato A, Plotegher N, Bubacco L, Moretto A. Photoresponsive Prion-Mimic Foldamer to Induce Controlled Protein Aggregation. Angew Chem Int Ed Engl 2021; 60:5173-5178. [PMID: 33180342 DOI: 10.1002/anie.202012995] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2020] [Indexed: 12/30/2022]
Abstract
Proteins reconfigure their 3D-structure, and consequently their function, under the control of specific molecular interactions that sense, process and transmit information from the surrounding environment. When this fundamental process is hampered, many pathologies occur as in the case of protein misfolding diseases. In this work, we follow the early steps of α-synuclein (aS) aggregation, a process associated with Parkinson's disease etiopathogenesis, that is promptly promoted by a light-mediated binding between the protein and a photoactive foldamer. The latter can switch between two conformations, one of which generates supramolecular fibrillar seeds that act as molecular templates able to induce a fast β-sheet transition for aS monomers that successively undergo fibrillar polymerization. The proposed method represents a powerful tool to study protein aggregation relevant to misfolding diseases in a controlled and inducible system.
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De Zotti M, Corvi G, Gatto E, Di Napoli B, Mazzuca C, Palleschi A, Placidi E, Biondi B, Crisma M, Formaggio F, Toniolo C, Venanzi M. Controlling the Formation of Peptide Films: Fully Developed Helical Peptides are Required to Obtain a Homogenous Coating over a Large Area. Chempluschem 2020; 84:1688-1696. [PMID: 31943881 DOI: 10.1002/cplu.201900456] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2019] [Revised: 10/18/2019] [Indexed: 01/17/2023]
Abstract
The influence of conformational dynamics on the self-assembly process of a conformationally constrained analogue of the natural antimicrobial peptide Trichogin GA IV was analysed by spectroscopic methods, microscopy imaging at nanometre resolution, and molecular dynamics simulations. The formation of peptide films at the air/water interface and their deposition on a graphite or a mica substrate were investigated. A combination of experimental evidence with molecular dynamics simulation was used to demonstrate that only the fully developed helical structure of the analogue promotes formation of ordered aggregates that nucleate the growth of micrometric rods, which give rise to homogenous coating over wide regions of the hydrophilic mica. This work proves the influence of helix flexibility on peptide self-organization and orientation on surfaces, key steps in the design of bioinspired organic/inorganic hybrid materials.
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Toniolo C, Moretto A, Formaggio F, Broxterman QB, Crisma M. From Amherst (Massachusetts, USA) to Padua (Italy) and back again: Louis A. Carpino's scientifically productive journey. Pept Sci (Hoboken) 2020. [DOI: 10.1002/pep2.24153] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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8
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Marafon G, Moretto A, Zanuy D, Alemán C, Crisma M, Toniolo C. Effect on the Conformation of a Terminally Blocked, ( E) β,γ-Unsaturated δ-Amino Acid Residue Induced by Carbon Methylation. J Org Chem 2020; 85:1513-1524. [PMID: 31769989 DOI: 10.1021/acs.joc.9b02544] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Peptides are well-known to play a fundamental therapeutic role and to represent building blocks for numerous useful biomaterials. Stabilizing their active 3D-structure by appropriate modifications remains, however, a challenge. In this study, we have expanded the available literature information on the conformational propensities of a promising backbone change of a terminally blocked δ-amino acid residue, a dipeptide mimic, by replacing its central amide moiety with an (E) Cβ═Cγ alkene unit. Specifically, we have examined by DFT calculations, X-ray diffraction in the crystalline state, and FT-IR absorption/NMR spectroscopies in solution the extended vs folded preferences of analogues of this prototype system either unmodified or possessing single or multiple methyl group substituents on each of its four -CH2-CH═CH-CH2- main-chain carbon atoms. The theoretical and experimental results obtained clearly point to the conclusion that increasing the number of adequately positioned methylations will enhance the preference of the original sequence to fold, thus opening interesting perspectives in the design of conformationally constrained peptidomimetics.
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Mir FM, Crisma M, Toniolo C, Lubell WD. Isolated α-turn and incipient γ-helix. Chem Sci 2019; 10:6908-6914. [PMID: 31391913 PMCID: PMC6640192 DOI: 10.1039/c9sc01683j] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2019] [Accepted: 06/07/2019] [Indexed: 12/17/2022] Open
Abstract
The unique abilities of homo-oligo-adamantyl peptides to adopt α- and γ-turn conformations are demonstrated by X-ray diffraction, and NMR and FT-IR absorption spectroscopies. Assembled by an Ugi multiple component reaction strategy, N α-formyl-adamantyl tripeptide iso-propyl and tert-butyl amides are respectively found to adopt an isolated α-turn and an incipient γ-helix conformation by X-ray diffraction crystallography. The shortest example of a single α-turn with ideal geometry is observed in the crystalline state. In solution both peptides predominantly assume γ-helical structures.
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Marafon G, Crisma M, Moretto A. Tunable E- Z Photoisomerization in α,β-Peptide Foldamers Featuring Multiple ( E/ Z)-3-Aminoprop-2-enoic Acid Units. Org Lett 2019; 21:4182-4186. [PMID: 31090420 DOI: 10.1021/acs.orglett.9b01360] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Systems in which an external stimulus elicits a response through some sort of modification at the molecular or supramolecular level bear potential for the development of smart materials and devices. This work describes a versatile synthetic approach suitable for the stepwise incorporation of multiple, even consecutive, units of the simplest Cα,β-unsaturated β-amino acid, ( E/ Z)-3-aminoprop-2-enoic acid, in peptide-based foldamers. The properties of these, including photoinduced E/ Z isomerizations, were investigated.
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Mir FM, Crisma M, Toniolo C, Lubell WD. Influence of the C‐terminal substituent on the crystal‐state conformation of Adm peptides. Pept Sci (Hoboken) 2019. [DOI: 10.1002/pep2.24121] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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12
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Drouillat B, Peggion C, Biondi B, Wright K, Couty F, Crisma M, Formaggio F, Toniolo C. Heterochiral Ala/(
αMe)Aze
sequential oligopeptides:
S
ynthesis and conformational study. J Pept Sci 2019; 25:e3165. [DOI: 10.1002/psc.3165] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2018] [Revised: 02/18/2019] [Accepted: 02/19/2019] [Indexed: 12/31/2022]
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13
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Drouillat B, Peggion C, Biondi B, Wright K, Couty F, Crisma M, Formaggio F, Toniolo C. A novel peptide conformation: the γ-bend ribbon. Org Biomol Chem 2018; 16:7947-7958. [PMID: 30318540 DOI: 10.1039/c8ob02279h] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Unlike the extensively investigated relationship between the peptide β-bend ribbon and its prototypical 310-helix conformation, the corresponding relationship between the narrower γ-bend ribbon and its regular γ-helix counterpart still remains to be studied, as the latter 3D-structures have not yet been experimentally authenticated. In this paper, we describe the results of the first characterization, both in the crystal state and in solution, of the γ-bend ribbon conformation using X-ray diffraction and FT-IR absorption, electronic CD and 2D-NMR spectroscopies applied to an appropriate set of synthetic, homo-chiral, sequential dipeptide oligomers based on (S)-Ala and the known γ-bend inducer, Cα-tetrasubstituted, N-alkylated α-amino acid residue (S)-Cα-methyl-azetidine-carboxylic acid.
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Marafon G, Crisma M, Moretto A. Intrinsically Photoswitchable α/β Peptides toward Two-State Foldamers. Angew Chem Int Ed Engl 2018; 57:10217-10220. [DOI: 10.1002/anie.201806035] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2018] [Indexed: 12/21/2022]
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15
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Marafon G, Crisma M, Moretto A. Intrinsically Photoswitchable α/β Peptides toward Two-State Foldamers. Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201806035] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
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16
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Boscutti G, Nardon C, Marchiò L, Crisma M, Biondi B, Dalzoppo D, Dalla Via L, Formaggio F, Casini A, Fregona D. Anticancer Gold(III) Peptidomimetics: From Synthesis to in vitro and ex vivo Biological Evaluations. ChemMedChem 2018; 13:1131-1145. [PMID: 29570944 DOI: 10.1002/cmdc.201800098] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2018] [Revised: 03/14/2018] [Indexed: 12/25/2022]
Abstract
Five new AuIII -peptidodithiocarbamato complexes of the type [AuIII Br2 (dtc-AA1 -AA2 -OR] (in which AA1 =N-methylglycine (Sar), l/d-Pro; AA2 =l/d-Ala, α-aminoisobutyric acid (Aib); R=OtBu, triethylene glycol methyl ether), differing with regard to the amino acid sequence and/or the chiral amino acid configuration, were designed to enhance tumor selectivity and bioavailability. The gold(III)-based moiety was functionalized to exploit the targeting properties of the peptidomimetic ligand toward two peptide transporters (namely PEPT1 and PEPT2), which are upregulated in several tumor cells. The compounds were synthesized and fully characterized, mainly by means of elemental analysis, one- and two-dimensional NMR spectroscopy, FT-IR, and UV/Vis spectrophotometry. The crystal structures of three compounds were also solved by X-ray diffraction. In vitro cytotoxicity studies using a panel of human tumor cell lines (A549 [non-small-cell lung carcinoma], MCF-7 [breast cancer], A2780 [ovarian carcinoma], H1975 [non-small-cell lung carcinoma], H460 [large-cell lung carcinoma], and A431 [human epidermoid carcinoma]) showed the dtc-Pro-Aib-OtBu derivative to be very effective, with GI50 values much lower than those of cisplatin. This complex was thus selected for evaluating stability under physiological conditions and possible interactions with serum albumin, as well in PARP-1 enzyme inhibition assays and preliminary ex vivo toxicity experiments on healthy rat tissues.
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Crisma M, Formaggio F, Alemán C, Torras J, Ramakrishnan C, Kalmankar N, Balaram P, Toniolo C. The fully‐extended conformation in peptides and proteins. Pept Sci (Hoboken) 2018. [DOI: 10.1002/bip.23100] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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18
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Aricò F, Aldoshin AS, Musolino M, Crisma M, Tundo P. β-Aminocarbonates in Regioselective and Ring Expansion Reactions. J Org Chem 2017; 83:236-243. [PMID: 29172495 DOI: 10.1021/acs.joc.7b02609] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
The reactivity of β-aminocarbonates as anisotropic electrophiles has been investigated with several phenols. Products distribution shows that the regioselectivity of the anchimerically driven alkylation reaction depends on the nucleophiles. The results suggest that in the presence of nucleophiles that are also good leaving groups, the reaction takes place under thermodynamic control favoring the attack on the most sterically hindered carbon of the cyclic aziridinium intermediate. Furthermore, when an enantiomerically pure pyrrolidine-based carbonate was used, the reaction with phenols proceeds via a bicyclic aziridinium intermediate leading to the stereoselective synthesis of optically active 3-substituted piperidines via ring expansion reaction. These results were confirmed both by NMR spectroscopy and X-ray diffraction analysis.
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Marafon G, Menegazzo I, De Zotti M, Crisma M, Toniolo C, Moretto A. Tuning morphological architectures generated through living supramolecular assembly of a helical foldamer end-capped with two complementary nucleobases. SOFT MATTER 2017; 13:4231-4240. [PMID: 28509927 DOI: 10.1039/c7sm00764g] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Two appropriately functionalized nucleobases, thymine and adenine, have been covalently linked at the N- and C-termini, respectively, of two α-aminoisobutyric acid-rich helical peptide foldamers, aiming at driving self-assembly through complementary recognition. A crystal-state analysis (by X-ray diffraction) on the shorter, achiral foldamer 1 unambiguously shows that adeninethymine base pairing, through Watson-Crick intermolecular H-bonding, does take place between either end of each peptide molecule. In the crystals, π-stacking between base pairs is also observed. Evidence for time-dependent foldameroldamer associations for the longer, chiral foldamer 2 in solution is provided by circular dichroism measurements. The self-assembly of foldamer 2, through living supramolecular polymerization, eventually leads to the formation of twisted fibers. Such a supramolecular organization can be affected by addition of either pristine adenine or thymine, that acts as a "terminator" by selectively matching a pairing nucleobase at one end of the foldamer. The co-assembly of foldamer 2 with a porphyrin-derivatized thymine, under appropriate experimental conditions, leads to the formation of vesicles which, in turn, can be converted to the fiber morphology by changing the environmental polarity. Conversely, dendrimeric, star polymer-like microstructures are generated when the supramolecular assembly of foldamer 2 is seeded by adenine-capped gold nanoparticles.
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Bisello A, Cardena R, Rossi S, Crisma M, Formaggio F, Santi S. Hydrogen-Bond-Assisted, Concentration-Dependent Molecular Dimerization of Ferrocenyl Hydantoins. Organometallics 2017. [DOI: 10.1021/acs.organomet.7b00248] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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21
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Toniolo C, Crisma M, Formaggio F, Alemán C, Ramakrishnan C, Kalmankar N, Balaram P. Intramolecular backbone···backbone hydrogen bonds in polypeptide conformations. The other way around: ɛ-turn. Biopolymers 2017; 108. [DOI: 10.1002/bip.22911] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2016] [Revised: 06/17/2016] [Accepted: 06/29/2016] [Indexed: 01/27/2023]
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22
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Mazzier D, Grassi L, Moretto A, Alemán C, Formaggio F, Toniolo C, Crisma M. En route towards the peptideγ-helix: X-ray diffraction analyses and conformational energy calculations of Adm-rich short peptides. J Pept Sci 2016; 23:346-362. [DOI: 10.1002/psc.2957] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2016] [Revised: 11/28/2016] [Accepted: 11/29/2016] [Indexed: 12/16/2022]
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23
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De Zotti M, Peggion C, Biondi B, Crisma M, Formaggio F, Toniolo C. Endothioxopeptides: A conformational overview. Biopolymers 2016; 106:697-713. [DOI: 10.1002/bip.22899] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2016] [Revised: 06/08/2016] [Accepted: 06/25/2016] [Indexed: 12/16/2022]
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24
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Mazzier D, Crisma M, De Poli M, Marafon G, Peggion C, Clayden J, Moretto A. Helical Foldamers Incorporating Photoswitchable Residues for Light-Mediated Modulation of Conformational Preference. J Am Chem Soc 2016; 138:8007-18. [DOI: 10.1021/jacs.6b04435] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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25
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Crisma M, Peggion C, Moretto A, Banerjee R, Supakar S, Formaggio F, Toniolo C. The 2.0₅-helix in hetero-oligopeptides entirely composed of C(α,α)-disubstituted glycines with both side chains longer than methyls. Biopolymers 2016; 102:145-58. [PMID: 24307568 DOI: 10.1002/bip.22450] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2013] [Revised: 11/15/2013] [Accepted: 12/02/2013] [Indexed: 02/02/2023]
Abstract
The existence of the very uncommon, but potentially quite interesting, multiple, consecutive fully-extended conformation (2.0₅-helix) has been already clearly demonstrated in homo-oligopeptides based on quaternary α-amino acids with both side chains longer than methyls, but not cyclized on the α-carbon atom. To extend the scope of this research, in this work we investigated the occurrence of this flat 3D-structure in hetero-oligopeptides, each composed of two or three different residues of that class. The synthesis of a terminally protected peptide series to the tetrapeptide level was carried out by solution methods. The resulting oligomers were chemically and conformationally characterized. The data obtained point to an overwhelming population of the fully-extended conformation in CDCl3. However, a solvent-driven switch to a predominant 3₁₀-helical structure was seen in CD3CN. A delicate, local balance between these two conformations is confirmed to occur in the crystalline state. Molecular dynamics simulations in CHCl3 on a hetero-tetrapeptide converged to the fully-extended conformation even starting from the 3₁₀-helical structure.
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