Solid-phase synthesis of triple-helical collagen-model peptides

Summary The triple-helical conformation of collagen has been proposed to be important for mediation of cellular activities, such as adhesion and activation, extracellular matrix assembly, and enzyme function. We have developed synthetic protocols that allow for the study of biological activities of specific collagen sequences in triple-helical conformation. These methods primarily involve solid-phase assembly and covalent linkage of three peptide chains. The resultant triple-helical peptides have sufficient thermal stabilities to permit structural and biological characterization under physiological conditions. The present article critically reviews the various approaches for constructing synthetic triple-helices.This paper is based on a presentation given at the Symposium on Peptide Structure and Design as part of the 31st Annual ACS Western Regional Meeting held in San Diego, CA, USA, October 18–21, 1995.     

Vibrational CD studies of interchain hydrogen-bonded tripodal peptides.

The solution conformations of three trispeptides--L,L,L-1,3,5-C6H3[CH2NHCOCH(X)-NHBoc++ +]3, X = CH3 (Ala) or CH2CH(CH3)2 (Leu), and L,L,L-N(CH2CH2NHCOCH[CH2-CH(CH3)2]NHBoc)3--have been determined from their ir and vibrational CD (VCD) spectra in the NH stretching and carbonyl stretching regions in apolar solution. The compounds containing L-Leu are shown to occur primarily in a propeller conformation with C3 symmetry that is stabilized by interchain hydrogen bonds. Through application of the coupled oscillator model of VCD, a right-handed sense for the hydrogen-bonded chains in the propeller is deduced, in agreement with previous empirical force field calculations. The spectra also provide evidence for interchain association between two chains, resulting in a C10-ring. For chains not involved in interchain association, the spectra reveal the presence of C7-rings within a chain. The trispeptide containing L-Ala is found to occur primarily in a random form.     

Synthesis and CD structural studies of CD52 peptides and glycopeptides

The syntheses of five natural and N-terminal acetylated peptides and glycopeptides of the CD52 antigen are described. Solid phase peptide synthesis was employed in the construction of the target compounds from Fmoc-protected commercial amino acids and synthetic glycan-asparagine conjugates. Circular dichroism studies of the synthetic targets showed that they exist as random coils in solution, and no significant change in secondary structure was observed when the CD52 peptide was either acetylated at the N-terminus or glycosylated at the Asn³ residue with a disaccharide or a fucose-containing branched trisaccharide.     

Structural consequences of D-amino acids in collagen triple-helical peptides

The effects of racemization of aspartic acid on triple-helical formation have been studied using a "host-guest" peptide approach where selected guest Gly-Xaa-Yaa triplets were included within a common acetyl-(Gly-Pro-Hyp)3-Gly-Xaa-Yaa-(Gly-Pro-Hyp)4-Gly-Gly-amide frame-work. Four guest triplets, Gly-Asp-Hyp and Gly-Asp-Ala where Asp is either L-Asp or D-Asp were studied. Thermal stability data indicated that incorporation of D-Asp residues prevented triple-helix formation in phosphate buffered saline, although triple-helical structures were formed in a stabilizing solvent, 67% aqueous ethylene glycol. In this solvent the melting temperatures of D-Asp containing peptides were more than 30 degrees C lower than the corresponding peptides containing L-Asp. For Gly-Asp-Ala peptides, but not Gly-Asp-Hyp, peptides, melting profiles indicated that a mixture of the D- and L-Asp containing peptides were able to form heterotrimer triple-helical molecules. These studies illustrate the dramatic destabilizing effect of D-amino acids on the triple-helix stability, but indicate that they can be accommodated in this conformation.     

Conformational solution studies of neuropeptide gamma using CD and NMR spectroscopy.

Neuropeptide gamma is one of the largest members of the tachykinin family of peptides, exhibiting strong agonistic activity towards the NK-2 tachykinin receptor. This peptide was synthesized by the solid-phase method using the Fmoc chemistry. Circular-dichroism spectroscopy (CD) investigations of this peptide were performed in phosphate buffer, in the presence of sodium dodecylsulphate (SDS) micelles and trifluoroethanol (TFE) solutions and in DMSO-d6 using the 2D NMR technique in conjunction with two different theoretical approaches. The first assumes multiconformational equilibrium of the peptide studied characterized by the values of statistical weights of low-energy conformations. These calculations were performed using three different force fields ECEPP/3, AMBER4.1 and CHARMM (implemented in the X-PLOR program). The second method incorporates interproton distance and dihedral angle constraints into the starting conformation using the Simulated Annealing algorithm (X-PLOR program). The CD experiments revealed that although the peptide studied is flexible in polar solvents, a tendency to adopt a helical structure was observed in the hydrophobic environment. The NMR data (NOE effects) indicate a helical or reverse structure in the Ile7-His12 fragment of the peptide studied in DMSO-d6 solution. The results obtained cannot be interpreted in terms of a single conformation. Most of the conformations obtained with the ECEPP/3 force field possess a high content of a helical structure. None of the conformers, obtained with the AMBER4.1 and CHARMM force fields, can be considered as the dominant one. In all conformations several beta-turns were detected and in some cases gamma-turns were also found. But in fact, it is rather difficult to select the position of the secondary element(s) present in the structure of NPgamma in solution. All conformers calculated with the X-PLOR program (with using NMR derived distance and torsion angle constraints) are stabilized by several beta-turns. Common structural motives are a type IV beta-turn in the Gln6-His12 fragment. All conformations obtained using two approaches adopt very similar turn shapes in the middle region of molecule and a random structure on the N- and C-terminal fragments.     

NMR and CD studies on an oligonucleotide containing N4-methylcytosine.

The hexamer d(CGm4CGCG) exists predominantly as a right handed B form helix at 20 degrees C in 150 mM NaCl, as shown by 2D NOE spectra. Under these conditions a minor species is also observed which corresponds to the single strand in slow exchange on a proton NMR time scale with the double strand. This exchange is unusually slow and separate resonances for the two species are seen up to 65 degrees C. At 50 degrees C the lifetime of the single strand species is 0.85 s. Under high salt conditions the hexamer is partly converted into the Z form, but the complete transition is only observed at 5M NaCl at -6 degrees C.     

Structural insight into the XTACC3/XMAP215 interaction from CD and NMR studies on model peptides

TACC3 is a centrosomal adaptor protein that plays important roles during mitotic spindle assembly. It interacts with chTOG/XMAP215, which catalyzes the addition of tubulin dimers during microtubule growth. A 3D coiled‐coil model for this interaction is available but the structural details are not well described. To characterize this interaction at atomic resolution, we have designed a simplified version of the system based on small peptides. Four different peptides have been studied by circular dichroism and nuclear magnetic resonance both singly and in all possible combinations; namely, five peptide pairs and two trios. In cosolvents, all single peptides tend to adopt helical conformations resembling those of the full‐length protein. However, neither the single peptides nor pairs of peptides form coiled coils. We show that the simultaneous presence of all preformed helices is a prerequisite for binding. The simplest 3D model for the interaction, based on the NMR results, is proposed. Interestingly, the peptide's structure remains unaffected by mutations at essential positions for TACC3 activity. This suggests that the lack of interaction of this TACC3 mutant with XMAP does not correlate with changes in the protein structure and that specific interactions are likely responsible for the interaction and stability of the complex.     

Solid-Phase synthesis and stability of triple-helical peptides incorporating native collagen sequences

A generally applicable solid-phase methodology has been developed for the synthesis of triple-helical polypeptides incorporating native collagen sequences. Three nascent peptide chains are C-terminal linked through one N^α-amino and two N^ε-amino groups of Lys, while repeating Gly-Pro-Hyp triplets induce triple helicity. Different protecting group strategies, including several three-dimensionally orthogonal schemes, have been utilized for the synthesis of four homotrimeric triple-helical polypeptides (THPs) of 79–124 residues, three of which incorporate native type IV collagen sequences. Highly efficient assemblies were achieved by 9-fluorenylmethoxycarbonyl (Fmoc) N^α-amino group protection, in situ 2-(1H-benzotriazole-l-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate mediated couplings, and 1,8-diazabicyclo [5.4.0] undec-7-ene mediated Fmoc group removal. THPs were characterized by Edman degradation sequencing, size-exclusion chromatography, mass spectrometry, reversed-phase high performance liquid chromatography, and CD spectroscopy. THP thermal stabilities ranged from 35 to 59°C, with chain length and Hyp content being the influential factors. Melting temperatures and van't Hoff enthalpies for peptide triple-helical denaturation could be correlated well to Hyp content. The THP synthetic protocol developed here will allow for the study of both structure and biological activity of specific collagen sequences in homotrimeric and heterotrimeric forms. © 1993 John Wiley & Sons, Inc.     

Influence of hydrogen bonding on the rotamer distribution of the histidine side chain in peptides: 1H NMR and CD studies.

Both 1H NMR and circular dichroism pH titration studies on histidine, His-Gly, Gly-His and Gly-His-Gly indicate that the side-chain spatial orientation depends strongly on the vicinal charges. The arrangement of the imidazole side-chain (rotamer population) is shown by the histidine beta and beta' and the glycine methylene proton chemical shifts as well as the vicinal 1H-1H coupling constants 3JCalpha-H-beta-H, beta'-H. For His-Gly and Gly-His-Gly a good correlation can be found between the ionization of the glycine COOH group and the increase of rotamer III (g-g) which is also visualized by circular dichroism through an enhancement of the ellipticity at 212 nm. In these two peptides a hydrogen bond between the imidazolium and the carboxylate group is supposed to stabilize rotamer III at pH 4-5.     

Designed triple-helical peptides as tools for collagen biochemistry and matrix engineering

Collagens, characterized by a unique triple-helical structure, are the predominant component of extracellular matrices (ECMs) existing in all multicellular animals. Collagens not only maintain structural integrity of tissues and organs, but also regulate a number of biological events, including cell attachment, migration and differentiation, tissue regeneration and animal development. The specific functions of collagens are generally triggered by specific interactions of collagen-binding molecules (membrane receptors, soluble factors and other ECM components) with certain structures displayed on the collagen triple helices. Thus, synthetic triple-helical peptides that mimic the structure of native collagens have been used to investigate the individual collagen-protein interactions, as well as collagen structure and stability. The first part of this article illustrates the design of various collagen-mimetic peptides and their recent applications in matrix biology. Collagen is also acknowledged as one of the most promising biomaterials in regenerative medicine and tissue engineering. However, the use of animal-derived collagens in human could put the recipients at risks of pathogen transmission or allergic reactions. Hence, the production of safe artificial collagen surrogates is currently of considerable interest. The latter part of this article reviews recent attempts to develop artificial collagens as novel biomaterials.     

Fluorescence determination of tryptophan side-chain accessibility and dynamics in triple-helical collagen-like peptides

We report tryptophan fluorescence measurements of emission intensity, iodide quenching, and anisotropy that describe the environment and dynamics at X and Y sites in stable collagen-like peptides of sequence (Gly-X-Y)(n). About 90% of tryptophans at both sites have similar solvent exposed fluorescence properties and a lifetime of 8.5-9 ns. Analysis of anisotropy decays using an associative model indicates that these long lifetime populations undergo rapid depolarizing motion with a 0.5 ns correlation time; however, the extent of fast motion at the Y site is considerably less than the essentially unrestricted motion at the X site. About 10% of tryptophans at both sites have a shorter ( approximately 3 ns) lifetime indicating proximity to a protein quenching group; these minor populations are immobile on the peptide surface, depolarizing only by overall trimer rotation. Iodide quenching indicates that tryptophans at the X site are more accessible to solvent. Side chains at X sites are more solvent accessible and considerably more mobile than residues at Y sites and can more readily fluctuate among alternate intermolecular interactions in collagen fibrils. This fluorescence analysis of collagen-like peptides lays a foundation for studies on the structure, dynamics, and function of collagen and of triple-helical junctions in gelatin gels.     

Solution and solid-state NMR structural studies of antimicrobial peptides LPcin-I and LPcin-II

Lactophoricin (LPcin-I) is an antimicrobial, amphiphatic, cationic peptide with 23-amino acid residues isolated from bovine milk. Its analogous peptide, LPcin-II, lacks six N-terminal amino acids compared to LPcin-I. Interestingly, LPcin-II does not display any antimicrobial activity, whereas LPcin-I inhibits the growth of both Gram-negative and Gram-positive bacteria without exhibiting any hemolytic activity. Uniformly ¹⁵N-labeled LPcin peptides were prepared by the recombinant expression of fusion proteins in Escherichia coli, and their properties were characterized by electrospray ionization mass spectrometry, circular dichroism spectroscopy, and antimicrobial activity tests. To understand the structure-activity relationship of these two peptides, they were studied in model membrane environments by a combination of solution and solid-state NMR spectroscopy. We determined the tertiary structure of LPcin-I and LPcin-II in the presence of dodecylphosphorylcholine micelles by solution NMR spectroscopy. Magnetically aligned unflipped bicelle samples were used to investigate the structure and topology of LPcin-I and LPcin-II by solid-state NMR spectroscopy.     

CD and NMR structural characterization of ceratotoxins,natural peptides with antimicrobial activity

Antibacterial properties of the secretion from the female reproductive accessory glands of medfly Ceratitis capitata are mostly ascribed to the presence of two peptides, ceratotoxin A and B, which exhibit a strong activity against gram-positive and gram-negative bacterial strains, and show sequence and function homology with cecropins, melittin, and magainins. CD experiments performed in different solvents indicate the presence of a significant content of helical structures in organic solvent. Two-dimensional nmr results for ceratotoxin A in methanol show a helical behavior for the 8–25 region of the peptide. A Ramachandran classification of each residue for the structures obtained from distance geometry calculations lead to the definition of four structural families in which the central segment 10–19 is always helical and differences refer to residues 8–9 and 19–23. A sequence analysis of the two ceratotoxins and a systematic search on the protein data bank revealed the occurrence of a KX-hydrophobic-hydrophobic-P motif that seems to be important for helix stabilization. © 1996 John Wiley & Sons, Inc.     

NMR and MD studies on the interaction between ligand peptides and alpha-bungarotoxin

The interaction between alpha-bungarotoxin and linear synthetic peptides, mimotope of the nicotinic acetylcholine receptor binding site, has been characterised extensively by several methods and a wealth of functional, kinetic and structural data are available. Hence, this system represents a suitable model to explore in detail the dynamics of a peptide-protein interaction. Here, the solution structure of a new complex of the protein toxin with a tridecapeptide ligand exhibiting high affinity has been determined by NMR. As observed for three other previously reported mimotope-alpha-bungarotoxin complexes, also in this case correlations between biological activity and kinetic data are not fully consistent with a static discussion of structural data. Molecular dynamics simulations of the four mimotope-toxin complexes indicate that a relevant contribution to the complex stability is given by the extent of the residual flexibility that the protein maintains upon peptide binding. This feature, limiting the entropy loss caused by protein folding and binding, ought to be generally considered in a rational design of specific protein ligands.     

Proton NMR studies of partially modified retro–inverso peptides

High-resolution ¹H-nmr analyses are presented for partially modified retro-inverso derivatives of peptides with emphasis on enkephalins. Studies in DMSO-d₆ reveal the unique as well as common characteristics of the non-amino acid residues incorporated into these modified peptides. The complete assignment of 1,1-diaminoalkyl and malonyl, as well as of amino acid, residues provides the basis for the exploration of the conformational features induced in peptides by such topochemical modifications.     

High-yield expression of isotopically labeled peptides for use in NMR studies

Fusion protein constructs of the 56 amino acid globular protein GB-1 with various peptide sequences, coupled with the incorporation of a histidine tag for affinity purification, have generated high-yield fusion protein constructs. Methionine residues were inserted into the constructs to generate pure peptides following CNBr cleavage, yielding a system that is efficient and cost effective for isotopic labeling of peptides for NMR studies and other disciplines such as mass spectroscopy. Six peptides of varying sequences and hydrophobicities were expressed using this GB-1 fusion protein technique and produced soluble fusion protein constructs in all cases. The ability to easily express and purify recombinant peptides in high yields is applicable for biomedical research and has medicinal and pharmaceutical applications.     

NMR studies of flexible peptides in cavities mimicking the synaptic cleft

The interaction of neuropeptides with post-synaptic receptors is characterised by a high entropic barrier originating from the combination of nanomolar concentration with low conformer population. The influence of high viscosity environments on conformer distribution can help overcome this difficulty. In an attempt to simulate the physicochemical conditions of the synaptic cleft, (15)N-labelled enkephalin has been studied in polyacrylamide gels swollen by different aqueous solutions in the temperature range 273-293 K. Nuclear Overhauser enhancement spectra in the gel pores are consistent with a conformational selection or a slowing down of internal motions that can favour the interaction of the peptide with the receptor.     

Structural comparison between oxidized and reduced Escherichia coli thioredoxin. Proton NMR and CD studies

Escherichia coli thioredoxin (Mr 11,700) usually functions as a hydrogen carrier protein that undergoes reversible oxidation/reduction reactions of its active-site disulfide linkage. By use of a number of assigned and identified resonances in one- and two-dimensional 1H NMR spectra, the two forms of the protein have been compared. Only groups that are relatively close to the active-site Cys-32, Cys-35 linkage such as Trp-28, Trp-31, Phe-27, Ala-29, and Val-25 undergo substantial changes in their 1H NMR chemical shift upon reduction. Various residues that are further removed from the active site, like Tyr-49, Tyr-70, His-6, Phe-12, Phe-81, and Phe-102, appear to be little affected (less than 0.02 ppm) by the reduction, suggesting that the rest of the protein structure is not much affected. Thus, the structural changes that occur upon reduction appear to be localized to the disulfide-containing turn and the central strand of the twisted beta-sheet that directly leads to this turn. Notwithstanding the apparent similarity in the secondary and tertiary structures of the oxidized and reduced forms of the protein, the thermal stability of the protein decreases by 10 degrees C upon the reduction of the single disulfide. This was found by both 1H NMR and near- and far-ultraviolet circular dichroism studies. Oxidized thioredoxin was also more resistant to alkaline denaturation. Furthermore, the exchange rate of the relatively stable slow-exchanging backbone amide protons that are part of the core of the twisted five-stranded beta-sheet of thioredoxin increases substantially after reduction.(ABSTRACT TRUNCATED AT 250 WORDS)     

CD and Fourier transform ir spectroscopic studies of peptides. II. Detection of β-turns in linear peptides

Comparative CD and Fourier transform ir (FTIR) spectroscopic data on N-Boc protected linear peptides with or without the (Pro-Gly) β-turn motif (e.g., Boc-Tyr-Pro-Gly-Phe-Leu-OH and Boc-Tyr-Gly-Pro-Phe-Leu-OH) are reported herein. The CD spectra, reflecting both backbone and aromatic contributions, were not found to be characteristic of the presence of β-turns. In the amide I region of the FTIR spectra, analyzed by self-deconvolution and curve-fitting methods, the β-turn band shewed up between 1639 and 1633 cm^?1 in trifluoroethanol (TFE) but only for models containing the (Pro-Gly) core. This band war-also present in the spectra in chloroform but absent in dimethylsulfoxide. These findings, in agreement with recent ir data on cyclic models and 3₁₀-helical polypeptides and protein in D₂O [see S. J. Prestrelski, D. M. Byler, and M. P. Thompson (1991), International Journal of Peptide and Protein Research, Vol. 37, pp. 508–512; H. H. Mantsch, A. Perczel. M. Hollósi, and G. D. Fasman (1992), FASEB Journal, Vol. 6, p. A341; H. H. Mantsch. A. Perczel, M. Hollósi, and G. Fasman (1992), Biopolymers. Vol. 33, pp. 201–207; S. M. Miick, G. V. Martinez, W. R. Fiori, A. P. Tedd, and G. L. Millhauser (1992). Nature, Vol. 359, pp. 653–655], suggest that the amide I band, with a major contribution from the acceptor C ? O of the 1 ← 4 intramolecular H bond of β-turns, appears near or below 1640 cm^?1, rather than above 1660 cm^?1. In TFE, bands between 1670 and 1660 cm^?1 are mainly due to “free” carbonyls, that is, C ? O's of amides that are solvated but not involved in the characteristic H bonds of periodic secondary structures or β-turns. © 1994 John Wiley & Sons, Inc.     

NMR studies of defensin antimicrobial peptides. 2. Three-dimensional structures of rabbit NP-2 and human HNP-1.

The solution structure of two homologous naturally occurring antimicrobial peptides, rabbit defensin NP-2 and human defensin HNP-1, have been determined by two-dimensional nuclear magnetic resonance spectroscopy, distance geometry, and restrained molecular dynamics calculations. The structure of these defensins consists of an antiparallel beta-sheet in a hairpin conformation, a short region of triple-stranded beta-sheet, several tight turns, and a loop region that has a well-defined local structure but with a global orientation that is not well-defined with respect to the rest of the molecule. The solution structures of these two peptides are compared with the solution and crystal structures of two other homologous defensins. The structures for the defensins are also compared with known structures of other naturally occurring antimicrobial peptides.