Design and synthesis of new types of oligonucleopeptides

Design and synthesis of oligonucleopeptides (ONPs),structural analogues of oligonucleotides, where the phosphodiester backbone is substituted by a peptidechain, are described. Oligonucleopeptides, in whichthe number of ordinary bonds between the nucleobasesis six and the number of bonds between the backboneand nucleobase is two or four were constructed usingtwo different approaches. The first way is based onincorporation of thyminylalanine residues into the peptidechain alternatively with glycine residues.Experimental studies of the stability ofoligonucleotide–oligonucleopeptide complexes as wellas model estimations of their potential surfacesindicated the low DNA binding efficiency of this typeof reagents. The second approach consists of synthesis of-ornithine peptides followed by modification of thebackbone with thyminylacetaldehyde attached to an-amino function of ornithine residues through Schiffbases. ONPs were synthesized using the solid-phase method.     

Counting peptide-water hydrogen bonds in unfolded proteins

It is often assumed that the peptide backbone forms a substantial number of additional hydrogen bonds when a protein unfolds. We challenge that assumption in this article. Early surveys of hydrogen bonding in proteins of known structure typically found that most, but not all, backbone polar groups are satisfied, either by intramolecular partners or by water. When the protein is folded, these groups form approximately two hydrogen bonds per peptide unit, one donor or acceptor for each carbonyl oxygen or amide hydrogen, respectively. But when unfolded, the backbone chain is often believed to form three hydrogen bonds per peptide unit, one partner for each oxygen lone pair or amide hydrogen. This assumption is based on the properties of small model compounds, like N-methylacetamide, or simply accepted as self-evident fact. If valid, a chain of N residues would have approximately 2N backbone hydrogen bonds when folded but 3N backbone hydrogen bonds when unfolded, a sufficient difference to overshadow any uncertainties involved in calculating these per-residue averages. Here, we use exhaustive conformational sampling to monitor the number of H-bonds in a statistically adequate population of blocked polyalanyl-six-mers as the solvent quality ranges from good to poor. Solvent quality is represented by a scalar parameter used to Boltzmann-weight the population energy. Recent experimental studies show that a repeating (Gly-Ser) polypeptide undergoes a denaturant-induced expansion accompanied by breaking intramolecular peptide H-bonds. Results from our simulations augment this experimental finding by showing that the number of H-bonds is approximately conserved during such expansion⇋compaction transitions.     

Prediction of Xaa-Pro peptide bond conformation from sequence and chemical shifts

We present a program, named Promega, to predict the Xaa-Pro peptide bond conformation on the basis of backbone chemical shifts and the amino acid sequence. Using a chemical shift database of proteins of known structure together with the PDB-extracted amino acid preference of cis Xaa-Pro peptide bonds, a cis/trans probability score is calculated from the backbone and ¹³C^β chemical shifts of the proline and its neighboring residues. For an arbitrary number of input chemical shifts, which may include Pro-¹³C^γ, Promega calculates the statistical probability that a Xaa-Pro peptide bond is cis. Besides its potential as a validation tool, Promega is particularly useful for studies of larger proteins where Pro-¹³C^γ assignments can be challenging, and for on-going efforts to determine protein structures exclusively on the basis of backbone and ¹³C^β chemical shifts.     

Conformation-Determining role for the N-acetyl group in the O-glycosidic linkage, α-GalNAc-Thr

An ¹H-nmr study of 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-D-galactopyranose (AcGalNAc) glycosylated Thr-containing tripeptides in Me₂SO-d₆ solution reveals two mutually exclusive intramolecular hydrogen bonds. In Z-Thr(AcGalNAc)-Ala-Ala-OMe, there is an intramolecular hydrogen bond between the Thr amide proton and the sugar N-acetyl carbonyl oxygen. The strength of this hydrogen bond will be dependent on the amino acid residues on the Thr C terminal side to some undetermined distance. In Ac-Thr(AcGalNAc)-Ala-Ala-OMe, a different intramolecular hydrogen bond between the sugar N-acetyl amide proton and the Thr carbonyl oxygen exists. The choice of hydrogen bonds seems dependent on the bulkiness of the residues on the Thr N terminal side. The consequence of such strong hydrogen bonds is a clearly defined orientation of the sugar moiety with respect to the peptide backbone. In the former, the plane of the sugar pyranose ring is roughly oriented perpendicularly to the peptide backbone. The latter orientation is where the plane of the sugar ring is roughly in line with the peptide backbone. In both orientations, the sugar moiety can increase the shielding of the neighboring amino acid residues from the solvent. The idea that the amino acid residues near the glycosylated Thr influence orientation of the sugar moiety with respect to the peptide backbone and in turn possibly hinder peptide backbone flexibility has interesting implications in the conformational as well as the biological role of O-glycoproteins.     

Mass spectrometric identification of the trypsin cleavage pathway in lysyl-proline containing oligotuftsin peptides.

Trypsin cleaves specifically peptide bonds at the C-terminal side of lysine and arginine residues, except for -Arg-Pro- and -Lys-Pro- bonds which are normally resistant to proteolysis. Here we report evidence for a -Lys-Pro- tryptic cleavage in modified oligotuftsin derivatives, Ac-[TKPKG]4-NH2) (1), using high-resolution mass spectrometry and HPLC as primary methods for analysis of proteolytic reactions. The proteolytic susceptibility of -Lys-Pro- bonds was strongly dependent on flanking residues, and the flexibility of the peptide backbone might be a prerequisite for this unusual cleavage. While -Lys-Gly- bonds in 1 were rapidly cleaved, the modification of these Lys residues by the attachment of a ss-amyloid(4-10) epitope to yield -Lys(X)-Gly derivatives prevented cleavage of this bond, and provided trypsin cleavage of -Lys-Pro- bonds, the pathway of this degradation being independent on the type of Lys-N(epsilon)-side chains (acetyl group, amino acid, peptide). Substitution of the Lys residues by Ala at the P'2 positions decreased the tryptic cleavage, while replacement of the bulky side chain of Thr at the P2 positions strongly increased the cleavage of -Lys-Pro- bonds. Circular dichroism (CD) data of the modified oligotuftsin derivatives are in accord with enhanced flexibility of the peptide backbone, as a prerequisite for increased susceptibility to cleavage of -Lys-Pro- bonds. These results obtained of oligotuftsin derivatives might have implications for the proteolytic degradation of target peptides that require specific conformational preconditions.     

Synthesis of differentially protected N-acylated reduced pseudodipeptides as building units for backbone cyclic peptides.

Backbone cyclization has become an important method for generating or stabilizing the bioactive conformation of peptides without affecting the amino acid side-chains. Up to now, backbone cyclic peptides were mostly synthesized with bridges between N-amino- and N-carboxy-functionalized peptide bonds. To study the influence of a more flexible backbone on the biological activity, we have developed a new type of backbone cyclization which is achieved via the N-functionalized moieties of acylated reduced peptide bonds. As described in our previous publications, the formation of N-functionalized dipeptide units facilitates the peptide assembly compared with the incorporation of N-alkyl amino acids. Besides the racemization-free synthesis of Fmoc-protected pseudodipeptide esters with reduced peptide bonds, the new type of backbone modification allows the use of a great variety of omega-amino- and alpha,omega-dicarboxylic acids differing in chain length and chemical properties. Best results for the coupling of the omega-amino- and alpha,omega-dicarboxylic acids to the reduced peptide bond were obtained by the formation of mixed anhydrides with alkyl chloroformates. Whereas the protecting group combination of Z/OBzl in the dipeptide unit and Boc/OtBu for the N-functionalized moiety leads to the formation of 2-ketopiperazine during hydrogenation, the combination of Fmoc/OtBu and Alloc/OAll is very suitable for the synthesis of backbone cyclic peptides on solid support.     

NMR characterization of the electrostatic interaction of the basic residues in HDGF and FGF2 during heparin binding

Electrostatic interaction is a major driving force in the binding of proteins to highly acidic glycosaminoglycan, such as heparin. Although NMR backbone chemical shifts have generally been used to identify the heparin-binding site on a protein, however, there is no correlation between the binding free energies and the perturbed backbone chemical shifts for individual residues. The binding event occurs at the end of a side chain of basic residue, and does not require causing significant alterations in the backbone environment at a distance of multiple bonds. We used the H₂CN NMR pulse sequence to detect heparin binding through the side-chain resonances Hε–Cε–Nζ of Lys and Hδ–Cδ–Nε of Arg in the two proteins of hepatoma-derived growth factor (HDGF) and basic fibroblast growth factor (FGF2). H₂CN titration experiments revealed chemical shift perturbations in the side chains, which were correlated with the free energy changes in various mutants. The residues K19 in HDGF and K125 in FGF2 demonstrated the most significant perturbations, consistent with our previous observation that the two residues are crucial for binding. The result suggests that H₂CN NMR provides a precise evaluation for the electrostatic interactions. The discrepancy observed between backbone and side chain chemical shifts is correlated to the solvent accessibility of residues that the K19 and K125 backbones are highly buried with the restricted backbone conformation and are not strongly affected by the events at the end of the side chains.     

Molecular dynamics simulations of a beta-hairpin fragment of protein G: balance between side-chain and backbone forces

How is the native structure encoded in the amino acid sequence? For the traditional backbone centric view, the dominant forces are hydrogen bonds (backbone) and phi-psi propensity. The role of hydrophobicity is non-specific. For the side-chain centric view, the dominant force of protein folding is hydrophobicity. In order to understand the balance between backbone and side-chain forces, we have studied the contributions of three components of a beta-hairpin peptide: turn, backbone hydrogen bonding and side-chain interactions, of a 16-residue fragment of protein G. The peptide folds rapidly and cooperatively to a conformation with a defined secondary structure and a packed hydrophobic cluster of aromatic side-chains. Our strategy is to observe the structural stability of the beta-hairpin under systematic perturbations of the turn region, backbone hydrogen bonds and the hydrophobic core formed by the side-chains, respectively. In our molecular dynamics simulations, the peptides are solvated. with explicit water molecules, and an all-atom force field (CFF91) is used. Starting from the original peptide (G41EWTYDDATKTFTVTE56), we carried out the following MD simulations. (1) unfolding at 350 K; (2) forcing the distance between the C(alpha) atoms of ASP47 and LYS50 to be 8 A; (3) deleting two turn residues (Ala48 and Thr49) to form a beta-sheet complex of two short peptides, GEWTYDD and KTFTVTE; (4) four hydrophobic residues (W43, Y45, F52 and T53) are replaced by a glycine residue step-by-step; and (5) most importantly, four amide hydrogen atoms (T44, D46, T53, and T55, which are crucial for backbone hydrogen bonding), are substituted by fluorine atoms. The fluorination not only makes it impossible to form attractive hydrogen bonding between the two beta-hairpin strands, but also introduces a repulsive force between the two strands due to the negative charges on the fluorine and oxygen atoms. Throughout all simulations, we observe that backbone hydrogen bonds are very sensitive to the perturbations and are easily broken. In contrast, the hydrophobic core survives most perturbations. In the decisive test of fluorination, the fluorinated peptide remains folded under our simulation conditions (5 ns, 278 K). Hydrophobic interactions keep the peptide folded, even with a repulsive force between the beta-strands. Thus, our results strongly support a side-chain centric view for protein folding.     

Stress and strain in staphylococcal nuclease.

Protein molecules generally adopt a tertiary structure in which all backbone and side chain conformations are arranged in local energy minima; however, in several well-refined protein structures examples of locally strained geometries, such as cis peptide bonds, have been observed. Staphylococcal nuclease A contains a single cis peptide bond between residues Lys 116 and Pro 117 within a type VIa beta-turn. Alternative native folded forms of nuclease A have been detected by NMR spectroscopy and attributed to a mixture of cis and trans isomers at the Lys 116-Pro 117 peptide bond. Analyses of nuclease variants K116G and K116A by NMR spectroscopy and X-ray crystallography are reported herein. The structure of K116A is indistinguishable from that of nuclease A, including a cis 116-117 peptide bond (92% populated in solution). The overall fold of K116G is also indistinguishable from nuclease A except in the region of the substitution (residues 112-117), which contains a predominantly trans Gly 116-Pro 117 peptide bond (80% populated in solution). Both Lys and Ala would be prohibited from adopting the backbone conformation of Gly 116 due to steric clashes between the beta-carbon and the surrounding residues. One explanation for these results is that the position of the ends of the residue 112-117 loop only allow trans conformations where the local backbone interactions associated with the phi and psi torsion angles are strained. When the 116-117 peptide bond is cis, less strained backbone conformations are available. Thus the relaxation of the backbone strain intrinsic to the trans conformation compensates for the energetically unfavorable cis X-Pro peptide bond. With the removal of the side chain from residue 116 (K116G), the backbone strain of the trans conformation is reduced to the point that the conformation associated with the cis peptide bond is no longer favorable.     

The dominant interaction between peptide and urea is electrostatic in nature: a molecular dynamics simulation study

The conformational equilibrium of a blocked valine peptide in water and aqueous urea solution is studied using molecular dynamics simulations. Pair correlation functions indicate enhanced concentration of urea near the peptide. Stronger hydrogen bonding of urea-peptide compared to water-peptide is observed with preference for helical conformation. The potential of mean force, computed using umbrella sampling, shows only small differences between urea and water solvation that are difficult to quantify. The changes in solvent structure around the peptide are explained by favorable electrostatic interactions (hydrogen bonds) of urea with the peptide backbone. There is no evidence for significant changes in hydrophobic interactions in the two conformations of the peptide in urea solution. Our simulations suggest that urea denatures proteins by preferentially forming hydrogen bonds to the peptide backbone, reducing the barrier for exposing protein residues to the solvent, and reaching the unfolded state.     

Influence of intramolecular interactions on conformational and dynamic properties of analogs of heptapeptide AFP<Subscript>14–20</Subscript>

Conformational and dynamic properties of proteins and peptides play an important role in their functioning. However, mechanisms that underlie this influence have not been fully elucidated. In the present work we computationally constructed analogs of heptapeptide AFP_14–20 (LDSYQCT) — one of the biologically active sites of human α-fetoprotein (AFP) — to study their conformational and dynamic properties using molecular dynamics simulation. Analogs were obtained by point substitutions of amino acid residues taking into account differences in their physicochemical properties and also on the basis of analysis of amino acid substitutions in the AFP_14–20-like motifs revealed in different physiologically active proteins. It is shown that changes in conformational mobility of amino acid residues of analogs are due to disruption or arising of intramolecular interactions that, in turn, determine existence of steric restrictions during rotation around covalent bonds of the peptide backbone. Substitution of an amino acid by another one with significant difference in physicochemical properties may not lead to remarkable changes in conformational and dynamic properties of the peptide if intramolecular interactions remain unchanged.     

The role of backbone conformation in deltorphin II binding: a QSAR study of new analogues modified in the 5-, 6-positions of the address domain

The delta selectivity of the opioid heptapeptides deltorphin I and II has been attributed to the C-terminal 'address' domain, the hydrophobic Val(5)-Val(6) residues apparently playing a topographical role. We now report the synthesis, opioid binding affinities, and a QSAR study of a series of peptides in which one of the valine side chains was altered. QSAR analyses included previously published models for a binding pocket interaction and an optimum size (Schullery, S.; Mohammedshah, T.; Makhlouf, H.; Marks, E.; Wilenkin, B.; Escobar, S.; Mousigian, C.; Heyl, D. Bioorg. Med. Chem. 1997, 5, 2221), and a new approach for backbone conformational effects using Langevin dynamics simulation (PM3 semi-empirical force field) of an isolated peptide fragment containing the side chain and flanking peptide bonds. No evidence is found of binding pocket interactions or optimum size for either the position-5 or -6 side chain. Rather, delta binding is generally disfavored while mu binding is either unaffected (position-5) or favored (position-6) by larger side chains. The dynamics results provide evidence of similar 'local' conformation roles for the positions 5 and 6 side chains. Specifically, delta binding is favored by side chains that maximize the extension of the backbone, measured as the through-space distance between peptide fragment ends, the angle between lines connecting the alpha-carbon with fragment ends, or the difference between the psi and phi peptide angles.     

A disulfide linked model of the complement protein C8γ complexed with C8α indel peptide

In a recent study C8γ (complement protein) with Cys40Ala substitution and a C8α derived peptide with Cys164Ala substitution were co-crystallized and their binding mode was revealed. Computer modeling and molecular dynamics simulations were performed in order to check the hypothesis that the residues Ala164 of C8α and Ala40 of C8γ occupied the right position if cysteine residues were in their place for disulfide bonding. Substitution of these two alanine residues with cysteine along with disulfide bond creation via molecular modeling and subsequent molecular dynamics simulation of the complex corroborated the hypothesis, which was also confirmed from recent crystallographic data. Average RMSD between backbone atoms of the indel peptide during the MD trajectory in comparison with the corresponding sequence of crystal structure of the C8α/γ complex was found only 0.085 nm. Figure Modeling the C*y/α comlexation. Ribbon representation of the C8y complexed with C8α indel peptide initial (green/cyan) X-ray structure and the final MD conformation (magenta/orange) after imposing the disulfide link. Average RMSD between backbone atoms of the indel peptide during MD trajectory in comparison with the corresponding sequence of crystal structure of the C8α/y complex was found only 0.085nm. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Insights into the structure of the LC13 TCR/HLA-B8-EBV peptide complex with molecular dynamics simulations

One key step in the immune response against infected or tumor cells is the recognition of the T-cell receptor (TCR) by class I major histocompatibility complexes. The complex between the HLA-B8 molecule and the immunodominant peptide with sequence FLRGRAYGL, derived from the Epstein-Barr virus, with the LC13 TCR has been determined by X-ray diffraction. The complex has been used as a starting point in a molecular dynamics study in order to investigate the dynamics of the complex association and to explore the specific interactions of the complex formation. The analyzed structures provided evidence that the peptide adopts an open type β-turn conformation close to C-terminal part, which dominates peptide/TCR interactions. Conformational energy landscape analysis indicated the presence of two conformational clusters in the peptide’s structure, underlying the backbone flexibility of the peptide despite being surrounded by two receptors. The peptide/MHC/TCR interface was found to hold significant number of solvent molecules, more specifically the peptide has been found to have approximately seventeen hydrogen bonds with water molecules. The molecular dynamics simulation indicated the disruption of some MHC/TCR contacts, mainly with the CDR1α loop. However, several other interactions emerged that resulted in a stable association during the 20 ns trajectory, as revealed by the buried surface area analysis.     

Deterministic features of side-chain main-chain hydrogen bonds in globular protein structures

A total of 19 835 polar residues from a data set of 250 non-homologous and highly resolved protein crystal structures were used to identify side-chain main-chain (SC-MC) hydrogen bonds. The ratio of the number of SC-MC hydrogen bonds to the total number of polar residues is close to 1:2, indicating the ubiquitous nature of such hydrogen bonds. Close to 56% of the SC-MC hydrogen bonds are local involving side-chain acceptor/donor ('i') and a main-chain donor/acceptor within the window i-5 to i+5. These short-range hydrogen bonds form well defined conformational motifs characterized by specific combinations of backbone and side-chain torsion angles. (a) The Ser/Thr residues show the greatest preference in forming intra-helical hydrogen bonds between the atoms O(gamma)(i) and O(i-4). More than half the examples of such hydrogen bonds are found at the middle of alpha-helices rather than at their ends. The most favoured motif of these examples is alpha(R)alpha(R)alpha(R)alpha(R)(g(-)). (b) These residues also show great preference to form hydrogen bonds between O(gamma)(i) and O(i-3), which are closely related to the previous type and though intra-helical, these hydrogen bonds are more often found at the C-termini of helices than at the middle. The motif represented by alpha(R)alpha(R)alpha(R)alpha(R)(g(+)) is most preferred in these cases. (c) The Ser, Thr and Glu are the most frequently found residues participating in intra-residue hydrogen bonds (between the side-chain and main-chain of the same residue) which are characterized by specific motifs of the form beta(g(+)) for Ser/Thr residues and alpha(R)(g(-)g(+)t) for Glu/Gln. (d) The side-chain acceptor atoms of Asn/Asp and Ser/Thr residues show high preference to form hydrogen bonds with acceptors two residues ahead in the chain, which are characterized by the motifs beta (tt')alphaR and beta(t)alpha(R), respectively. These hydrogen bonded segments, referred to as Asx turns, are known to provide stability to type I and type I' beta-turns. (e) Ser/Thr residues often form a combination of SC-MC hydrogen bonds, with the side-chain donor hydrogen bonded to the carbonyl oxygen of its own peptide backbone and the side-chain acceptor hydrogen bonded to an amide hydrogen three residues ahead in the sequence. Such motifs are quite often seen at the beginning of alpha-helices, which are characterized by the beta(g(+))alpha(R)alpha(R) motif. A remarkable majority of all these hydrogen bonds are buried from the protein surface, away from the surrounding solvent. This strongly indicates the possibility of side-chains playing the role of the backbone, in the protein interiors, to satisfy the potential hydrogen bonding sites and maintaining the network of hydrogen bonds which is crucial to the structure of the protein.     

Two proposed general configurations for bacterial cell wall peptidoglycans shown by space-filling molecular models

Bacterial cell wall peptidoglycans are built from unbranched β-(1 → 4)-linked glycan chains composed of alternately repeating units of N-acetylglucosamine and N-acetylmuramic acid residues, with peptide side chains attached to the muramic acid residues. The glycan chains are interconnected by peptide bonds formed between the peptide side chains. Through the use of three-dimensional molecular models, two configurations of the glycan strands and the peptide side chains are described, which by their constancy of form reflect the fundamental constancies of the covalent structures. Each of these two models will accommodate any chemical modification that has been observed in bacteria without change in the configuration of the peptide backbone. Some alterations in the chemical structure, which have been sought in bacteria, but not found, would not be tolerated by the models. In these models, glycan strands are parallel, with their lengths and widths predominantly in the plane of the cell wall. The cross-bridging portions of the peptide side chains are at right angles to the glycan strand, in a separate, parallel plane. A compact model is presented in which the peptide side chain is closely appressed to the glycan strand and is stabilized by three hydrogen bonds per disaccharide–peptide subunit. In a second model, the peptide side chain is raised away from the glycan strand in an entirely extended configuration. The compact and extended forms are interconvertible. The thickness of a sheet of peptidoglycan would be from 10.6 to 11.1 Å for the compact model, and 19.1 Å for the extended model.     

Three-dimensional structure of ectatomin from Ectatomma tuberculatum ant venom

Summary Two-dimensional ¹H NMR techniques were used to determine the spatial structure of ectatomin, a toxin from the venom of the ant Ectatomma tuberculatum. Nearly complete proton resonance assignments for two chains of ectatomin (37 and 34 amino acid residues, respectively) were obtained using 2D TOCSY, DQF-COSY and NOESY experiments. The cross-peak volumes in NOESY spectra were used to define the local structure of the protein and generate accurate proton-proton distance constraints employing the MARDIGRAS program. Disulfide bonds were located by analyzing the global fold of ectatomin, calculated with the distance geometry program DIANA. These data, combined with data on the rate of exchange of amide protons with deuterium, were used to obtain a final set of 20 structures by DIANA. These structures were refined by unrestrained energy minimization using the CHARMm program. The resulting rms deviations over 20 structures (excluding the mobile N- and C-termini of each chain) are 0.75 ? for backbone heavy atoms, and 1.25 ? for all heavy atoms. The conformations of the two chains are similar. Each chain consists of two α-helices and a hinge region of four residues; this forms a hairpin structure which is stabilized by disulfide bridges. The hinge regions of the two chains are connected together by a third disulfide bridge. Thus, ectatomin forms a four-α-helical bundle structure.     

Hydrogen bonds between short polar side chains and peptide backbone: prevalence in proteins and effects on helix-forming propensities

A survey of 322 proteins showed that the short polar (SP) side chains of four residues, Thr, Ser, Asp, and Asn, have a very strong tendency to form hydrogen bonds with neighboring backbone amides. Specifically, 32% of Thr, 29% of Ser, 26% of Asp, and 19% of Asn engage in such hydrogen bonds. When an SP residue caps the N terminal of a helix, the contribution to helix stability by a hydrogen bond with the amide of the N3 or N2 residue is well established. When an SP residue is in the middle of a helix, the side chain is unlikely to form hydrogen bonds with neighboring backbone amides for steric and geometric reasons. In essence the SP side chain competes with the backbone carbonyl for the same hydrogen-bonding partner (i.e., the backbone amide) and thus SP residues tend to break backbone carbonyl-amide hydrogen bonds. The proposition that this is the origin for the low propensities of SP residues in the middle of alpha helices (relative to those of nonpolar residues) was tested. The combined effects of restricting side-chain rotamer conformations (documented by Creamer and Rose, Proc Acad Sci USA, 1992;89:5937-5941; Proteins, 1994;19:85-97) and excluding side- chain to backbone hydrogen bonds by the helix were quantitatively analyzed. These were found to correlate strongly with four experimentally determined scales of helix-forming propensities. The correlation coefficients ranged from 0.72 to 0.87, which are comparable to those found for nonpolar residues (for which only the loss of side-chain conformational entropy needs to be considered).     

The synthesis and spectroscopic analysis of the neurotoxic prion peptide 106-126: Comparative use of manual Boc and Fmoc chemistry

Summary A peptide corresponding to residues 106–126 of the human prion protein (PrP) possesses the neurotoxic and amyloidogenic properties of the infectious form of the parental protein. This peptide is now identified as a ‘difficult sequence’ and synthesis using conventional manual Fmoc chemistry was unsuccessful with acylation terminating at a central core of hydrophobic amino acids. The use of tetramethylfluoroformamidinium hexafluorophosphate and 1-methyl-2-pyrrolidone as anti-aggregatory agents in the coupling steps improved the synthesis but still resulted in an incomplete peptide. The incorporation ofN-(2-hydroxy-4-methoxybenzyl)protection at glycine residues 119 and 124 enabled synthesis of the full length peptide in low yield. Synthesis using Boc chemistry within situ neutralisation gave the full length peptide in high yield.     

Major histocompatibility complex class II binding characteristics of peptoid-peptide hybrids

The major histocompatibility complex (MHC) class II binding requirements for solvent-exposed peptide residues were systematically studied using amino acid and peptoid substitutions. In a peptoid residue, the side chain is present on the backbone nitrogen atom as opposed to the alpha-carbon atom in an amino acid residue. To investigate the effect of this side chain shifting on MHC binding, three amino acids in the central part of the peptide sticking out of the binding groove were replaced by corresponding peptoid residues. Two peptoid-peptide hybrids showed large affinity decreases in the MHC-peptide binding assay. To investigate this affinity loss, the individual contributions to MHC binding affinity of the side chain (position), the putative hydrogen bond, and the flexibility were dissected. We conclude that the side chain position as well as the backbone nitrogen atom hydrogen bonding features of solvent-exposed residues in the peptide can be important for MHC binding affinity.