In order to elucidate the hydrogen-bonding behavior of cross-linked resin-bound peptides in the swollen state, conformational properties of cross-linked polystyrene resin bound oligoleucines were studied by infrared spectroscopy. The conformational analysis of H-Leu"-Phe-resins ( = 3, 6, 9, and 15) swollen in methylene chloride revealed that the peptide chains had various conformations depending on the chain length: = 3, essentially an unordered structure; = 6 and 9, mainly the /3-sheet structure; and = 15, predominantly the -helical structure. The formation of the /3-sheet structure indicates that cross-linked resin-bound peptides in the swollen state can easily interact with each other through intermolecular hydrogen bonds even at a low loading of peptide chains and that the intraresin site separation of the peptide chains is not achieved even when the peptide chain length is not so long. Effects of solvent polarity on conformational transformations of H-Leu"-Phe-resins ( = 3, 6, 9, and 15) were also examined. Cross-linking effects on the conformational properties of cross-linked H-Leu"-Phe-resins ( = 3, 6, 9, and 15) were further investigated by comparison with the conformational properties of soluble H-Leu"-Phe-resins ( = 3, 6, 9, and 20).
ABSTRACT:Cross-linked and soluble polystyrene resin-bound oligo(leucine)s were prepared by the step-by-step coupling of Boc-Leu3 -0H to H-Phe-resins using dicyclohexylcarbodiimide and 1-hydroxy-IH-benzotriazole. The solid state conformational analysis of HfLeu 3 j,Phe-resins (n = 1-5, cross-linked resin; n = 1-3, soluble resin) by infrared spectroscopy indicated that the peptide chains had various conformations depending on the chain length: n =I, essentially an unordered conformation; n = 2--4, mainly the jl-structure; and n = 5, predominantly the a-helix. The formation of the jl-structure indicates that resin-bound peptide chains can easily interact with each other through hydrogen bonds even in polymer networks. The correlation of the conformation in the swollen state with that in the solid state was examined using various solvents. Effects of shear stress on conformational transformations of HfLeu 3 j,Phe-resins (n = 1-3) was also examined.
The influence of both the peptide-chain lengths of Boc-oligopeptides and the degree of crosslinking of polymer supports on the coupling efficiencies of the C-terminal amino acids of Bocoligopeptides in the coupling reactions with terminal free amino groups of amino acids linked to polymer supports was investigated. The coupling efficiencies of Boc-oligopeptides (consisting of 3 to I0 amino acid residues) relative to those of Boc-Val-OH or Boc-Met-OH were determined by competitive coupling reactions with amino acids anchored to soluble and crosslinked polystyrene supports, using four systems of coupling reagents dicyclohexylcarbodiimide (DCC) + I -hydroxy-I H-benzotriazole, DCC + N-hydroxysuccinimide, DCC + N-hydroxy-5norbornene-2,3-dicarboximide, and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. In the coupling reactions on soluble polystyrene supports, containing I mol-Vo of an amino acid (per styrene monomeric unit), and on poly(styrene-co-divinylbenzene ( I Yo)), containing I or I0 mol-Vo of an amino acid, little influence of peptide-chain lengths was observed. In reactions with amino acids anchored to poly(styrene-co-divinylbenzene (2'70)) (containing 2 or 20 mol-Vo of terminal amino groups), an increase in peptide-chain length of Boc-oligopeptides was found to reduce remarkably the coupling efficiencies of their C-terminal amino acids. With poly-(styrene-co-divinylbenzene (270))s as supports, it was detected that amino groups are present which are sterically inaccessible for peptide fragments such as Boc-Ala-Ala-Leu-Leu-Leu-OH and Boc-Ala-Ala-Leu-Leu-Leu-Pro-Pro-Leu-Leu-Leu-OH.
Fragment condensations between amino- and carboxyl-components of hydrophobic decapeptides, which have a β-sheet structure in the solid state and are insoluble in DMF, NMP, and DMSO, were examined in a mixture of CH2Cl2 and TFE (4/1, v/v) using various coupling reagents. By the use of DCC and HOBt as coupling reagents, the reaction proceeded smoothly in moderate yield to give eicosapeptides. These have a stable helical structure in the solid state and are easily soluble in a variety of organic solvents. The hydrophobic eicosapeptides obtained were subjected to a successive coupling reaction in CH2Cl2 alone to give helical tetracontapeptides in high yield, which also have high solubility in various organic solvents of low polarity. The solubility of hydrophobic helical peptides presents a solubility feature of helical peptides which are obtained as peptide intermediates in protein synthesis. The synthetic strategy for helical peptides and proteins is discussed in connection with the solubility prediction method.
A series of small peptides consisting of the repeating sequence Asn‐Pro‐Asn‐Ala, and their Gly‐substituted analogs, having one of the Asn residues replaced by Gly, have been synthesized to investigate the role of Asn side‐chain amido groups in stabilizing the secondary structure of the Asn‐Pro‐Asn‐Ala tetrapeptide unit. Analyses of the 1H NMR data obtained in dimethyl sulfoxide (Me2SO) revealed that the secondary structure of the tetrapeptide sequence is stabilized by virtue of the two Asn residues preceding and following Pro. The sequence‐specific turn structure, which is proposed here as the most probable conformer for the tetrapeptide unit, contains at most three intra‐molecular hydrogen bonds: between backbone CO of Asn preceding Pro and backbone NH of Asn following Pro (inverse γ turn), between side‐chain NHE and backbone CO of the same Asn following Pro, and between side chain CO of Asn preceding Pro and backbone NH of X(Ala).
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