Secondary structure of peptides. 4: ¹³C‐Nmr CP/MAS investigation of solid oligo‐ and poly(L‐alanines)

Abstract

SynopsisPrimary and tertiary amine-initiated polymerizations of L-alanine-N-carboxyanhydride (L-Ala-NCA) were conducted a t 20 or 100°C in a variety of solvents. The 75.5-MHz '3C-nmr CP/MAS spectra of the resulting poly(L-alanines) revealed that all samples contain both &-helix and pleated-sheet structures. Depending on the reaction conditions the a-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the a-helix/@-sheet ratio, indicating that this ratio is thermodynamiccy controlled.Since relatively large amounts of oligopeptides of degree of polymerization (DP) 4-6 can be extracted by means of acetic acid, it is concluded that (a) most poly(L-alanines) possess a bimodal molecular weight distribution, (b) the oligopeptide fraction with 5 11 is responsible for the @-sheet fraction of all samples, and (c) the two-stage crystal growth proposed by Komoto and Kawai is not correct. Solubilizing initiators such as poly(ethy1ene oxide) NH;I prevent the precipitation of oligoalanine and, thus, the formation of a @-sheet structure. 13C-nmr CP/MAS measurements also show that tri-and tetra-L-alanines form insoluble @-sheet structures.

Secondary structure of peptides 16th. Characterization of proteins by means of13C NMR CP/MAS spectroscopy

“…First, in the case of polypeptides the chemical shift differences of peaks representing different periodic secondary structures, such as a-helices, 31 helices or #-sheets, decrease with decreasing difference of the angles ~ and l/J characteristic foI the secondary structures under comparison. So the main chain atoms of poly(L-proline) give identical 13C or 15N NMR signals when the 31 helix and 103 helix are compared [3,20], in agreement with nearly identical ~0 and l/J values. In contrast, the two peaks of CO and Ca signals representing the c~-helix and #-sheet form of polypeptide differ by 4-7 ppm corresponding to a substantial difference of the ~ and ~ pairs [1,6].…”

Determination of α‐helix and β‐sheet stability in the solid state: A solid‐state NMR investigation of poly(L‐alanine)

“…It has been well established that both the polymerization conditions and the initiator used in polymerization determine the length and initial conformation of PLA. 22,35 The results of reprecipitation from DCA, a helixpromoting solvent, 32 are shown in Figure 1. The 13 C CPMAS spectra of commercial 15-kDa PLA before ( Figure 1A) and after ( Figure 1B) reprecipitation from DCA reveal that the ␤-sheet content was greatly reduced and the final conformation was predominantly ␣-helical.…”

“…The 13 C isotropic chemical shift depends on the backbone torsion angles and hydrogen-bonding pattern (carbonyl carbon only), resulting in a strong correlation with secondary structure. 16 -21 13 C CP-MAS of polypeptides was developed by comparing spectra of PLA and other peptide samples with their known secondary structure (obtained from x-ray diffraction), [22][23][24][25] and is a well-established method for quantitative secondary structure determination in solid-state peptide and protein samples. 16,24,26 -28 The conformation of PLA depends on the polymerization conditions used in synthesis, 22,24,25 and the interconversion of ␣-helical and ␤-sheet structure in PLA was demonstrated by stretching or rolling PLA fibers in steam or with a plasticizing agent.…”

“…16 -21 13 C CP-MAS of polypeptides was developed by comparing spectra of PLA and other peptide samples with their known secondary structure (obtained from x-ray diffraction), [22][23][24][25] and is a well-established method for quantitative secondary structure determination in solid-state peptide and protein samples. 16,24,26 -28 The conformation of PLA depends on the polymerization conditions used in synthesis, 22,24,25 and the interconversion of ␣-helical and ␤-sheet structure in PLA was demonstrated by stretching or rolling PLA fibers in steam or with a plasticizing agent. 13,29 In solution NMR, certain solvents are known to induce ␣-helical secondary structure, and this has been established for PLA as well.…”

Simultaneous Chain‐Growth and Step‐Growth Polymerization—A New Route to Cyclic Polymers

“…A remarkable result of these studies was the finding that even in the case of L-alanine NCA almost exclusively cyclic oligo-and polypeptides were formed. Oligo (L-alanines) have high tendency to precipitate from the reaction mixture after reaching a DP of 4 or 5 [21] and in the case of imidazolecatalyzed polymerizations the majority of the reaction products were linear chains hindered by association and precipitation to cyclize. Why gave the solvent-catalyzed polymerizations high yields of cycles?…”