Utility of Triethyloxonium Tetrafluoroborate for Chloride Removal during Sarcosine N-Carboxyanhydride Synthesis: Improving NCA Purity

The clinical translation of polysarcosine (pSar) as polyethylene glycol (PEG) replacement in the development of novel nanomedicines creates a broad demand of polymeric material in high-quality making high-purity sarcosine N-carboxyanhydride (Sar-NCA) as monomer for its production inevitable. Within this report, we present the use of triethyloxonium tetrafluoroborate in Sar-NCA synthesis with focus on amino acid and chloride impurities to avoid the sublimation of Sar-NCAs. With a view towards upscaling into kilogram or ton scale, a new methodology of monomer purification is introduced by utilizing the Meerwein's Salt triethyloxonium tetrafluoroborate to remove chloride impurities by covalent binding and converting chloride ions into volatile products within a single step. The novel straightforward technique enables access to monomers with significantly reduced chloride content (<100 ppm) compared to Sar-NCA derived by synthesis or sublimation. The derived monomers enable the controlled-living polymerization in DMF and provide access to pSar polymers with Poisson-like molecular weight distribution within a high range of chain lengths (Xn 25-200). In conclusion, the reported method can be easily applied to Sar-NCA synthesis or purification of commercially available pSar-NCAs and eases access to well-defined hetero-telechelic pSar polymers.

Recent progress on polySarcosine as an alternative to PEGylation: Synthesis and biomedical applications

Biotherapeutic PEGylation to prolong action of medications has gained popularity over the last decades. Various hydrophilic natural polymers have been developed to tackle the drawbacks of PEGylation, such as its accelerated blood clearance and non-biodegradability. Polypeptoides, such as polysarcosine (pSar), have been explored as hydrophilic substitutes for PEG. pSar has PEG-like physicochemical characteristics such as water solubility and no reported cytotoxicity and immunogenicity. This review discusses pSar derivatives, synthesis, characterization approaches, biomedical applications, in addition to the challenges and future perspectives of pSar based biomaterials as an alternative to PEG.

The structure of synthetic polypeptides III. The molecular configurations and physical properties of some regenerated synthetic polypeptides

An examination of some physical properties of the α ll and β forms of three synthetic polypeptides (the 1:1 copolymer of DL- β -phenylalanine and DL-leucine, the 1:1 copolymer of DL- β -phenylalanine and L-leucine, and the 1:1:1 copolymer of DL- β -phenylalanine, DL-leucine, and γ -benzyl-DL-glutamate) has been carried out. The two modifications were found to exhibit strikingly different solubilities in non-polar liquids. This result is in agreement with the structures suggested in part I for the α ll and β forms, in which the peptide hydrogen bonds are intra- and inter-chain respectively. The results of a study of the regeneration of the polypeptides from carboxylic acid solutions under various conditions have enabled us to elaborate the mechanism of the action of formic acid given in part I. Further, it has been found possible to effect an α ll → β change solely in the amorphous region of the polymer. The β forms of the two polypeptides have been found to be unstable at high temperatures and to revert to the α ll . The α ll is stable at temperatures up to 280°C, and may readily be obtained in a highly crystalline and oriented form by a suitable heat treatment. X-ray photographs of such specimens show resolution of the polar arcs at 5·2 to 5·3 Å, indicating that the unit cells may have mono- or triclinic symmetry.

Studies in polymerization, VIII. Reactions of N -carboxy-α-amino acid anhydrides initiated by metal cations

In the course of an investigation of the reactions of N -carboxy-α-amino-acid anhydrides in polar media (mainly NN -dimethyl formamide solution), it has been found that soluble metallic salts can initiate reactions, including polymerization. This appears to result from the displacement of protons from such organic molecules by metal cations under suitable conditions. The cation forms a complex with the N -carbonic anhydride, which then loses a proton from the —NH— group if this is present. The nitrogen atom in the ring is thus made sufficiently basic to react with another molecule of the carbonic anhydride (see equation (2)). Simple reactions, summarized in equations (3) and (4), then lead to the observed products, cyclic polypeptides and substituted hydantoin 3-acetic acids. A detailed study of the productsobtained from glycine N -carbonic anhydride has been made, and pure cyclic hexaglycine has been isolated in the crystalline state. If the N -carbonic anhydride is N -substituted these reactions cannot occur. A similar kind of initiation can, however, be brought about by the addition of 3-methyl hydantoin. This substance resembles glycine N -carbonic anhydride structurally, and it behaves similarly with metal cations (see equation (5)). The ensuing reaction in the case of sarcosine N-carbonic anhydride leads to a linear polymer, with terminal 3-methyl hydantoin residues and —NH Me groups. The stability of the hydantoin ring precludes the possibility of cyclization reactions. A detailed kinetic study of this reaction of sarcosine N -carbonic anhydride supports the mechanism summarized in equations (13a to ƒ). The molecular weight distributions obtained from polymerizations of this kind have been deduced. The bearing of these results on the spontaneous polymerization of the N -carbonic anhydrides in polar media is discussed briefly.

The structure of synthetic polypeptides I. X-ray investigation

Oriented fibres and films of a number of synthetic polypeptides have been prepared, and examined by X-ray diffraction. It is shown that the polypeptides can exist in a folded (α) and an extended (β) configuration. The observations on the α form are consistent with the presence of a twofold screw axis in symmetrical polypeptides, and lead to a value of about 5.5 Ǻ for the true fibre repeat distance. The fibre period in the copolymers is shown to be greater than 5.26 Ǻ. It is considered that the X-ray evidence favours a ribbon-like molecule, in agreement with the structure shown in I. The β forms of the polypeptides give X-ray diagrams similar to those of β proteins, and probably consist of extended polypeptide chains. It is shown that the polypeptides can undergo an α→β transformation similar to that encountered in the fibrous α proteins. The medium from which the peptide is regenerated has a very important influence in determining which form is obtained. Formic acid appears to be unique in producing the β modification of polypeptides which swell or dissolve in this liquid. A mechanism for this transformation is suggested. A number of copolymers containing glycine have been examined. These appear to exist mainly in the β form. The similarities in the behaviour of synthetic polypeptides and fibrous proteins are discussed.