Syntheses and polymerizations of alpha-amino acid N-carboxyanhydrides (NCAs) were reported for the first time by Hermann Leuchs in 1906. Since that time, these cyclic and highly reactive amino acid derivatives were used for stepwise peptide syntheses but mainly for the formation of polypeptides by ring-opening polymerizations. This review summarizes the literature after 1985 and reports on new aspects of the polymerization processes, such as the formation of cyclic polypeptides or novel organometal catalysts. Polypeptides with various architectures, such as diblock, triblock, and multiblock sequences, and star-shaped or dendritic structures are also mentioned. Furthermore, lyotropic and thermotropic liquid-crystalline polypeptides will be discussed and the role of polypeptides as drugs or drug carriers are reviewed. Finally, the hypothetical role of NCAs in molecular evolution on the prebiotic Earth is discussed.

Polypeptides and 100 years of chemistry of alpha-amino acid N-carboxyanhydrides.

Since 1906, when Leuchs synthesized the first R-amino acid N-carboxyanhydrides (NCAs),1 later referred to as Leuchs’ anhydrides, a great number of publications dealing with the ring-opening polymerization (ROP) of these monomers (Scheme 1) has accumulated. This interest stems from the wide variety of polypeptides that this polymerization can generate. The synthetic polypeptides produced from the NCAs, although far from being monodisperse or constructed from a precise sequence and composition of R-amino acid residues, possess the ability, as their natural relative-proteins, to form R-helix and -sheet motifs. These secondary structures contribute significantly to the self-assembling character of polypeptide chains, leading to novel supramolecular structures with potential biomedical and pharmaceutical applications.2 As for their natural counterparts, it is important for such synthetic polypeptides to be well-defined with high molecular and structural homogeneity in order to favor their selfassembly into precisely defined nanostructures, a requirement for appropriate functionality. It was not until 1997, when Deming3 reported the first living initiating system for the ROP of NCAs, that the synthesis of well-defined polypeptides was achieved. Following this first report, other alternative living initiating systems or methods have also been developed. These living systems lead to well-defined homo-/copolypeptides and hybrids, with high molecular weight and structural homogeneity. Nevertheless, the earlier studies served as the springboard for developments in the whole area of polypeptide synthesis. Several excellent reviews4 have been dedicated to the ROP of NCAs, elucidating the mechanistic aspects of this polymerization. However, only a few have addressed the synthesis of polypeptide-based materials with different macromolecular architectures.4c,5,6 This review is divided into three parts. The first highlights the mechanistic developments of the ROP of NCAs from the conventional to the living initiating systems/methods; the second is dedicated to the synthesis of polypeptides and polypeptide hybrids with different macromolecular architectures; and the third deals with surface-bound polypeptides. Surface-bound polypeptides were incorporated in the review due to the great interest in biologically active surfaces for medical diagnostics and sensors.7

Synthesis of Well-Defined Polypeptide-Based Materials via the Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydrides

Interchange reactions involving condensation polymers

Polysarcosine-containing copolymers: Synthesis, characterization, self-assembly, and applications

15N-NMR Spectroscopy—New Methods and Applications [New Analytical Methods (28)]

Ternary copolyamides were obtained either by anionic copolym-erization of ∊-caprolactam, ω-capryllactam, and ω-laurinlactam or by cocondensation of AH-salt with e-caprolactam and ω-laurinlactam. These copolyamides contain nine different amide groups which can be identified by their CO-signals in the 90.5 MHz 13C-NMR spectra. From the intensities of the CO signals, the ratio of monomer units in the copolymers and hence the reactivity of the monomers can be estimated. Furthermore, the average length of the homogeneous blocks can be calculated.

13C-NMR Sequence Analysis. XIV. Ternary Aliphatic Copolyamides

15N NMR spectroscopy, 2. Detection of tacticity in polypeptides†

The natural abundance 15N nmr spectra of linear polysarcosine (DP = 35) has been recorded in Me2SO and H2O solution. Because of cis/trans isomerization at the peptide bond, a broad signal with several splittings was observed. These splittings appear to reflect the influence of three peptide bonds on a single N atom. The 15N signals from the sequence polypeptides (β-Ala-Sar-Gly)n and (β-Ala-Sar-D,L-Ala)n also show a cis/trans splitting, as well as chemical shifts which are dependent on the peptide sequence. The tertiary nitrogen of the sarcosyl residue has a T1 relaxation time which is longer than the T1 for secondary nitrogens of the other amino acids. The nuclear Overhauser effect is also discussed.

15N nmr spectroscopy. I. Polysarcosine and related sequence polypeptides.

Secondary structure of peptides: 19. Primary/secondary structure relationships of leucine/valine copolypeptides prepared from α-amino acid N-carboxyanhydrides

Benzylamine-, triethylamine-, and pyridine-initiated terpolymerizations of alanine N- carboxyanhydride (Ala-NCA), glycine N-carboxyanhydride (Gly-NCA), and valine N-carboxyanhydride (Val-NCA) were conducted in various solvents. The molar compositions of the resulting terpolypeptides were determined from 'H NMR spectra. Their sequences were analyzed by means of 15N NMR spectroscopy, and nearly random sequences were found in all cases. The secondary structure of the solid "as-polymerized samples was characterized by means of 13C NMR CP/MAS spectroscopy. The a-helixlP-sheet ratio was determined for all three amino acid units separately. In contrast to glycine a substantial fraction (up to 60%) of Val units assumes the a-helical conformation. The composition of the secondary structure partially changes upon reprecipitation. In contrast to the random terpolypeptides the two isomeric sequence polypeptides (Ala-Gly-Val), and (Gly-Ala-Val), exclusively possess the &sheet structure. However, in the block copolypeptide (Ala))- (Gly),-(Val), the (Ala)l blocks assume the a-helical conformation, whereas the (Gly), and (Val), blocks exclusively take on the p-sheet structure.

Secondary structure of peptides. 20. Primary-secondary structure relationships of Ala/Gly/Val copolypeptides

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