Poly-L-a-amino acids

The extension of the term isotactic to condensation polymers was made by Natta in his first article discussing poly-L-a-amino acids (22). In itself the term isotactic is redundant here as the configuration of the repeating unit is sufficient to identify the macromolecular stmcture. It is, however, useful to distinguish a system, racemic or not, in which each macromolecule is composed of only l or only D residues from a mixture of macromolecules made up of random or alternate sequences of L or d units. Similarly the term syndiotactic serves in the identification of oligopeptides or polypeptides composed of alternate sequences of D and L units, like those synthesized by Lorenzi and Tomasic (77). 

In principle, polymers consisting of a single kind of chiral monomeric unit can produce left- and right-handed helices. But the two kinds of helix are diastereomeric to each other that is, they are not energetically equal. For this reason, either left or right-handedness is a preferred form for such polymers. For example, polymers of chiral (S)-a-olefins and most poly(D-saccharides) form exclusively left-handed helices. On the other hand, deoxyribonucleic acids and almost all poly(L-a-amino acids) occur as right-handed helices. Polymers of the corresponding monomer antipodes form helices of opposite turn. 

Polymers with a single kind of chiral monomeric unit are always optically active, that is, they rotate the plane of polarized light. In general, the effect of end groups disappears for degrees of polymerization above about 10-20 thus, the measured optical activity of high-molar-mass polymers results from that of the chiral monomeric units. An example of this consists of poly(L-a-amino acids) in the coiled state, for example, in dichloroacetic acid. 

Poly(L-a-amino acids) generally form right-handed helices, whereas poly(D-a-amino acids) form left-handed helices. An exception is poly()8-benzyl-L-aspartate), which occurs as left-handed helices. The helical structure is retained in solvents such as dioxan or dimethyl formamide. But in dichloroacetic acid or hydrazine, coiled molecules are present. 

The collective effects of the double-bond character of the peptide bond, the intramolecular hydrogen bonding, the hydrophobic bonding, and the L configuration of the peptide residues induce most poly(L-a-amino acids) and protein sequences to adopt the shape of a right-handed a-helix. But there are also exceptions. Despite the l configuration of the peptide residues, poly(L-j8-benzyl aspartate) forms a left-handed a helix. In addition to a helices and the pleated-sheet structures 03 structures), other secondary structures are also known for poly(a-amino acids) (Table 30-2). 

The advantage of a polymer thus prqjared is that it contains the smallest munber of bonds susceptible to oizymatic attack and that the rate of scission of these bonds can be controlled by changing the structure, amount, and length of the crosslinks. Alternatively, biodegradable synthetic polymers are obtained by synthesizing analogues of natural polymers, e.g. of synthetic poly(L-a-amino acids)... 

Pd on poly-L-leucine and other poly-L-a-amino-acids hydrogena- tion Precursors (S)Phenyl-alanine, (R)dehydro-oc-methylcinnamic acid (see Figure 7) 1.16-5.94 [83-86]... 

Catalytic Activities of Thermal Poly-anbydro-a-Amino Acids Duane L. Rohlting and Sidney W. Fox... 

Itoh et al. (1969) have measured the far-infrared spectra (700 to 60 cm ) of the x-helix structures of poly(L-a-amino-n-butyric acid), poly-L-norvaline, poly-L-nor-leucine, and poly-L-leucine, as well as the spectra of the ) -form structures of poly(L-a-amino-n-butyric acid), poly-L-valine, poly(DL-a-amino-n-butyric acid), poly-DL-norvaline, and poly-DL-norleucine. The a-helix has characteristic bands near 690, 650, 610, 380, 150, and 100cm . The -form has characteristic bands near 700, 240, and 120 cm . The vibrations of the main chain are strongly coupled with the deformation vibrations of the side chains. An earlier study (Itoh et al., 1968) involved various polyalanines and correlations of spectra with a- and j8-structures. 

With regards to new polymeric biomaterial, Gomurashvilli and co-workers (2) have developed new biodegradable and tissue-resorbable co-poly(ester amides) (PEAs) using a versatile Active PolyCondensation (APC) method which involves di-p-toluenesulfonic acid salts of bis-(L-a-amino acid)-a,(o-alkylene diesters and active diesters of dicarboxylic acids as monomers. A wide range of... 

S. Mallakpour, A. Zadehnazari, Novel optically active poly(amide-thioester-imide)s containing L-a-amino acids and thiadiazol anticorrosion group production and characterization. High Perform. Poly. 25, 377-386 (2012)... 

Malcolm (1973, 1975) reported possible observable differences between the pressure-area curves of poly(L-alanine) mixed with poly(D-a-amino-n-butyric acid) and the corresponding mixture containing poly(D-alanine). Shafer (1974) observed differences in the film pressure between (/ )-phosphatidyl serine with poly-L-lysine and the corresponding film with poly-D-lysine injected under the film. 

Benoit, H., Freund,L., Spach,G. In Fasman,G.D. (Ed.) Poly-a-amino acids, Chapter 3. New York Marcel Dekker 1967. 

Polypeptides obtained by the anionic polymerization of optically active N-carboxy-a-amino acid anhydrides are apt to have such an ordered structures as a-helices, which is useful for investigation on the relationship between the physical structure and the permeability of the membrane. Takizawa et al.44 46) studied the water permeation and solute separation through poly(n-alkyl L-glutamate) membranes 3. It was concluded that water molecules permeate through relatively large free spaces... 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

A photochromic polymer containing azobenzene units has also been prepared by modification of a naturally occurring microbial poly(E-L-lysine) (Scheme 5, Structure IX), and investigated by means of absorption and circular dichroism spectroscopy.1431 The structure of this polymer, however, does not correspond to those of polypeptides, which are poly(amide)s of a-amino acids, and therefore the results cannot be discussed in terms of the typical polypeptide structures (a-helix, P-structure, random coil) and their standard CD spectra. 

The kinetics and mechanism of polymerization of IV-carboxy-a-amino acid anhydrides as well as the biological properties of poly-a-amino acids have been studied and reviewed extensively (70,79,80). The critical chain length for helix formation varies from one amino acid to another (81,82). The critical chain length in L-methionine oligopeptides is the heptamer (83). 

From the CD spectra of PLL-A-67 (poly-L-lysine having 67% adenine units) in acidic aqueous solution, the residual elliptidty at 222 nm ([ J222)is plotted against the pH of the system (Fig. 21). The value [0)222 is known to be related to the helix content of poly-(a-amino acid)73. ... 

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