Poly a-amino Acids

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 dependence of the molar optical activity of coils on the wavelength of polarized light can be described by a one-term Drude equation. In the case of helices, the Moffitt-Yang equation is better. The constants Xo are completely unaffected by the solvent, while aa and ba assume different values (Table 4-9). For a given polymer in different helicogenic solvents, bo is found to be more or less constant, while m still depends on the solvent. For various poly(a-aminocarboxylic acids), bo has approximately the same value so long as these are in the helix conformation, bo is therefore a typical constant for the helix conformation of poly(a-aminocarboxylic acids), while gq contains contributions from the helix and the asymmetric carbon atoms. 

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One particular type of polyamide produced from intermediates containing lateral side groups are the poly-(a-amino acids). The a-amino acids have the structure shown in Figure 18.23 (I) and give polymers of the type shown in Figure 18.23 (II). The proteins may be considered as a special class of such... 

Sela, M., and Katchalski, E., Biological properties of poly-a-amino acids, Adv. Prot. Chem.. 14. 391-479, 1959. 

UETAL. Poly(a-amino acid) Drug Conjugates... 

Synthesis and Chemical Properties of Poly-a-Amino Acids Ephraim Katchalski and Michael Sela... 

S. W. Englander, Poly-a-Amino Acids, p. 339. Dekker, New York, 1967. 

Stereosequences in polymers having the general formula — CHR—X—Y— [e.g., poly-a-amino acids, polymers derived from substituted oxiranes, thi-iranes, aziridines, lactones or lactams, and 1,4 polymers of 1- (or 4) monosub-stituted butadienes] are not affected by the degeneracy phenomena existing in vinyl polymers. Their number is indicated in the first column of Table 2. 

Fasman,G.D. In Fasman,G.D. (Ed.) Poly-a-amino acids, Chapter 11. New York Marcel Dekker 1967. 

Experiments with poly-a-amino acids or proteins have been in perfect agreement with the above when A0 = 212 nm and b0 = —630. Equation (23) is also called Moffitt s equation or Moffitt-Yang s equation. 

Complexes between chiral polymers having ionizable groups, and achiral small molecules become, under certain conditions, optically active for the absorption regions of the achiral small molecules. Dyes such as acridine orange and methyl orange have been used as achiral species, since they are in rapport with biopolymers through ionic coupling. This phenomenon has been applied to the detection of the helix chirality in poly-a-amino acids, polynucleotides, or polysaccharides when instrumental limitations prevent direct detection of the helices. 

Fig. 35. The pH dependence of Moffitt s bQ values of poly(a-amino acids) in aqueous solution 102) ...

Benoit, H., Freund, L., Spach, G., Poly-a-Amino Acids, 1967, Marcel... 

Sela, M., and A. Berger The terminal groups of poly-a-amino acids. J. Amer. Chem. Soc. 77, 1893 (1955). 

Ribbed helices (costal helices) are important in organic chemistry because linear polymers contain side chains as well as backbones. We may, then, discern not only the catenal helix of the backbone, but the intercostal helix formed by all of the ribs and the infracostal helicesof the individual side chains. The intercostal helix may be iterative (as in an isotactic head-to-tail vinyl polymer or homogeneous poly-a-amino acid) or non-iterative (as in a random copolymer, an atactic polymer or typical protein). The intracostal helices can best be analyzed as short-chain crooked lines, as in Section III. Important as costal helicity is, it is secondary to catenal helicity and we therefore limit our attention to the primary helicity, that of long chains. Indeed, we limit our attention to catenal helices having chain motifs of two atoms and two bonds as found in head-to-tail vinyl homopolymers ... 

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