Poly Polypeptide, cyclic

A variety of important monomers are polymerized via anionic polymerization including cyclic ones like ethylene oxide as base for poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) and A-carboxyanhydrides (NCAs) used for making polypeptides and caprolactam, the monomer of nylon 6 (Eig. 3.9). Whereas in all cases rather stringent conditions have to be used to allow at least an efficient polymerization, for technical products often not fully living conditions are achieved, which leads to broader molar mass distributions. 

Turning to natural polypeptides and proteins, we may cite our computations on gramicidin S, collagen-like poly (tripeptides), a-lactalbumin, melittin, and bovine pancreatic trypsin inhibitor (BPTI) as examples. In the case of gramicidin S, a cyclic decapeptide, the computed structure [27, 28] was found to agree [29, 30] with a subsequently-determined X-ray structure [31] (see Fig. 2). Likewise, the computed triple-stranded helical structure of poly(Gly-Pro-Pro) [32] (Fig. 3) has cartesian coordinates that agree with those from a single-crystal X-ray diffraction study of (Pro-Pro-Gly)io [33] within 0.3 A. 

In the calculations of the electronic structure of these polypeptides, we applied the all-valence electron MINDO crystal-orbital method in its MINDO/3 parametrization. The calculations involved atomic coordinates corresponding to the antiparallel-) -pleated-sheet structure of Pauling and Corey S hence we had a combined symmetry operation in the case of poly( y) each gly unit was obtained in this polymer from its neighboring one by a translation along the helical axis through 3.25 A and by a simultaneous 180° rotation around it. In order to preserve the cyclic property of the Fock hypermatrix of the whole polymer (which is... 

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