Polypeptides synthetic conditions, conformational

Table 4. Synthetic conditions, conformational characteristics, and H and chemical shifts of natural fibrous proteins and their model polypeptide samples (from refs 64 and 67)...

A well-defined monodisperse penta(L-alanine)- -butylamide H-[Ala]5-NHBu was synthesized by an activated ester method " and other natural abundant polypeptides, [Ala]n-5, [Leu]n-1 and [Leu]n-2, were synthesized by the N-carboxy a-amino-acid anhydride (NCA) method.Fully N-labelled homopolypeptides, [Ala ]n (99 at.% of N purity MASSTRACE, Inc.) and [Leu ]n (99 at.% of N purity MASSTRACE, Inc.), which show characteristic differences in conformation such as the a-helix and /3-sheet forms, were prepared by the heterogeneous polymerization of the corresponding NCAs in acetonitrile with -butylamine as an initiator. Conformational characterization of these samples was made on the basis of the conformation-dependent C and chemical shifts determined from the CP-MAS NMR method and from the characteristic bands in the IR and far-IR spectra. Figs. 38 and 39 show the 75.5 MHz C and 30.4 MHz N CP-MAS NMR spectra respectively of these fully N-labelled (99 at.% purity of N) homopolypeptides adopting the a-helical and /3-sheet forms (A) [Ala ]n-2 (a-helix), (B) [Ala ]n-1 (/3-sheet), (C) [Leu ]n-2 (a-helix), (D) [Leu ]n-1 (/3-sheet) in the solid state. Synthetic conditions and conformational characteristics of these samples are summarized... 

The research on polypeptides and their assembly behaviors is important and beneficial for several areas. First, polypeptides can be used as a model polymer with various chain rigidities. Polypeptides can adopt conformations of a-helix, p-sheet and random coU, which can transform into each other under controlled conditions. The a-helix to random coil transition in solutions is especially interesting. Thus, polypeptides can serve as an ideal model for investigating the influence of polymer rigidity on the assembly behavior of polymers. Second, the synthetic polypeptides... 

In summary, we have therefore seen that poly-L-lysine presents a valuable model for a partially helical polypeptide chain, one which is amenable to conformational analysis by optical rotatory dispersion. The method by which residues in a helical conformation may be discerned and counted against a background of disordered regions has been illustrated with this polypeptide under almost ideal conditions. The adequacy of the method is corroborated by copolymers a step closer to proteins in complexity, but some of the limitations that will be encountered in its application to proteins are already foreshadowed. Before this application is discussed, however, two other phenomena relevant to protein structure that are clearly exhibited in synthetic polypeptides, the helix-coil transition and the /3-conformation, will be considered. 

What accommodations, then, must be made in the pattern of analysis developed for standard synthetic polypeptides if it is to yield quantitative estimates of partial helical content in proteins The requirements are much the same as those set out for poly-L-lysine (see Section III, G, 3), yet since globular proteins, like copolymers of L-lysine and L-glutamic acid, cannot be made completely helical in aqueous solution, a helical reference conformation must be taken either from other standard molecules or from the nonaqueous behavior of the protein in question. This latter procedure involves solvent changes with little bearing on native conditions and may be impossible to carry out in the face of restraints imposed by proline and cross-links, so that standard helical dispersion can more feasibly serve as this reference conformation. 

A value of be = —630 is well established for synthetic polypeptides and fibrous proteins. The considerations leading to the reasonable approximation that be of the disordered chain is zero, together with the implied equality of Xo and for this state, have been discussed in Section III, C, 1, and incorporated into the pattern of analysis for partial helical content as set out in Section III, G, 2. A value of ao = 4-650 has been obtained for poly-L-glutamic acid, poly-L-lysine, and Pinna nobilis tropomyosin under the appropriate conditions. As has been stressed, these constants have conformational significance only when a value of 212 nm is used for Xo. 

Because of rotational flexibility in the polypeptide backbone, primarily around the N— (p) and C —C (f/r) angles, there is a very large number of possible conformations that any one polypeptide molecule may adopt. Unlike most synthetic polymers, however, proteins have the ability to fold up (under the right conditions) into specific conformations, and it is these conformations (structures) that give rise to their individual properties. 

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