Hydrogen bonding polyglycine

Examine the structure of the short strand of Nylon 6 in which all amide bonds are Z. What is the monomer unit How many monomers are in the strand Note Each end of the polymer strand has been capped by one or more atoms. Do not count these caps as monomers. Compare the strand to that of a simple polypeptide, for example, polyglycine (see also Chapter 16, Problem 9), and point out any obvious similarities. Pay particular attention to hydrogen bonds. 

From such a background, some kinds of polypeptide blend samples have been studied by solid state NMR.27,72 74 Especially, detailed information for four kinds of blend samples such as poly(L-alanine) (PLA)/poly(L-valine) (PLV), PLA/poly(L-isoleucine) (PLIL), poly(D-alanine) (PDA)/PLV and polyglycine (PG)/PLV blends, have been reported. Here, let us describe some reasons why PLA/PLV, PDA/PLV, PLA/PLIL and PG/PLV blends are interesting systems. PLA and PDA in the solid-state can take the a-helix and (3-sheet forms due to intra- and intermolecular HBs, respectively. PG in the solid-state can take the 3j-helix (PG-II) and (3-sheet (PG-I) forms due to intra- and intermolecular HBs, respectively. However, PLIL and PLV in the solid state can predominantly take the (3-sheet form as the stable conformation. For this reason, it is interesting to know whether an isolated a-helix or 3i-helix form polypeptide surrounded by a major polypeptide in the (3-sheet form can take the helical conformation, or not, due to the balance between intramolecular and intermolecular hydrogen bonds. In addition, we would like to know whether a polypeptide in the (3-sheet form surrounded by a major polypeptide in the a-helix or 3 -helix form can take the (3-sheet form. 

II Open systems in two dimensions in which chains are linked to usually six other chains forming a two-dimensional hydrogen-bonded framework. The only known example of this type is fi polyglycine. 

Krimm S, Kurowa K, Rebare T (1967) Infra-red studies of C-H—0=C hydrogen bonding in polyglycine, II. In Ramachandran GN (ed) Conformation of biopolymers. Academic Press, London, pp 439-447... 

Fig. 4b. Structure of polyglycine II (from Crick and Rich, 1955). A projection of the structure with the screw axis vertical. The chain on the right is nearer the reader than that on the left. The planar peptide groups are edge-on at the bottom of the figure with hydrogen bonds from these groups virtually perpendicular to the plane of the paper.

In the polyglycine II structure proposed by Crick and Rich, all of the polyglycine chains are parallel and are packed in an hexagonal array. Each chain has a threefold screw axis and is hydrogen bonded to each of its six neighbors as shown in Fig. 4a, which is a projection down the screw axis. The hydrogen bonds are linear with an 0 N distance of 2.76 A, which is within the range of values found for simple compounds. 

These chains lie side by side to form a flat sheet. Each chain is held by hydrogen bonds to the two neighboring chains (Fig. 36.2). This structure has a repeat distance of 7.2 A, the distance between alternate amino acid residues. (Notice that alternate side chains lie on the same side of the sheet.) However, crowding between side chains makes this idealized flat structure impossible, except perhaps for synthetic polyglycine. 

Nakano et al) reported a C CP/MAS NMR study on poly(L-Alanine) (PLA)/Polyglycine (PC) blends. The comparison between the C CP/MAS spectra of the blends and those of the parent polymers discloses that upon blending, new conformations of PLA and PC are fonned, which is closely related to the presence of intermolecular hydrogen-bonding interactions. Tip measurements demonstrate that the major parts of PC and PLA in the blends, which are in 3i-helix and a-helix conformation forms respectively, are phase separated. On the other hand, the p-sheet forms of PC and PLA, which are newly formed upon blending, exhibit similar values, demonstrating that these two parts are miscible at the scale of 3-4 nm. 

Polyglycine, H(-CO-CH2-NH)n-H, is the simplest polypeptide. It has two solid state structures. Form I consists of chains of polymer that are hydrogen-bonded into two-dimensional sheets. This form (including selectively deuterated isotopomers) has been studied by INS [53-55]. The spectra were interpreted similarly to those of N-methylacetamide. Reexamination of the spectra with periodic-DFT calculations is necessary. Acetanilide, C6H5-NH-C(=0)-CH3, forms hydrogen-bonded chains similar to N-methylacetamide and as such is a potential model for phenyl substituted peptides. However, the interest in acetanilide is that the... 

Figure 5.9 Biological hydrogen-bonding motifs helical DNA (left), a-helical polyglycine (centre) and / -sheet polyglycine (right)...

Fig. 13. Conformational map for helices with i to i-3 hydrogen bonding (variants of the 3io-helix). The shaded areas represent those conformations which allow acceptable hydrogen bonding. Conformations inside the area (marked by 1) enclosed by the line are sterically allowed for single-stranded helical polyglycine. The positions of the right- and left-handed standard 3io-helix are indicated by circles (Leach et al., 1966b).

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