Peptide bonds, polyglycine

Early studies on metal-peptide complexes were generally concerned with polyglycines, and there are several reviews on these complexes. In the first part of this article, we will discuss various metal-peptide complexes that have coordinating side chains. Comparison of these metal-peptide complexes will be made with their glycine analogs, and the differences will be noted. The second part of the article will deal with metal-mediated peptide-bond cleavage. 

Later, the peptide linkage itself began to be modified (Fig. 1.11). For example, the classical mode of stabilization against peptidase cleavage by conversion of the peptide bond NH into N-methyl subsequently evolved into the preparation of peptoids (polyglycine chains with each NH replaced by a variety of N-alkyl groups of the type that resemble the side-chains found in normal amino acids). Libraries of these compounds were very popular for a while but interest has decreased as time passes (114,115). 

Romiplostim (AMG 531) is a member of new class of therapeutics called "peptibodies," which are peptides with key biologic activities covalently linked to antibody fragments that serve to extend the peptide s half-life. Romiplostim contains two disulfide-bonded human Fc fragments, each covalently attached through a polyglycine sequence to a peptide... 

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.

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... 

Pauling and Corey also came to the conclusion that in stable arrangements of peptide chains, e.g. polyglycine, in which no side chain interactions are involved (i) as many hydrogen bonds as possible are formed (ii) all the amino acids are arranged symmetrically with respect to one another, and all play an equivalent part in the structure. 

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