Penta-glycine

Data from Ref. [94C2] are Ks,a values determined over the concentration range 1-3 mg mL, except for penta-glycine for which the concentration was about 1 mg mL. ... 

Calcium in this case would coordinate with acyl oxygens from the polypeptide backbone of the protein, because of its unique amino acid sequences and potential conformations348). Non-polar peptides have been isolated from elastin which consists almost entirely of the three non-polar amino acids glycine, valine, and proline349). Portions of porcine tropoelastin have been partially sequenced350). Repeating tetra-, penta-, and hexapeptides have been observed. The tetrapeptide contains the sequence —Gly—Gly—L—Val—L—Pro— the pentapeptide —L—Val—L—Pro-... 

Hexacoordinated structures prevail in the case of complexonates of the triamine series [764]. However, a great variety of CN of metals is also observed. Moreover, many coordination modes are observed for the complexones of the triamine series, penta-, hexa-, hepta-, and octa-types with chelate and chelate-bridge function of mono-, di-, tri-, and tetra-deprotonated ligands. As a result, a number of metal-cycles are formed. Thus, maximum dentacity of diethylenetriami-noacetic acid 431 (m = n = p = q = 1), with respect to a metal atom, equals 8 (3N+50), leading to seven metal-cycles (two ethylenediamine and five glycine) being formed [764]. 

Various cyclization techniques have been used in the past, which include disulfide bond [61], side-chain lactam [59], and head-to-tail amide bond formation [62], In most cases, the readiness of an open-chain precursor to cyclize depends on the size of the ring and the sequence of the peptide to be cyclized. Usually, ring closure with hexa- and pentapeptides is somewhat hampered however, the cyclization may be enhanced by the presence of turn structure inducing amino acids such as glycine, proline, and D-amino acids, etc., as was the case in the study of McBride et al. [63], Spatola et al. [64] have recently conducted extensive studies in these aspects and several cyclic penta-, hexa-, and heptapeptide libraries were prepared and analyzed. Their results showed that rapid cyclization rates can be achieved with optimized synthesis and cyclization procedures, and many combinations of cyclic peptides can be formed in high quality if they contain structural features that make cyclization more facile. 

The actual formation step is the reaction of carbon dioxide itself with the conjugate base of the penta-ammine-aquo-complex. For this process the second-order rate constant in Scheme 2) is 2.2 x 10 1 mol s at 25 °C the activation parameters are A/f = 15.3 0.9 kcal mol and LS = + 3.6 3,0 cal deg mol. The activation parameters for the reaction of [Co(NH3)5(OH2)] + with the neutral ligand glycine are = 27.6 kcal... 

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