Peptide hydrogens, ionization

Further studies of the C-peptide showed that a helix is very strongly affected by pH, due to the presence of ionizable side chains. Helical stabilization occurs when favorable interactions can occur between ionized side chains and the helix dipole, and also when ionization can promote hydrogen bonding between side chains. In the case of C-peptide, two ionized groups (with p/fas near 3.5 and 6.5) promote helix formation and helix content follows a bellshaped curve, with a maximum near pH 5. 81 Increasing concentrations of salt (NaCl) also increases conformational stability. 98 ... 

The influence of proline residues on coordination has been examined by Kozlowski and coworkers. Where Pro occurs in a peptide chain there is no peptide hydrogen (20), and from a variety of spectroscopic studies these workers have established that a Pro residue causes disruption of metal ion-promoted peptide ionizations, i.e. Pro serves as a breaking point in metal ion coordination. On... 

The copper(II)-promoted hydrolysis of glycylglycine has been studied in some detail.120 Copper(II) ions catalyze the hydrolysis of glycylglycine in the pH range 3.5 to 6 at 85 °C.120 The pH rate profile has a maximum at pH 4.2, consistent with the view that the catalytically active species in the reaction is the carbonyl-bonded complex. The decrease in rate at higher pH is associated with the formation of a catalytically inactive complex produced by ionization of the peptide hydrogen atom. This view has subsequently been confirmed by other workers,121 in conjunction with an IR investigation of the structures of the copper(II) and zinc(II) complexes in D20 solution.122 Catalysis by cobalt(II),123 and zinc(II), nickel(II) and manganese(II) has also been studied.124-126... 

Tables 1 and 2 Hst the important physical properties of formamide. Form amide is more highly hydrogen bonded than water at temperatures below 80°C but the degree of molecular association decreases rapidly with increa sing temperature. Because of its high dielectric constant, formamide is an excellent ionizing solvent for many inorganic salts and also for peptides, proteias (eg, keratin), polysaccharides (eg, cellulose [9004-34-6] starch [9005-25-8]) and resias.

Kamel AM, Zandi KS, Massefski WW. 2003. Identification of the degradation product of ezlopitant, a non-peptidic substance P antagonist receptor, by hydrogen deuterium exchange, electrospray ionization tandem mass spectrometry (ESI/MS/MM) and nuclear magnetic resonance (NMR) spectroscopy. J Pharm Biomed Anal 31 1211. 

The prediction of the partition properties of peptide molecules is difficult, owing to their conformational flexibility, and the possible presence of multiple intramolecular hydrogen bonds and ionizable groups. Richards and coworkers (40-42) were the first to consider explicitly the effects of the population of accessible conformational minima in both phases. These types of calculation are, however, computationally intensive. The introduction of the solvent-accessible surface area in the prediction of log Poct for steric isomers (43,44) also constitutes a promising approach. 

Table VII. Ionization Constants for the Loss of Coordinated Amine Hydrogens from Cu(III)—Peptide Complexes0...

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