Polypeptides, cationic interactions

Although the order of affinity of PF-4 for different glycosaminogly-cans, and dissociation of their complexes with salts, are typical of nonspecific, electrostatic interactions, PF-4 is not strictly a cationic protein.452 It is probable that heparin binds to clusters of basic amino acids (two lysine pairs) near the carboxyl terminal of a polypeptide chain that has an overall preponderance of acidic amino acid residues.457 High-molecular-weight heparin species can bind two PF-4 molecules, with formation of complexes 10 to 100 times as strong as those with antithrombin.217... 

Blackwell and his co-workers have used circular dichroism spectroscopy to study the interactions of glycosaminoglycans with collagen, and with synthetic cationic polypeptides. In the absence of glycosaminoglycans, poly-L-lysine and poly-L-arginine exist in an extended charged coil conformation. Glycosaminoglycans bind to these cationic polypeptides and cause them to assume an a-helical conformation. In a series of systematic studies (76-83). Blackwell and his co-workers... 

For a displacing salt with a divalent cation but a monovalent anion, each cation (e.g., Ca2+) will cause the neutralization of two charge group interactions between the polypeptide or protein and the electrostatic ligate, with the concentration of the M2+ cation D0 (in moles per liter) required to maintain electroneutrality exactly equal to half of the concentration of the accompanying monovalent (e.g., Cl ) counterions, C, (in moles per liter), associated with the positively charged polypeptide or protein when it is desorbed from the cation exchange HP-IEX sorbent, and hence... 

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