Nmr studies on complex formation of poly(L‐lysine HBr) with carbonate ion in aqueous solution

Abstract

AbstractDiemer formation of poly(L‐lysine HBr) in carbonate buffer at pD 10.5 was reported in our previous paper [Biopolymers(1981) 20, 345–357]. The mechanism of the dimer formation was investigated employing carbon‐13 and proton nmr. pD dependence of the 13C‐nmr spectrum of poly(L‐lysine HBr) in the presence of carbonate ion clearly shows that a complex formation between the CO ion and protonated ε‐amino group is involved in the stabilization of the dimer form. The lifetime of the complex is longer than 10−2 s. A stoichiometric evaluation suggests that CO bridges two lysyl side chains. A molecular model of the dimer form designated as a single antiparallel β‐ribbon was proposed and discussed in the light of hydrodynamic and ir spectroscopic properties reported earlier. Concentration change experiments indicate that CO binds not only to the dimer formation is inferred as stabilization of the single antiparallel β‐ribbon as an intermediate structure in the conversion between the α‐helix and β‐sheet. The α‐CH proton signal of the lysine residue located in the ordered structure (α‐helix and β‐form) was observed to be separate from that in the random‐coil region.