MEASUREMENT OF PEPTIDE AGGREGATION WITH PULSED-FIELD GRADIENT NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Interactions between hydrophobic patches in proteins are often a drivi ng force for denaturation and aggregation. The aggregation of the beta -amyloid peptide fragment, VHHQKLVFFAEDVGSNK (beta(12-28)), has been i nvestigated in aqueous solution at low pH. This peptide contains a cen tral hydrophobic patch spanning residues 17-21. Diffusion coefficients measured with pulsed-field gradient NMR as a function of peptide solu tion concentration were used to assess the extent of aggregation. Foll owing the hypothesis that hydrophobic interactions are an important dr iving force in the aggregation of this peptide at low pH, a non-aggreg ating analog of the beta(12-28) peptide, [Gly(19,20)]beta(12-28) was s ynthesized. In the [Gly(19,20)]beta(12-28) peptide, the replacement of the two phenylalanine residues disrupts the hydrophobic interactions which drive the aggregation of beta(12-28). The diffusion coefficient of the [Gly(19,20)]beta(12-28) peptide is invariant over the concentra tion range studied and provides a good estimate of the monomeric diffu sion coefficient of beta(12-28). A second peptide analog was synthesiz ed in which the phenylalanine at position 20 was replaced with a cyste ine residue. The disulfide-linked dimer, ([Cys(20)]beta(12-28))(2), wa s formed upon air oxidation of this peptide. The diffusion coefficient of the ([Cys(20)]beta(12-28))(2) peptide was measured and used to est imate the diffusion coefficient of the beta(12-28) dimer. Using the mo nomeric and dimeric diffusion coefficients measured for the glycine an d cysteine analogs, the concentration dependence of the beta(12-28) di ffusion coefficient was found to be consistent with a monomer-dimer ag gregation model. (C) 1998 Elsevier Science B.V.