Alteration of Water Structure by Peptide Clusters Revealed by Neutron Scattering in the Small-Angle Region (below 1 Å-1)

Solution scattering of neutrons and x-rays can provide direct information on local interactions of importance for biomolecular folding and structure. Here, neutron scattering experiments are combined with molecular-dynamics simulation to interpret the scattering signal of a series of dipeptides with varying degrees of hydrophobicity (GlyAla, GlyPro, and AlaPro) in concentrated aqueous solution (1:20 solute/water ratio) in which the peptides form large segregates (up to 50-60 amino acids). Two main results are found: 1), the shift to lower Q of the so-called water-ring peak (Q approximate to 2 angstrom(-1)) arises mainly from an overlap of water-peptide and peptide-peptide correlations in the region of 1.3 < Q < 2 angstrom(-1), rather than from a shift of the water signal induced by the presence of the clusters; and 2), in the low-Q region (Q approximate to 0.6 angstrom(-1)) a positive peak is observed originating from both the solute-solute correlations and changes in the water structure induced by the formation of the clusters. In particular, the water molecules are found to be more connected than in the bulk with hydrogen-bonding directions tangential to the exposed hydrophobic surfaces, and this effect increases with increasing peptide hydrophobicity. This work demonstrates that important information on the (hydrophobic) hydration of biomolecules can be obtained in the very-small-angle region.