Cooperative hydrogen-bonding in models of antiparallel β-sheets

We present fully geometrically optimized density functional theory calculations at the B3LYP/D95(d,p) level on antiparallel beta-sheet models consisting of two or four strands of two or four glycine residues and artificial nylon-like two- or four-strand models of two glycine residues separated by two methylene groups. Unlike the H-bonds in alpha-helices and chains of H-bonding amides, the association of polyglycine strands shows little or no H-bond cooperativity. We show that C-5 intrastrand H-bonds are either disrupted or enhanced upon formation of interstrand H-bonds, depending upon the H-bonding pattern in the glycine (but not the nylon-like) structures. The apparent relative absence of H-bond cooperativity in beta-sheet models of polyglycine derives from the weakening and strengthening of these intrastrand H-bonds. Normal cooperative H-bonding occurs when the nylon-like strands (which cannot form the intrastrand H-bond) form two- and four-strand sheets. The H-bonding interactions are stronger and the H-bonding distances shorter (on average) than previously reported for similar calculations on alpha-helical structures, consistent with the observations that amyloid diseases, such as Alzheimer's and prion diseases, involve conversion of a-helical secondary structures to (presumably more stable) beta-sheets.