The importance of hydrogen bonding between the glutamine side chains to the formation of amyloid VQIVYK parallel beta-sheets: an ONIOM DFT/AM1 study.

We report DFT calculations indicating that beta-sheet formation involving the capped amino acid sequence VQIVYK is due (at least in part) to cooperative H-bonding between the glutamine side chains. The sequence VQIVYK has been reported to be essential for the aggregation of the tau protein into the amyloids associated with Alzheimer's disease and has been crystallized. Sheets containing only capped Q's form cooperative H-bonds between the side chains that enhance stabilization while keeping the backbones of the individual strands close to the quasi-planarity expected for a beta-sheet. Sheets containing only capped A's cannot form H-bonds between the side chains, do not interact cooperatively, and form helical structures that deviate considerably from the quasi-planarity expected for beta-sheets. Comparisons between the sheets made from capped VQIVYK's, Q's, and A's illustrate the importance of the cooperative H-bonds between the Q's to the stability of tau amyloids.