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It is known that steric blocking by peptide sidechains of hydrogen bonding,HB, between water and peptide groups, PGs, in beta sheets accords with an aminoacid intrinsic beta sheet preference. The present observations with QuantumMolecular Dynamics, QMD, simulation with quantum mechanical treatment of everywater molecule solvating a beta sheet that would be transient in nature suggestthat this steric blocking is not applicable in a hydrophobic region unless acation is present, so that the amino acid beta sheet preference due to thissteric blocking is only effective in the presence of a cation. We observedbackbone hydration in a polyalanine and to a lesser extent polyvaline alphahelix without a cation being present, but a cation could increase the strengthof these HBs.  Parallel beta sheets have a greater tendency than antiparallel beta sheets ofequivalent small size to retain regular structure in solvated QMD, and a 4strand 4 inter-PG HB chain parallel beta sheet was used. Stability wasreinforced by one surface being polyvaline, which buttressed the oppositesurface which was used for experimentation. A single Ca2+ ion was used forinvestigation of individual binding events rather than bulk properties. Nodirect binding between Ca2+ and the PG oxygen was observed in thesesimulations, but perhaps it occurs at longer time scales as the transient betasheet unfolds.  When linear scaling QMD methods that are accurate for peptide resonance,Resonance-Assisted Hydrogen Bonding and the properties of water becomeavailable, more extensive experiments having multiple ions of multiple typescould be performed at acceptable computational cost. It important that suchinvestigations be performed on protein secondary structures rather than modelamides so that sidechain limitation of backbone hydration and hence intrinsicamino acid propensity is captured.

Amino acid preference against beta sheet through allowing backbone hydration enabled by the presence of cation