Abstract
The infrared dichroic ratios of the amide bands from oriented β-barrels yield an experimental value for the mean orientation, β, of the β-strands, relative to the barrel axis. For a barrel of n strands, this then gives the shear number, S, that characterizes the stagger of the β-sheet. Combining values of β and n specifies the barrel geometry by using the optimized model of Murzin, Lesk & Chothia for regular barrels. Application to published infrared data on the Escherichia coli outer membrane protein, OmpA yields S=9−10 ( n=8), a barrel radius of 0.81(±0.01) nm, and an internal free volume of 0.031 nm 3 per residue, where the average twist of the β-sheets is θ≈28 °, and their coiling angle is ϵ≈1 °. Hydrophobic matching of the 2.6 nm transmembrane stretch partly determines the shear number of the OmpA β-barrel.
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