Biophysical characterization of betabellin 16D: A beta-sandwich protein that forms narrow fibrils which associate into broad ribbons

The betabellin structure is a de novo designed beta-sandwich protein consis ting of two 32-residue beta sheets packed against one another by hydrophobi c interactions. Betabellin 16S (B16S), a 32-residue peptide chain (HSLTAKIa kLTFSIAahTYTCAVakYTAKVSH, where a is DA1a, h is DHis, and k is DLys), did n ot have beta structure in water at pH 6.5. Air oxidation of B16S furnished betabellin 16D (B16D), a 64-residue disulfide-bridged two-chain protein, wh ich also did not fold in water at pH 6.5. However, the extent of beta struc ture observed for B16D increased with pH and ionic strength of the solution and the B16D concentration as observed by circular dichroism spectropolari metry, Transmission electron microscopy showed that B16D formed narrow fibr ils that associated into broad ribbons in 5.0 mM Mops and 0.25 M NaCl at pH 6.9. (C) 1999 Academic Press.