SYNTHESIS, SECONDARY STRUCTURE AND FOLDING OF THE BEND REGION OF LUNGSURFACTANT PROTEIN-B

Previous theoretical analysis of the primary structure of lung surfact ant protein SP-B indicates a disulfide-linked, hydrophobic midsequence that forms a hair-pin-like motif. Here, we experimentally investigate the secondary structure of the disulfide-stabilized bend region by sy nthesizing two 12-residue analogs of the SP-B midsequence. The native peptide has the same sequence for residues 35 to 46 as native human SP -B, while, in the second mimic peptide Leu(40) and Val(41) were replac ed with D-Ser and L-His. Both peptides contain cysteine residues at th e N- and C-terminus (Cys(35) and Cys(46), respectively). Oxidation/red uction experiments with fast atom bombardment mass spectrometry showed mass shifts of approximately 2 daltons, consistent with the oxidized peptides existing in solution as monomers, each with one internal disu lfide bond (Cys(35)-Cys(46)). Since circular dichroism and Fourier-tra nsform infrared measurements show that both peptides assume turn confo rmations in structure-promoting solvents such as trifluoroethanol (TFE ), a structural model is proposed in which Cys(35) and Cys(46). For th e double spin-labeled mimic peptide in TFE, ESR spectra indicated broa dening characteristic of either radical interactions or decreased mobi lity, or both. Increases in radical interactions for the double spin-l abeled mimic peptide would be expected for Cys(35) and Cys(46) approac hing within 14 Angstrom in structure-promoting solvents, while decreas es in spin-label mobility could be due to the formation of a loop. Bas ed on these observations with peptide analogs, residues 35 to 46 proba bly form a similar bend in the full-length protein.