Conformational mapping of the N-terminal segment of surfactant protein B in lipid using C-13-enhanced Fourier transform infrared spectroscopy

Synthetic peptides based on the N-terminal domain of human surfactant prote in B (SP-B1-25; 25 amino acid residues; NH2-FPIPLPYCWLCRALIKRIQAMIPKG) reta in important lung activities of the full-length, 79-residue protein. Here, we used physical techniques to examine the secondary conformation of SP-B1- 25 in aqueous, lipid and structure-promoting environments. Circular dichroi sm and conventional, C-12-Fourier transform infrared (FTIR) spectroscopy ea ch indicated a predominate alpha-helical conformation for SP-B1-25 in phosp hate-buffered saline, liposomes of 1-palmitoyl-2-oleoyl phosphatidylglycero l and the structure-promoting solvent hexafluoroisopropanol; FTIR spectra a lso showed significant beta- and random conformations for peptide in these three environments. In further experiments designed to map secondary struct ure to specific residues, isotope-enhanced FTIR spectroscopy was performed with 1-palmitoyl-2-oleoyl phosphatidylglycerol liposomes and a suite of SP- B1-25 peptides labeled with C-13-carbonyl groups at either single or multip le sites. Combining these C-13-enhanced FTIR results with energy minimizati ons and molecular simulations indicated the following model for SP-B1-25 in 1-palmitoyl-2-oleoyl phosphatidylglycerol: beta-sheet (residues 1-6), alph a-helix (residues 8-22) and random (residues 23-25) conformations. Analogou s structural motifs are observed in the corresponding homologous N-terminal regions of several proteins that also share the 'saposin-like' (i.e. 5-hel ix bundle) folding pattern of full-length, human SP-B. In future studies, C -13-enhanced FTIR spectroscopy and energy minimizations may be of general u se in defining backbone conformations at amino add resolution, particularly for peptides or proteins in membrane environments.