Biophys J, October 2002, p. 2126-2141, Vol. 83, No. 4

Effect of Hydrophobic Surfactant Proteins SP-B and SP-C on Phospholipid Monolayers. Protein Structure Studied Using 2D IR and Correlation Analysis

Saratchandra Shanmukh,^* Phillip Howell,^* John E. Baatz, and Richard A. Dluhy^*

 ^*University of Georgia, Department of Chemistry, Athens, Georgia 30602-2556 and  Medical University of South Carolina, Department of Pediatrics, Charleston, South Carolina 29425 USA

We have applied two-dimensional infrared (2D IR) and correlation spectroscopy to in-situ IR spectroscopy of pulmonary surfactant proteins SP-B and SP-C in lipid-protein monolayers at the airwater interface. For both SP-B and SP-C, a statistical windowed autocorrelation method identified two separate surface pressure regions that contained maximum amide I intensity changes: 4-25 mN/m and 25-40 mN/m. For SP-C, 2D IR and correlation analyses of these regions indicated that SP-C adopts a variety of secondary structure conformations, including -helix, -sheet, and an intermolecular aggregation of extended -sheet structure. The main -helix band split into two peaks at high surface pressures, indicative of two different helix conformations. At low surface pressures, all conformations of the SP-C molecule reacted identically to increasing surface pressure and reoriented in phase with each other. Above 25 mN/m, however, the increasing surface pressure selectively affected the coexisting protein conformations, leading to an independent reorientation of the protein conformations. The asynchronous 2D IR spectrum of SP-B showed the presence of two -helix components, consistent with two separate populations of -helix in SP-Ba hydrophobic fraction associated with the lipid chains and a hydrophilic fraction parallel to the membrane surface. The distribution of correlation intensity between the two -helix cross peaks indicated that the more hydrophobic helix fraction predominates at low surface pressures whereas the more hydrophilic helix fraction predominates at high surface pressures. The different SP-B secondary structures reacted identically to increasing surface pressure, leading to a reorientation of all SP-B subunits in phase with one another.

Biophys J, October 2002, p. 2126-2141, Vol. 83, No. 4
© 2002 by the Biophysical Society   0006-3495/02/10/2126/16  $2.00

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