Infrared reflection-absorption spectroscopy: principles and applications to lipid-protein interaction in Langmuir films

Infrared reflection-absorption spectroscopy (IRRAS) of lipid/protein monolayer films in situ at the air/water interface provides unique molecular structure and orientation information from the film constituents. The technique is thus well suited for studies of lipid/protein interaction in a physiologically relevant environment. Initially, the nature of the IRRAS experiment is described and the molecular structure information that may be obtained is recapitulated. Subsequently, several types of applications, including the determination of lipid chain conformation and tilt as well as elucidation of protein secondary structure are reviewed. The current article attempts to provide the reader with an understanding of the current capabilities of IRRAS instrumentation and the type of results that have been achieved to date from IRRAS studies of lipids, proteins, and lipid/protein films of progressively increasing complexity. Finally, possible extensions of the technology are briefly considered.

A coordinated approach to cutaneous wound healing: vibrational microscopy and molecular biology

The repair of cutaneous wounds in the adult body involves a complex series of spatially and temporally organized processes to prevent infection and restore homeostasis. Three characteristic phases of wound repair (inflammation, proliferation including re-epithelialization and remodelling) overlap in time and space. We have utilized a human skin wound-healing model to correlate changes in genotype and pheno-type with infrared (IR) and confocal Raman spectroscopic images during the re-epithelialization of excisional wounds. The experimental protocols validated as IR images clearly delineate the keratin-rich migrating epithelial tongue from the collagen-rich wound bed. Multivariate statistical analysis of IR datasets acquired 6 days post-wounding reveal subtle spectral differences that map to distinct spatial distributions, which are correlated with immunofluorescent staining patterns of different keratin types. Images computed within collagen-rich regions expose complementary spatial patterns and identify elastin in the wound bed. The temporal sequence of events is explored through a comparison of gene array analysis with confocal Raman microscopy. Our approach demonstrates the feasibility of acquiring detailed molecular structure information from the various proteins and their subclasses involved in the wound-healing process.

Interaction of recombinant surfactant protein D with lipopolysaccharide: conformation and orientation of bound protein by IRRAS and simulations

Effective innate host defense requires early recognition of pathogens. Surfactant protein D (SP-D), shown to play a role in host defense, binds to the lipopolysaccharide (LPS) component of Gram-negative bacterial membranes. Binding takes place via the carbohydrate recognition domain (CRD) of SP-D. Recombinant trimeric neck+CRDs (NCRD) have proven valuable in biophysical studies of specific interactions. Although X-ray crystallography has provided atomic level information on NCRD binding to carbohydrates and other ligands, molecular level information about interactions between SP-D and biological ligands under physiologically relevant conditions is lacking. Infrared reflection-absorption spectroscopy (IRRAS) provides molecular structure information from films at the air/water interface where protein adsorption to LPS monolayers serves as a model for protein-lipid interaction. In the current studies, we examine the adsorption of NCRDs to Rd 1 LPS monolayers using surface pressure measurements and IRRAS. Measurements of surface pressure, Amide I band intensities, and LPS acyl chain conformational ordering, along with the introduction of EDTA, permit discrimination of Ca (2+)-mediated binding from nonspecific protein adsorption. The findings support the concept of specific binding between the CRD and heptoses in the core region of LPS. In addition, a novel simulation method that accurately predicts the IR Amide I contour from X-ray coordinates of NCRD SP-D is applied and coupled to quantitative IRRAS equations providing information on protein orientation. Marked differences in orientation are found when the NCRD binds to LPS compared to nonspecific adsorption. The geometry suggests that all three CRDs are simultaneously bound to LPS under conditions that support the Ca (2+)-mediated interaction.

Structural characterization of the monolayer-multilayer transition in a pulmonary surfactant model: IR studies of films transferred at continuously varying surface pressures

The four-component system acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC)/1,2-dipalmitoylphosphatidylglycerol/ (DPPG)/pulmonary surfactant protein SP-C/cholesterol provides a useful model for in vitro biophysical studies of the reversible monolayer to multilayer transition that occurs during compression <--> expansion cycles in the lung. Monolayer films of this mixture (with chain perdeuterated DPPC-d62) at the air/water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP (continuously varying surface pressures) (Langmuir 2007, 23, 4958). The thermodynamic properties of the Langmuir films have been examined with pressure-area isotherms, while the molecular properties of the film constituents in the transferred films in the monolayer and multilayer phases have been examined with IR spectroscopy. Quantitative intensity measurements of the DPPC-d62, DPPG, and SP-C components in each phase reveal that the DPPG and SP-C constituents are relatively enriched in the multilayer compared with the DPPC-d62, although all three species are present in both phases. Some molecular structure information is available from the surface-pressure-induced variation in IR parameters. The DPPC-d62 exhibits slightly increased conformational order in the multilayer phase as detected from decreases in the CD2 stretching frequencies upon compression, while the lipid phosphate residues become dehydrated, as deduced from increases in the 1245 cm-1 symmetric PO2- stretching frequency. A small increase is observed in the protein amide I frequency; possible interpretations of these changes are presented. The current observations are compared with ideas contained in the "squeeze-out hypothesis" (Handbook of Physiology, The Respiratory System; American Physiological Society Press: Bethesda, MD, 1986; Vol. III, p 247) and in the "liquid crystalline collapse" model (Biophys. J. 2003, 84, 3792). Within the limitation of the current procedures, the data contain elements from both these descriptions of the monolayer transformation. Extensions and possible limitations of the COVASP-IR method are discussed.

Tracking the dephosphorylation of resveratrol triphosphate in skin by confocal Raman microscopy

Polyphenolic resveratrol has been identified as a potent antioxidant acting as both a free radical scavenger and an inhibitor of enzyme oxidative activity. However, the reactive propensity of resveratrol also limits its use in topical formulations. A transient derivative of resveratrol, resveratrol triphosphate, has been designed to provide a means for the delayed delivery of the active compound in skin tissue where endogenous enzymes capable of dephosphorylation reside. Confocal Raman microscopy studies of intact pigskin biopsies treated with modified resveratrol provided information about the spatial distribution and time-dependence of permeation and conversion to the native active form. Conversion to the active form was not observed when skin samples were exposed to steam, a procedure that likely inactivates endogenous skin enzymes. In addition, treatment with the triphosphate compared to the parent compound revealed a more homogeneous distribution of resveratrol throughout the stratum corneum and viable epidermis when the former was applied. Thus, the bioavailability of resveratrol in the epidermis appears to be enhanced upon application of the pro-molecule compared to resveratrol.

FTIR studies of collagen model peptides: complementary experimental and simulation approaches to conformation and unfolding

X-ray crystallography of collagen model peptides has provided high-resolution structures of the basic triple-helical conformation and its water-mediated hydration network. Vibrational spectroscopy provides a useful bridge for transferring the structural information from X-ray diffraction to collagen in its native environment. The vibrational mode most useful for this purpose is the amide I mode (mostly peptide bond C=O stretch) near 1650 cm-1. The current study refines and extends the range of utility of a novel simulation method that accurately predicts the infrared (IR) amide I spectral contour from the three-dimensional structure of a protein or peptide. The approach is demonstrated through accurate simulation of the experimental amide I contour in solution for both a standard triple helix, (Pro-Pro-Gly)10, and a second peptide with a Gly --> Ala substitution in the middle of the chain that models the effect of a mutation in the native collagen sequence. Monitoring the major amide I peak as a function of temperature gives sharp thermal transitions for both peptides, similar to those obtained by circular dichroism spectroscopy, and the Fourier transform infrared (FTIR) spectra of the unfolded states were compared with polyproline II. The simulation studies were extended to model early stages of thermal denaturation of (Pro-Pro-Gly)10. Dihedral angle changes suggested by molecular dynamics simulations were made in a stepwise fashion to generate peptide unwinding from each end, which emulates the effect of increasing temperature. Simulated bands from these new structures were then compared to the experimental bands obtained as temperature was increased. The similarity between the simulated and experimental IR spectra lends credence to the simulation method and paves the way for a variety of applications.

Langmuir-Blodgett films formed by continuously varying surface pressure. Characterization by IR spectroscopy and epifluorescence microscopy

Monolayer films of phospholipids at the air-water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP. The molecular and supramolecular properties of the film constituents have been characterized with two complementary techniques. IR spectroscopy was used to monitor chain conformation as a function of transfer surface pressure. Results were compared to those from Langmuir films determined directly at the A/W interface by IR reflection-absorption spectroscopy (IRRAS). The methylene stretching frequencies for both proteated and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC and DPPC-d62) in the transferred molecules indicate that the phospholipids retain at least, in part, their surface pressure-dependent chain-conformational order characteristics. The line widths of these modes are somewhat reduced, suggestive of slower rates of reorientational motion in the Langmuir-Blodgett (LB) films. Epifluorescence microscopy reveals a progressive condensation gradient, including nucleation and growth of probe-excluding condensed domains along the transfer line. DPPC condensation, observed along a single LB film, was qualitatively comparable to compression-driven condensation as observed in situ or in conventional LB films transferred at constant pressures. However, condensation along the compression isotherm in COVASP-LB films was reduced by 15-20% as compared to films equilibrated at different constant pressures, probably the result of kinetic differences in equilibration processes. As a preliminary demonstration of the utility of this new approach, the monolayer --> multilayer transition known to occur (Eur. Biophys. J. 2005, 34, 243) in a four-component model for pulmonary surfactant has been examined. IR parameters from both the lipid and the protein constituents of the film all indicate that the transition persists during the transfer process. This new approach for the study of transferred films will permit the efficient characterization of lipid-protein interactions and structural transitions occurring in pulmonary surfactant films subjected to dynamic compression.

Imaging the prodrug-to-drug transformation of a 5-fluorouracil derivative in skin by confocal Raman microscopy

The widespread adoption of transdermal drug delivery has been limited by the barrier properties of the outermost layer of the epidermis, the stratum corneum (SC). A variety of approaches have been developed to overcome the barrier, including the use of a prodrug form of an active therapeutic agent to enhance transdermal delivery. Once in the epidermis, the pro-molecule is converted to the active drug by endogenous enzymes or simple chemical hydrolysis. The prodrug selected for the current studies, 1-ethyloxycarbonyl-5-fluorouracil, is known to enhance transdermal delivery of 5-fluorouracil, an important systemic antitumor drug. Using confocal Raman microscopy on pigskin biopsies treated with prodrug, we are able to image the spatial distribution of both prodrug and drug in the SC and viable epidermis, thereby providing information about permeation and metabolism. This approach may readily be extended to a variety of dermatological processes.

Vibrational microscopy and imaging of skin: from single cells to intact tissue

Vibrational microscopy and imaging offer several advantages for a variety of dermatological applications, ranging from studies of isolated single cells (corneocytes) to characterization of endogenous components in intact tissue. Two applications are described to illustrate the power of these techniques for skin research. First, the feasibility of tracking structural alterations in the components of individual corneocytes is demonstrated. Two solvents, DMSO and chloroform/methanol, commonly used in dermatological research, are shown to induce large reversible alterations (α-helix to β-sheet) in the secondary structure of keratin in isolated corneocytes. Second, factor analysis of image planes acquired with confocal Raman microscopy to a depth of 70 μm in intact pigskin, demonstrates the delineation of specific skin regions. Two particular components that are difficult to identify by other means were observed in the epidermis. One small region was formed from a conformationally ordered lipid phase containing cholesterol. In addition, the presence of nucleated cells in the tissue (most likely keratinocytes) was revealed by the spectral signatures of the phosphodiester and cytosine moieties of cellular DNA.

Surfactant protein C and lung function: new insights into the role of α-helical length and palmitoylation

Surfactant protein C (SP-C) is known to be essential for lung function and the formation of a surface confined reservoir at the alveolar interface. The structural features relevant for the peptide's extraordinary ability to form extended three-dimensional structures were systematically investigated and are summarized in the present paper. The influence of palmitoylation was studied for full length SP-Cs as well as truncated variants with the N-terminal residues 1-17 and 1-13, respectively. The combined results from film balance measurements, fluorescence microscopy (FLM) and scanning force microscopy (SFM) reveal a fine-tuned balance between the influence of the palmitoyl chains and alpha-helical length. Native SP-C added to DPPC/DPPG monolayers (molar ratio 80:20) induced the formation of the surface confined reservoir independent of its palmitoylation degree. However, topographic images revealed that only bilayers and not multilayers where formed when the acyl chains were missing. The influence of palmitoylation increased when alpha-helical length was considerably reduced to 17 or even 13 amino acid residues. In these strongly truncated SP-C peptides palmitoyl chains increased monolayer stability and anchored the peptides in the lipid film. However, no multilayer formation was observed at all for all shortened peptides. The alpha-helix of SP-C seems to be a prerequisite for the formation of extended three-dimensional structures and obviously has to be able to span a lipid bilayer. Palmitoylation obviously mediates interactions between lipids and/or peptides not only within a protein/lipid film but also between neighbouring layers and induces a stacking of bilayers.

Vibrational microspectroscopy and imaging of molecular composition and structure during human corneocyte maturation

The outermost region of the epidermis, the stratum corneum (SC), provides an essential barrier to water loss and protects against exogenous substances. The functional integrity of the SC depends on a complex maturation and exfoliation process, which is often perturbed in skin diseases. The maturation of corneocytes isolated from different depths in healthy human SC was investigated using infrared (IR) spectroscopic imaging and Raman microscopy. Both IR and Raman spectral quality of individual corneocytes was high and revealed depth-dependent variations in molecular composition. Spectral changes were identified as arising from alterations in the concentration of the major constituents of natural moisturizing factor (NMF), important in maintaining SC hydration. A significant decrease in the concentration of NMF was observed for corneocytes isolated from superficial compared to deeper SC layers (layer 3 vs. layer 11, respectively). An IR parameter that measures the relative NMF concentration in corneocytes is introduced. The potential role of vibrational imaging to evaluate corneocyte composition and molecular structure in the treatment of NMF-related diseases is discussed.

Determination of molecular conformation and permeation in skin via IR spectroscopy, microscopy, and imaging

Skin tissue, in addition to its specific use in dermal research, provides an excellent model for developing the techniques of vibrational microscopy and imaging for biomedical applications. In addition to permitting characterization of various regions of skin, the relative paucity of major biological constituents in the stratum corneum (the outermost layer of skin), permits us to image, with microscopic resolution, conformational alterations and concentration variations in both the lipid and protein components. Thus we are able to monitor the effects of exogenous materials such as models for drug delivery agents (liposomes) and permeation enhancers (DMSO) on stratum corneum lipid organization and protein structure. In addition, we are able to monitor protein conformational changes in single corneocytes. The current article demonstrates these procedures, ranging from direct univariate measures of lipid chain conformational disorder, to factor analysis which permits us to image conformational differences between liposomes that have permeated through the stratum corneum from those which have remained on the surface in a reservoir outside the skin.

Feasibility of Tracking Phospholipid Permeation into Skin Using Infrared and Raman Microscopic Imaging

The feasibility of monitoring the permeation of chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC-d62) and 1-palmitoyl-d31, 2-oleoylphosphatidylcholine (P-d31OPC) vesicles into pigskin using infrared (IR) microscopic imaging and confocal Raman microscopy was demonstrated. The former technique permits the examination of the relative concentration of molecular species (e.g., endogenous and exogenous lipids and proteins) over spatial areas, approximately 1 mm, with a spatial resolution of approximately 10-12 microm. In contrast, Raman microscopy allows the confocal examination of tissue at depths up to 100 microm with a pixel size of about 2-3 microm3. Spectral signal/noise, however, is reduced from IR and significantly smaller areas are generally monitored. The permeation of the gel phase DPPC-d62 was limited to approximately 5-15 microm, whereas the liquid-crystalline phase P-d31OPC permeated to substantially greater depths (35-100 microm), at times ranging up to 24 h after application. The results are generally in accord with literature values. In addition, the state of the P-d31OPC (intact vesicles or molecularly dispersed with skin constituents) was evaluated from the spatial dependence of the deuteriopalmitate chain conformational order. Upon permeation, the chains became more ordered. The advantages and limitations of these imaging technologies are discussed.

Monolayer–multilayer transitions in a lung surfactant model: IR reflection–absorption spectroscopy and atomic force microscopy

A hydrophobic pulmonary surfactant protein, SP-C, has been implicated in surface-associated activities thought to facilitate the work of breathing. Model surfactant films composed of lipids and SP-C display a reversible transition from a monolayer to surface-associated multilayers upon compression and expansion at the air/water (A/W) interface. The molecular-level mechanics of this process are not yet fully understood. The current work uses atomic force microscopy on Langmuir-Blodgett films to verify the formation of multilayers in a dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, cholesterol, and SP-C model system. Isotherms of SP-C-containing films are consistent with exclusion and essentially complete respreading during compression and expansion, respectively. Multilayer formation was not detected in the absence of SP-C. Most notable are the results from IR reflection-absorption spectroscopy (IRRAS) conducted at the A/W interface, where the position and intensity of the Amide I band of SP-C reveal that the predominantly helical structure changes its orientation in monolayers versus multilayers. IRRAS measurements indicate that the helix tilt angle changed from approximately 80 degrees in monolayers to a transmembrane orientation in multilayers. The results constitute the first quantitative measure of helix orientation in mixed monolayer/multilamellar domains at the A/W interface and provide insight into the molecular mechanism for SP-C-facilitated respreading of surfactant.

A Quantitative Reconstruction of the Amide I Contour in the IR Spectra of Globular Proteins: From Structure to Spectrum

The Amide I contours of six globular proteins of varied secondary structure content along with a peptide model for collagen and pulmonary surfactant protein C have been simulated very closely by using a modified GF matrix method. The starting point for the method uses the three-dimensional structure as obtained from the Protein Data Bank. Elements of the interactions between peptide groups (e.g., transition dipole coupling) are very sensitive to tertiary structure, thus the current formalism demonstrates that the Amide I contour may be useful for a more detailed probe of 3-D conformation that goes beyond the traditional use of this band to probe the percentages of particular elements of secondary structure. For example, postulated changes to a known structure can be tested by comparing the new simulated band to the experimental band. A number of refinements to the transition dipole interaction calculation have been made. Most of the important interactions between the C=O oscillators that define the Amide I mode appear to have been identified, including through space transition dipole coupling, through valence bond and through hydrogen bond coupling. The eigenvector matrix produced by the method permits the contribution of each peptide group to the spectrum to be precisely determined. Analysis of the results shows that the often-used structure-frequency correlations are at best approximate and at worst misleading. The subbands from helices, sheets, turns, and loops are much broader and more overlapped than has been commonly assumed. Furthermore, the traditional alpha-helical marker band may be substantially distorted in short segments. Difference spectra based on isotope editing, a technique thought capable of revealing the spectral contributions of individual peptide groups, are shown to be prone to misinterpretation.

Orientation of peptides in aqueous monolayer films. Infrared reflection-absorption spectroscopy studies of a synthetic amphipathic beta-sheet

Infrared reflection-absorption spectroscopy (IRRAS) intensities of the Amide I vibration are used to develop a quantitative approach for determining the Euler angles that describe the orientation of protein beta-sheets in aqueous monolayer films. A synthetic amphipathic peptide, Val-Glu-Val-Orn-Val-Glu-Val-Orn-Val-Glu-Val-Orn-Val-OH is used as a test case. The pattern of Amide I frequencies suggests that the molecule is organized as an antiparallel beta-sheet at the air/water interface. The model used to simulate the Amide I intensities reveals that the beta-sheet has a slight preferential alignment parallel to the direction of compression; i.e., deviation from uniaxial symmetry is observed. In addition, the sheet is found to lie flat on the aqueous surface, with (presumably) the polar side chains interacting with the aqueous subphase. Limitations and advantages of the theoretical approach are discussed.

Uncertainties in depth determination and comparison of multivariate with univariate analysis in confocal Raman studies of a laminated polymer and skin

Confocal Raman microscopy data are reported for a laminated polymer (Paramount) and for pigskin. The nature of the laminated structure of the polymer provides a useful test for evaluation of thickness distortions in confocal measurements in soft samples, which are found to be quite significant. The spatial variation in line profiles generated from univariate analyses with scores derived from factor loadings are consistent for both samples and provide distinct diagnostic markers for stratum corneum and epidermis regions of skin. Univariate analysis of the C-C stretching region of skin reveals a spatial dependence of chain conformational order. In addition, variations in keratin-containing areas of the stratum corneum are readily identified from area maps of the S-S stretching vibrations. These data indicate that confocal Raman imaging studies of molecular structure changes in particular regions of skin during pathological processes will prove quite valuable in dermatology.

An infrared reflection-absorption spectroscopy study of the secondary structure in (KL4)4K, a therapeutic agent for respiratory distress syndrome, in aqueous monolayers with phospholipids

KLLLLKLLLLKLLLLKLLLLK (KL(4)) has been suggested to mimic some aspects of the pulmonary surfactant protein SP-B and has been tested clinically as a therapeutic agent for respiratory distress syndrome in premature infants [Cochrane, C. G., and Revak, S. D. (1991) Science 254, 566-568]. It is of obvious interest to understand the mechanism of KL(4) function as a guide for design of improved therapeutic agents. Attenuated total reflection (ATR) IR measurements have indicated that KL(4) is predominantly alpha-helical with a transmembrane orientation in lipid multilayers (1), a geometry quite different from the originally proposed peripheral membrane lipid interaction. However, the lipid multilayer model required for ATR may not be the best experimental paradigm to mimic the in vivo function of KL(4). In the current experiments, IR reflection-absorption spectroscopy (IRRAS) was used to evaluate peptide secondary structure in monolayers at the air/water interface, the physical state that best approximates the alveolar lining. In contrast to the ATR-IR results, KL(4) (2.5-5 mol %) films with either DPPC or DPPC/DPPG (7/3 mol ratio) adopted an antiparallel beta-sheet structure at all surface pressures studied > or =5 mN/m, including pressures physiologically relevant for lung function (40-72 mN/m). In contrast, in DPPG/KL(4) films, the dominant conformation was the alpha-helix over the entire pressure range, a possible consequence of enhanced electrostatic interactions. IRRAS has thus provided unique molecular structure information and insight into KL(4)/lipid interaction in a physiologically relevant state. A structural model is proposed for the response of the peptide to surface pressure changes.

Location of structural transitions in an isotopically labeled lung surfactant SP-B peptide by IRRAS

Pulmonary surfactant, a lipid/protein complex that lines the air/water interface in the mammalian lung, functions to reduce the work of breathing. Surfactant protein B (SP-B) is a small, hydrophobic protein that is an essential component of this mixture. Structure-function relationships of SP-B are currently under investigation as the protein and its peptide analogs are being incorporated into surfactant replacement therapies. Knowledge of the structure of SP-B and its related peptides in bulk and monolayer phases will facilitate the design of later generation therapeutic agents. Prior infrared reflection-absorption spectroscopic studies reported notable, reversible surface pressure-induced antiparallel beta-sheet formation in a synthetic peptide derived from human SP-B, residues 9-36 (SP-B(9-36)). In the current work, infrared reflection-absorption spectroscopy is applied in conjunction with isotopic labeling to detect the site and pressure dependence of the conformational change. SP-B(9-36), synthesized with (13)C=O-labeled Ala residues in positions 26, 28, 30, and 32, shifted the beta-sheet marker band to approximately 1600 cm(-1) and thus immediately identified this structural element within the labeled region. Surface pressure-induced alterations in the relative intensities of Amide I band constituents are interpreted using a semiempirical transition dipole coupling model. In addition, electron micrographs reveal the formation of tubular myelin structures from in vitro preparations using SP-B(9-36) in place of porcine SP-B indicating that the peptide has the potential to mimic this property of the native protein.

Secondary structure and lipid interactions of the N-terminal segment of pulmonary surfactant SP-C in Langmuir films: IR reflection-absorption spectroscopy and surface pressure studies

Pulmonary surfactant, a thin lipid/protein film lining mammalian lungs, functions in vivo to reduce the work of breathing and to prevent alveolar collapse. Analogues of two hydrophobic surfactant proteins, SP-B and SP-C, have been incorporated into therapeutic agents for respiratory distress syndrome, a pathological condition resulting from deficiency in surfactant. To facilitate rational design of therapeutic agents, a molecular level understanding of lipid interaction with surfactant proteins or their analogues in aqueous monolayer films is necessary. The current work uses infrared reflection-absorption spectroscopy (IRRAS) to determine peptide conformation and the effects of S-palmitoylation on the lipid interactions of a synthetic 13 residue N-terminal peptide [SP-C13(palm)(2)] of SP-C, in mixtures with 1,2-dipalmitoylphosphatidylcholine (DPPC) or 1,2-dipalmitoylphosphatidylglycerol (DPPG). Two Amide I' features, at approximately 1655 and approximately 1639 cm(-1) in the peptide IRRAS spectra, are assigned to alpha-helical peptide bonds in hydrophobic and aqueous environments, respectively. In binary DPPC/SP-C13(palm)(2) films, the proportion of hydrated/hydrophobic helix increases reversibly with surface pressure (pi), suggestive of the peptide being squeezed out from hydrophobic regions of the monolayer. No such effect was observed for DPPG/peptide monolayers, indicative of stronger, probably electrostatic, interactions. Depalmitoylation produced a weakened interaction with either phospholipid as deduced from IRRAS spectra and from pi-area isotherms. S-Palmitoylation may modulate peptide hydration and conformation in the N-terminal region of SP-C and may thus permit the peptide to remain in the film at the high surface pressures present during lung compression. The unique capability of IRRAS to detect the surface pressure dependence of protein or peptide structure/interactions in a physiologically relevant model for surfactant is clearly demonstrated.

Thermal stability and DPPC/Ca2+ interactions of pulmonary surfactant SP-A from bulk-phase and monolayer IR spectroscopy

Surfactant protein A (SP-A), the most abundant pulmonary surfactant protein, is implicated in multiple biological functions including surfactant homeostasis, biophysical activity, and host defense. SP-A forms ternary complexes with lipids and Ca2+ which are important for protein function. The current study uses infrared (IR) transmission spectroscopy to investigate the bulk-phase interaction between SP-A, 1,2-dipalmitoylphosphatidylcholine (DPPC), and Ca2+ ions along with IR reflection-absorption spectroscopy (IRRAS) to examine protein secondary structure and lipid orientational order in monolayer films in situ at the air/water interface. The amide I contour of SP-A reveals two features at 1653 and 1636 cm(-1) arising from the collagen-like domain and a broad feature at 1645 cm(-1) suggested to arise from the carbohydrate recognition domain (CRD). SP-A secondary structure is unchanged in lipid monolayers. Thermal denaturation of SP-A in the presence of either DPPC or Ca2+ ion reveals a sequence of events involving the initial melting of the collagen-like region, followed by formation of intermolecular extended forms. Interestingly, these spectral changes were inhibited in the ternary system, showing that the combined presence of both DPPC and Ca2+ confers a remarkable thermal stability upon SP-A. The ternary interaction was revealed by the enhanced intensity of the asymmetric carboxylate stretching vibration. The IRRAS measurements indicated that incorporation of SP-A into preformed DPPC monolayers at a surface pressure of 10 mN/m induced a decrease in the average acyl chain tilt angle from 35 degrees to 28 degrees. In contrast, little change in chain tilt was observed at surface pressures of 25 or 40 mN/m. These results are consistent with and extend the fluorescence microscopy studies of Keough and co-workers [Ruano, M. L. F., et al. (1998) Biophys. J. 74, 1101-1109] in which SP-A was suggested to accumulate at the liquid-expanded/liquid-condensed boundary. Overall these experiments reveal the remarkable stability of SP-A in diverse, biologically relevant environments.