Infrared spectroscopic investigations of pulmonary surfactant. Surface film transitions at the air-water interface and bulk phase thermotropism

Abstract

The molecular structure of the phospholipid component of intact pulmonary surfactant isolated from bovine lung lavage has been examined by Fourier transform infrared spectroscopy. Two different physical states of the surfactant were examined by means of different infrared spectroscopic sampling techniques. Transmission infrared experiments were used to study the surfactant in the bulk phase. In these experiments, the thermotropic behavior of the bulk surfactant was monitored by temperature-induced variations in the phospholipid acyl chain CH2 stretching frequencies. A broad phase transition (confirmed by differential scanning calorimetry) was noted with an onset temperature near 15 degrees C and a completion temperature near 42 degrees C. In addition to the bulk transmission experiments, external reflection infrared spectroscopy was used to examine surfactant films in situ at the air-water interface. As surface pressure was increased from 0 to 43 dyn/cm, a gradual and continuous decrease in the CH2 stretching frequency was noted for the surfactant. Thus, under surface pressures which correspond to large lung volumes in vivo, the surfactant acyl chains exist mostly in the ordered (trans) configuration. The frequency shift in the CH2 stretching mode is consistent with a continuous ordering of the acyl chains upon compression over the pressure range 0-43 dyn/cm, and implies that a weakly cooperative phase transition occurs in the hydrocarbon region of the surface film. The surface film transition is especially noted in the pressure-area curve of the surfactant and approximates in two dimensions the broad thermotropic phase transition of the bulk phase surfactant. Substantial differences were observed between the response to surface pressure changes of intact surfactant compared with the main surfactant phospholipid, 1,2-dipalmitoyl-sn--glycero-3-phosphocholine. The changes in response are attributed to the presence of additional surfactant components. The current work demonstrates the ability of infrared spectroscopy to obtain structural information on the surfactant in physical states that directly relate to those in vivo.

Bulk flow driven by a viscous monolayer

“…DPPC is an amphiphilic molecule with twin hydrophobic tails which can form highly insoluble films on the surface of water. There is a vast body of literature on DPPC films (von Tscharner & McConnell 1981;Helm et al 1987;Dluhy et al 1989;McConnell 1991;Sundaram et al 1998;Dynarowicz-Latka, Dhanabalan & Oliveira 2001;Wüstneck et al 2001;Ding, Warriner & Zasadzinski 2002a;Ding et al 2002b;Roke et al 2003;Aroti et al 2004;Baoukina et al 2008;Nishimura et al 2008;Walder, Levine & Dennin 2008;Choi et al 2011;Hermans & Vermant 2014), the motivation being in part because it is the most prevalent component of lung surfactant. Furthermore, DPPC is widely studied because it is ubiquitous in bilayers that form cell walls.…”

Reflection–absorption FT-IR spectroscopy of pentadecanoic acid at the air/water interface

“…Amphiphiles that have been examined in this way include the fatty acids [8], their esters [9], long-chain alcohols [9], and phospholipids [2][3][4]10,11]. All of these materials possess a long alkyl chain and it is the RA bands arising from these chains that provide the clearest spectra and thus the most reliable information about molecular conformation in the monolayer.…”

Comparison between the behavior of different hydrophobic peptides allowing membrane anchoring of proteins

“…Moreover, there is a direct thermodynamic relationship between bilayers and monolayers [126,127]. In addition, a large number of spectroscopic (UV-visible absorption [128][129][130][131][132][133][134], fluorescence [128,[131][132][133][134][135][136], and vibrational [137][138][139][140][141][142][143], (see [120,[144][145][146][147][148][149][150][151][152] for a review)), microscopic (Brewster angle [153,154] and fluorescence [155][156][157], (see [158][159][160][161][162][163][164] for a review)), and different additional physical methods [165][166][167]…”

“…Two approaches have been developed to determine the structure and orientation of peptides in monolayers at the air-water interface by infrared spectroscopy : infrared reflection absorption spectroscopy (IRRAS) [141][142][143] and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS, Fig. 8) [137].…”