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Unraveling tryptophan modulated 2D DPPC lattices: An approach toward stimuli responsiveness of the pulmonary surfactant
Date Issued
24-11-2011
Author(s)
Sarangi, Nirod Kumar
Indian Institute of Technology, Madras
Abstract
A molecular understanding on the preferential and selective interactions of L-tryptophan, a major component of surfactant proteins, with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) is important in the metabolic cycle of the pulmonary surfactant. In view of this, interfacial signals of interest in real time were tapped with aligned DPPC monolayers over a physiological tryptophan subphase using extremely surface sensitive 2D vibrational spectroscopy. Polarization-modulated and angle dependent Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) of DPPC monolayers on water and L-tryptophan subphases depicted fine structure/conformation differences in the interaction modes, evidenced from changes in the vibrational band intensities and frequencies under conditions of controlled 2D surface pressure. The computed 1:1 adducts of DPPC/H2O and DPPC/tryptophan in support of FT-IRRAS fine structure characteristics demonstrated binding in interfacial DPPC-tryptophan adducts to be driven by cation - π interactions alongside hydrogen bonding of carbonyl and phosphate groups of the lipid with NH3+ of the zwitterionic tryptophan. In situ spectroscopy enabled assignment of relative orientations of the equivalent - CH2 functional groups from the polarized XY plane transition moments with component intensities of the split orthorhombic CH 2 mode. A larger molecular tilt of 37° for the DPPC monolayer over tryptophan subphase in comparison with that over water (26°) substantiated the DPPC headgroup interaction with tryptophan, complemented through δ (N+(CH3)3), νas (PO2-), νs (PO2-), νas (C - N+ - C), and ν (C=O) vibrational features. The IRRAS spectral features of the DPPC 2D condensed phase showed distinct tryptophan-induced temperature dependent lattice phase transitions: hexagonal → orthorhombic → triclinic → hexagonal packing of the hydrocarbon chains was noted over a subphase temperature range from 20 to 43 °C. The temperature dependent 2D DPPC lattice characteristics cited in this work will aid in understanding the impact of a temperature pulse toward the membrane functionality. © 2011 American Chemical Society.
Volume
115