Unraveling tryptophan modulated 2D DPPC lattices: an approach toward stimuli responsiveness of the pulmonary surfactant.

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/H(2)O 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 NH(3)(+) of the zwitterionic tryptophan. In situ spectroscopy enabled assignment of relative orientations of the equivalent -CH(2) 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(+)(CH(3))(3)), ν(as) (PO(2)(-)), ν(s) (PO(2)(-)), ν(as) (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.