Order and fluidity in the terminal methyl region of dipalmitoyl phosphatidylcholine multibilayers: A comparison of raman and 2H-NMR spectroscopy

Abstract

Deuterium magnetic resonance ( 2H-NMR) and Raman spectroscopy are used to investigate order and fluidity at the terminal methyl position in 16- d 3, 16′- d 3 dipalmitoylphosphatidylcholine (16- d 6 DPPC) multibilayers. These methods reveal substantial motion and disorder in the gel phase, 5–10°C below the gel-liquid crystal phase transition temperature ( T m ). The phase transition is sensed in the 2H-NMR spectrum as a reduction in the quadrupole splitting from 14 kHz to ∼3 kHz. In contrast, the Raman parameter used to characterize the CD 3 vibrations is quite insensitive to the melting process, although an analogous parameter does sense disordering at T m at the 10 and 10′ position in 10- d 2, 10′- d 2 DPPC. The difference in the response of the NMR and Raman parameters may arise because the vibrational spectrum of the CD 3 group is inhomogenously broadened and is therefore quite sensitive to alterations in the local environment around the methyl group. In contrast, the NMR quadrupole splitting is sensitive to both local motion of the methyl group and, near Tm, to motions of the CD 2 group induced by transgauche isomerizations further up the chain. The difficulties that arise when results from different spectroscopic techniques are compared are demonstrated.