Spectral distortion and sample heating in 1064 nm FT-Raman spectra of aqueous dispersions of dipalmitoylphosphatidylcholine

Abstract

To estimate the spectral distortion and the sample heating in the 1150-700 cm −1 NIR FT - Raman spectrum of NIR-absorptive scattering sample, we measured 1064-nm excited FT - Raman spectra of H 2O and D 2O dispersions of dipalmitoylphosphatidylcholine (DPPC). To evaluate the differential NIR Raman signal absorption by sample, which may result in the spectral distortion, and the absorptions of Rayleigh scattering and exciting laser, which may cause sample heating, we measured the NIR absorption spectra of DPPC - H 2O and DPPC - D 2O dispersions. Turbidity measurements at 1064, 1200 and 1540 nm as a function of temperature were performed for the H 2O and D 2O dispersions to evaluate turbidity changes in the phase transitions. The following observations were made: (1) The NIR Raman signal absorptions free from the sample heating effect were estimated to be about 20% and 4–8% in the H 2O and D 2O dispersions, respectively, for the CC skeletal optical modes (SOMs) and for the 874 and 889 cm −1 vibrations. (2) The Raman intensity attenuations caused by sample heating were about 10% for the CC SOMs, while an anomalous spectral artifact was observed in the 874 and 889 cm −1 features in comparison to those in the 514.5 nm Raman spectrum. It was interpreted in terms of the difference in the thermal behavior between the 874 and 889 cm −1 bands, that is, the thermal behavior of the 874 cm −1 band was similar to that of the CC SOMs, while the 889 cm −1 Raman intensity anomalously changed in comparison to that of the corresponding 514.5 nm Raman band, which is free from sample heating resulting from visible-region absorption by sample: about 35% decrease was estimated for both of the H 2O and D 2O dispersions. The local laser sample heating in the 1064 nm Raman measurement more significantly affected the 870–890 cm −1 spectral features than the CC SOM vibrations. (3) From the observed differential turbidity at 1200 nm in the two phases, about the 6 and 5% changes in the Raman intensities of the CC SOMs were estimated during the pre-transition for the H 2O and D 2O dispersions, respectively, and about 5% and 4% changes were estimated during the main transition. Since it may be estimated that the energy which had been absorbed from the NIR laser was near, but not beyond the Δ H for the pretransition of DPPC, it was concluded that usual discussions in model and natural biomembrane studies using the relative intensities of the CC SOMs will be still useful in the 1064 nm FT - Raman investigation of DPPC as long as the sample temperature is thermostatically set up so that the temperature of the sample spot irradiated by laser does not elevate over the pretransition temperature during Raman measurement.