SPECTRAL DISTORTION AND SAMPLE HEATING IN 1064 NM FT-RAMAN SPECTRA OFAQUEOUS DISPERSIONS OF DIPALMITOYLPHOSPHATIDYLCHOLINE

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 H2O and D2O dispersio ns of dipalmitoylphosphatidylcholine (DPPC). To evaluate the different ial NIR Raman signal absorption by sample, which may result in the spe ctral distortion, and the absorptions of Rayleigh scattering and excit ing laser, which may cause sample heating, we measured the NIR absorpt ion spectra of DPPC-H2O and DPPC-D2O dispersions. Turbidity measuremen ts at 1064, 1200 and 1540 nm as a function of temperature were perform ed for the H2O and D2O dispersions to evaluate turbidity changes in th e phase transitions. The following observations were made: (1) The NIR Raman signal absorptions free from the sample heating effect were est imated to be about 20% and 4-8% in the H2O and D2O dispersions, respec tively, for the CC skeletal optical modes (SOMs) and for the 874 and 8 89 cm(-1) vibrations. (2) The Raman intensity attenuations caused by s ample heating were about 10% for the CC SOMs, while an anomalous spect ral artifact was observed in the 874 and 889 cm(-1) features in compar ison to those in the 514.5 nm Raman spectrum. It was interpreted in te rms 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 n m Raman band, which is free from sample heating resulting from visible -region absorption by sample: about 35% decrease was estimated for bot h of the H2O and D2O dispersions. The local laser sample heating in th e 1064 nm Raman measurement more significantly affected the 870-890 cm (-1) spectral features than the CC SOM vibrations. (3) From the observ ed differential turbidity al 1200 nm in the two phases, about the 6 an d 5% changes in the Raman intensities of the CC SOMs were estimated du ring the pre-transition for the H2O and D2O 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 fr om the NIR laser was near, but not beyond the Delta H for the pretrans ition of DPPC, it was concluded that usual discussions in model and na tural biomembrane studies using the relative intensities of the CC SOM s will be still useful in the 1064 nm FT-Raman investigation of DPPC a s long as the sample temperature is thermostatically set up so that th e temperature of the sample spot irradiated by laser does not elevate over the pretransition temperature during Raman measurement. (C) 1997 Elsevier Science B.V.