Quantitative Correlation of Raman Spectral Indicators in Determining Conformational Order in Alkyl Chains

Abstract

The correlation of Raman spectral indicators for the determination of alkyl chain interactions and conformational order is presented. These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions. Spectral indicators are quantitatively correlated to the I[ν a (CH 2 )]/I[ν s (CH 2 )], as this is the primary indicator of rotational and conformational order obtained empirically from Raman spectra. These indicators are interpreted in terms of alkane intramolecular motion, intermolecular interactions between alkyl chains, crystal structure of these solid materials, and the presence of methylene conformers. Results demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation. These results can be used to understand molecular interactions and structure-function relationships in alkane-based materials.

Vibrational spectroscopic studies of newly developed synthetic biopolymers

“…The peak frequencies increase slightly and the chains further decouple during the phase transition. 45 For GDM-12, we observed a frequency shift of 4-6 cm 21 for the transition from the gel to liquid stage. (CH 2 )] ratio decreases from 1.31 to 1.22.…”

“…27,41 Several techniques, including variable-temperature infrared and Raman spectroscopy, have been extensively applied in order to monitor the change of the inter-and intramolecular order of lipid molecules during thermal transitions. 27,[41][42][43][44][45][46] In temperature-dependent infrared spectroscopy, the shifts in the vibrational peak positions in the CÀ ÀH stretching region as a function of temperature is utilized to derive the phase transition temperature profiles of PEGylated lipids. Furthermore, the frequencies corresponding to the CÀ ÀH stretching vibrational region (3000-2800 cm 21 ) can be used as indicators of order-disorder transitions of acyl chains because of their strong sensitivities to the conformation of the hydrocarbon chain.…”

“…51 The conformational order analysis presented in this article has been based on the prediction of Larsson and Rand, 42 which were subsequently expanded by Orendorff et al 45 From The plot is divided into three different sections by dashed lines representing the conformational order of the lipid before phase transition, during phase transition and after phase transition. As exhibited in this figure, the gel phase spans a temperature range from 24 to 2.48C, the biphasic (during phase transition) region lies between 2.4 to 5.68C, and the liquid phase ranges from 5.6 to 128C.…”

“…The peak frequencies are highly sensitive to subtle changes in chain interactions that may dampen or otherwise affect m s (CH 2 ) vibrations while the I[m s (CH 3 )]/I[m s (CH 2 )] ratio is sensitive to larger coupling interactions that affect the terminal methyl group in the lipids. 45 The m s (CH 2 ) vibrations are damped by highly coupled acyl chains in the gel stage and are shifted to a lower frequency. As the thermal energy of the system increases in the gel, the chains decouple slightly and the frequencies increases.…”

“…The width of this peak continues to increase as the rotational disorder remains at its maximum throughout the liquid stage of the lipid in the higher temperature region. 45 …”