Calorimetry-Derived Composition Vectors to Resolve Component Raman Spectra in Phospholipid Phase Transitions

Abstract

Multidimensional least squares analysis is a well-established technique for resolving component vibrational spectra from mixed samples or systems. Component resolution of temperature-dependent vibrational spectra is challenging, however, due to the lack of a suitable model for the variation in sample composition with temperature. In this work, analysis of temperature-dependent Raman spectra of lipid membranes is accomplished by using "concentration" vectors independently derived from enthalpy changes determined by differential scanning calorimetry. Specifically, the lipid-bilayer phase transitions of DMPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) are investigated through Raman spectra acquired from individual, optically trapped vesicles in suspension as a function of temperature. Heat capacity profiles of the same vesicle suspension are measured using differential scanning calorimetry and numerically integrated to generate enthalpy change curves of each phase transition, which are in turn used to construct composition vectors. Multidimensional least squares analysis optimized for a fit to these composition vectors allows resolution of the component spectra corresponding to gel, ripple, and liquid-crystalline phases of the DMPC. The quality of fit of the calorimetry-derived results is confirmed by unstructured residual differences between the data and the model, and a composition variation predicted by the resolved spectra that matches the calorimetry results. This approach to analysis of temperature-dependent spectral data could be readily applied in other areas of materials characterization, where one is seeking to learn about structural changes that occur through temperature-dependent phase transitions.

Multivariate Analysis of Mixed Lipid Aggregate Phase Transitions Monitored Using Raman Spectroscopy

“…Qualitative comparisons with the spectra of single-phase lipids in the literature 32,34,61 indicate that the conformational order of the lipids in Component 8 (8LC/dliq) is very similar to that of the liquid crystalline L a state of long-chain lipids, but the chain packing is looser. Similarly, the lipids in Component 5 (5GL/ice) exhibit more conformational order and closer packing than gel phase lipids at comparable temperatures.…”

“…Kitt et al. 34 surveyed the use of physical models to resolve the spectra, X , from vibrational spectral matrices by incorporating additional information from complementary measurements into the curve-resolution scheme. They used composition information from calorimetric measurements to augment temperature-dependent Raman spectra collected during the lipid bilayer phase transition of optically trapped lipid vesicles.…”

“…31,32 Raman bands characteristic of gel and liquid crystalline lipids can be observed in the C-H and C-C stretching regions, even when collected from single vesicles. 33,34 The O-H stretch of water hydrogen-bonded to the lipids also provides also information regarding the structure of hydrated lipids. [35][36][37] One of the challenges of using vibrational spectroscopy to monitor lipid structural transitions is the difficulty of isolating components in overlapped mixture spectra.…”

“…Analysts use a range approaches including numerical constraints, 41 expanded measurement dimensions, 42 and concatenation or stacking of related measurements 43 to reduce this rotational ambiguity in analysis of spectral matrices of mixtures. Kitt et al 34 surveyed the use of physical models to resolve the spectra, X, from vibrational spectral matrices by incorporating additional information from complementary measurements into the curve-resolution scheme. They used composition information from calorimetric measurements to augment temperature-dependent Raman spectra collected during the lipid bilayer phase transition of optically trapped lipid vesicles.…”

Lipid droplet phase transition in freezing cat embryos and oocytes probed by Raman spectroscopy

“…To study the temperature evolution of the CC stretching region and avoid the problems with all-trans peak analysis, we used a simplified approach based on the CC spectrum linear decomposition into spectral components. This concept is already successfully tested on synthetic lipids (58,59). In approximation that the acyl chains of lipid molecules are in ordered all-trans conformation state at the low-temperature limit (below −100 °C) and the completely disordered at the high-temperature limit (above 10 °C), the CC region was described as the combination of spectral components corresponding to ordered, S o ( ω ), and disordered, S d ( ω ), states and linear background.…”