Monomer-to-Micelle Transition of Dihexanoylphosphatidylcholine: 13C NMR and Raman Studies

Academic Article

Abstract

  • The monomer-to-micelle transition of dihexanoylphosphatidylcholine has been studied by 13C NMR and Raman spectroscopy. Lipid 13C chemical shifts in a variety of solvents indicate that micellization chemical shift differences for many carbons can be successfully modeled as a simple transfer from a strong hydrogen bond donating solvent (methanol, CHCl3) to a hydrogen bond donor deficient/acceptor solvent (CCl4, THF, dioxane). This solvent transfer model fails to predict the micellization shift trends at three points in the lipid molecule: the acyl chain peaks, the carbonyls, and the glyceryl backbone methine carbon. The temperature dependence of the phospholipid 13C chemical shifts suggests that the acyl chain micellization shifts are due to conformational changes through the γ effect. This implies a 5% or 7% increase in Pt, the probability of a trans conformer in the acyl chain, for micellar vs. monomer lipid. A comparison of the micellization shifts of acyl chain carbons in dihexanoylphosphatidylcholine and its ether-linked analogue rac-dihexylphosphatidylcholine suggests similar small ordering effects in both micelles. Solvent transfer and the γ effect cannot account for the 13C micellization chemical shifts of the carbonyl and glyceryl backbone methine carbons. Therefore, a possible source of these shift discrepancies is a conformational change in the glyceryl backbone between monomer and micelle. Two regions of the Raman spectrum of dihexanoyl-PC micelles, namely the C-H and C-C stretching regions, are used to obtain independent structural information about the acyl chains. Data from the C-H stretching regions suggest that lateral interactions between the chains in the micelle are disrupted compared to ordered forms that occur at low temperature, thereby leading to reduced intensity for spectral features which depend on interchain interaction for their intensity. In addition, both the C-H and C-C stretching regions indicate the existence of substantial disorder in the hydrocarbon chain conformation. Comparison of the monomer and micelle spectra suggests that a slight increase in chain order may occur upon micelle formation. Thus, both techniques are consistent with the following changes in short-chain lecithin molecules upon micellization: the acyl chains become very slightly ordered and are in a less polar environment; at most only minor conformational changes occur in the glycerol backbone. This view of the monomer-to-micelle transition is discussed in relation to the interfacial activation of soluble phospholipases. © 1982, American Chemical Society. All rights reserved.
  • Authors

    Digital Object Identifier (doi)

    Pubmed Id

  • 23918230
  • Author List

  • Burns RA; Roberts MF; Dluhy R; Mendelsohn R
  • Start Page

  • 430
  • End Page

  • 438
  • Volume

  • 104
  • Issue

  • 2