Thermodynamic and structural characterization of amino acid-linked dialkyl lipids

Abstract

Using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), we determined some thermodynamic and structural parameters for a series of amino acid-linked dialkyl lipids containing a glutamic acid-succinate headgroup and di-alkyl chains: C12, C14, C16 and C18 in CHES buffer, pH 10. Upon heating, DSC shows that the C12, C14 and annealed C16 lipids undergo a single transition which XRD shows is from a lamellar, chain ordered subgel phase to a fluid phase. This single transition splits into two transitions for C18, and FTIR shows that the upper main transition is predominantly the melting of the hydrocarbon chains whereas the lower transition involves changes in the headgroup ordering as well as changes in the lateral packing of the chains. For short incubation times at low temperature, the C16 lipid appears to behave like the C18 lipid, but appropriate annealing at low temperatures indicates that its true equilibrium behavior is like the shorter chain lipids. XRD shows that the C12 lipid readily converts into a highly ordered subgel phase upon cooling and suggests a model with untilted, interdigitated chains and an area of 77.2A(2)/4 chains, with a distorted orthorhombic unit subcell, a=9.0A, b=4.3A and beta=92.7 degrees . As the chain length n increases, subgel formation is slowed, but untilted, interdigitated chains prevail.

Fig. 1

Sketch of the n = 14 member of the (Cn)₂-Glu-C2-COOH family.

Fig. 2

Calorimetric transition temperature as a function of pH and chain length. Circles are from the data in Fig. 3 at pH 10; triangles from pH 7.4; squares from pH 4.

Fig. 3

DSC heating scans (upward peaks) after brief incubation at −7 °C and cooling scans (downward peaks) of (Cn)₂-Glu-C₂-COOH lipids in CHES buffer at pH 10 as a function of chain length. The data shown were acquired with the Multicell DSC at 9−10 °C/h and are displaced on the ordinate for clarity.

Fig. 4

Main transition enthalpies (open circles) and also the sum of the enthalpies of both the lower and main transitions (solid squares) for the pH 10 data of Fig. 3 (initial scans). Lines are to guide the eye.

Fig. 5

X-ray diffraction patterns of fully hydrated lipids in pH 10 CHES buffer in capillaries. (A) (C12)₂-Glu-C₂-COOH at 10 °C. (B) (C12)₂-Glu-C₂-COOH at 48 °C. (C) (C12)₂-Glu-C₂-COOH held at 10 °C for one day after heating to 48 °C.

Fig. 6

Radially averaged X-ray intensities. (A) Low-angle data with eight orders of lamellar diffraction for (C18)₂-Glu-C₂-COOH lipid at 10 °C obtained from a capillary sample in pH 10 CHES buffer. (B) Wide-angle data collected at 10 °C in pH 10 CHES buffer, except that (C14)₂-Glu-C₂-COOH data were collected at 25 °C.

Fig. 7

FTIR spectra of (C12)₂-Glu-C₂-COOH and (C18)₂-Glu-C₂-COOH in the subgel phase (0 °C) and in the fluid phase.

Fig. 8

FTIR data (top) showing ester C=O bandwidth Δν (triangles) and CH₂ stretching ν (squares) during=heating of (C12)₂-Glu-C₂-COOH (open symbols) and (C18)₂-Glu-C₂-COOH (solid symbols). On the bottom are two heating scans from the DSC data of Fig. 3.

Fig. 9

Model of (C12)₂-Glu-C₂-COOH lipid at 10 °C.