A calorimetric study of binary mixtures of dihydrosphingomyelin and sterols, sphingomyelin, or phosphatidylcholine

Abstract

The thermotropic properties of binary mixtures of D-erythro-n-palmitoyl-dihydrosphingomyelin (16:0-DHSM), D-erythro-n-palmitoyl-sphingomyelin (16:0-SM), cholesterol, lathosterol, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were studied by differential scanning calorimetry. Addition of sterol to 16:0-DHSM and 16:0-SM bilayers resulted in a progressive decrease in both the T(m) and the enthalpy of the main transition. The sterol-induced broad components in 16:0-DHSM endotherms had markedly lower enthalpies than those induced in 16:0-SM. Pretransitions recorded in 16:0-DHSM and 16:0-SM membranes responded differently to low concentrations of cholesterol. The presence of 5 mol % cholesterol increased the pretransition temperature in 16:0-SM bilayers, whereas it decreased the temperature in 16:0-DHSM membranes. Lathosterol behaved in general as cholesterol with regard to its effects on the thermotropic behavior of both sphingolipids, but it appeared to form more stable sterol-rich domains, as seen from the higher T(m) of the broad component, in comparison to cholesterol. Thermograms recorded on binary mixtures of 16:0-SM:16:0-DHSM and DPPC:16:0-DHSM showed that 16:0-SM mixed nearly ideally with 16:0-DHSM, whereas DPPC mixing was less ideal in a 16:0-DHSM membrane. In conclusion, we observed that 16:0-DHSM interactions with sterols differed from that seen with 16:0-SM, and that 16:0-DHSM mixed better with 16:0-SM than DPPC, which indicates that DHSM could function as a membrane organizer within laterally condensed domains.

FIGURE 1

Representative DSC heating thermograms of 16:0-DHSM and 16:0-SM bilayers containing cholesterol. Binary cholesterol:phospholipid mixtures containing 0–25 mol % cholesterol were heated at a rate of 0.5°C/min. The sterol mol % is indicated in the figure.

FIGURE 2

Transition temperatures of binary mixtures of sterol and sphingomyelins as a function of sterol content. Phospholipid membranes containing 0–25 mol % cholesterol or lathosterol were heated at a rate of 0.5°C/min. The bimodal transition was deconvoluted into a phospholipid main transition component (A), and a sterol-induced broad transition component (B).

FIGURE 3

Transition enthalpies of binary mixtures of sterol and sphingomyelins as a function of sterol content. Phospholipid membranes containing 0–25 mol % cholesterol or lathosterol were heated at a rate of 0.5°C/min. The enthalpy of the phospholipid main transition is shown in A, and the sterol-induced broad transition in B.

FIGURE 4

Effect of cholesterol on the pretransitions of 16:0-DHSM and 16:0-SM bilayers. 16:0-SM (A) and 16:0-DHSM (B) membranes containing 0 or 5 mol % cholesterol were heated at a rate of 0.5°C/min.

FIGURE 5

Representative DSC heating thermograms of binary mixtures of 16:0-DHSM and 16:0-SM or DPPC. The phospholipid mixtures containing 0, 25, 50, 75, or 100 mol % 16:0-DHSM were heated at a rate of 0.5°C/min. The molar ratios of the mixtures are indicated in the figure as X_DHSM.

FIGURE 6

Effect of DHSM inclusion into SM and PC membranes. Transition temperature (A) and enthalpy (B) of 16:0-DHSM membranes containing 0, 25, 50, 75, or 100 mol % 16:0-SM or DPPC. Phospholipid membranes were heated at a rate of 0.5°C/min. The data are the average values from three experiments mean ± SD.

FIGURE 7

Binary phase diagram of phospholipid mixtures. Phase diagrams of DPPC:16:0-DHSM, 16:0-SM:16:0-DHSM, and 16:0-SM:DPPC mixtures constructed from DSC data. Solid lines represent observed data that was corrected as described in the text. The dashed lines are ideal phase diagrams calculated according to Eqs. 1 and 2.

FIGURE 8

Pretransitions of SM and PC membranes as a function of 16:0-DHSM content. 16:0-SM or DPPC membranes containing 0, 25, 50, 75, or 100 mol % 16:0-DHSM were heated at a rate of 0.5°C/min. All data points are the average value from three experiments mean ± SD.