Structure and thermotropic properties of 1,3-dipalmitoyl-glycero-2-phosphocholine.

The structure and thermotropic properties of hydrated 1,3-dipalmitoyl-glycero-2-phosphocholine (beta-DPPC) have been studied by X-ray diffraction and differential scanning calorimetry. After prolonged storage at -3 degrees C, differential scanning calorimetry heating scans exhibit endothermic transitions at 27 degrees C and 37 degrees C, with transition enthalpies, delta H = 9.1 and 10.5 kcal/mol beta-DPPC, respectively (1 cal = 4.184 J). Upon cooling, the high temperature transition is completely reversible, whereas the low temperature transition is not. Prolonged incubation of hydrated beta-DPPC at low temperatures is necessary in order to regain the full enthalpy of the low temperature transition, indicating metastability of the low temperature form. X-ray diffraction studies indicate three different lamellar phases upon heating equilibrated beta-DPPC from -3 degrees C: (1) below 18 degrees C, a hydrated (14 mol water/mol beta-DPPC) "crystalline" bilayer phase, Lc, with an ordered hydrocarbon chain-packing mode and a bilayer periodicity d = 58 A; (2) between 30 degrees C and 35 degrees C, a hydrated (22 mol water/mol beta-DPPC) gel phase, L beta, with hexagonal chain-packing and d = 47 A; hydrocarbon chain interdigitation in this phase is suggested by the small bilayer periodicity, a sharp, symmetric wide-angle reflection at 1/4.2 A-1, an area per mol beta-DPPC at the interface of approximately 80 A2, electron density profiles and structure factor calculations using strip electron density models; (3) above 37 degrees C, a highly hydrated (48 mol water/mol beta-DPPC) liquid crystalline bilayer phase, L alpha, with d = 65 A. Previous nuclear magnetic resonance and neutron diffraction studies have suggested that in beta-DPPC the glycerol backbone adopts an orientation parallel to the bilayer surface, in contrast to its usual perpendicular orientation in alpha-DPPC. This conformation presumably results in an increased intramolecular chain separation, with consequent changes in the molecular packing, hydration and thermotropic behavior of beta-DPPC, compared to its positional isomer alpha-DPPC.

Fibrinogen: agreement of experimental and calculated hydrodynamic data with electron-microscopic models.

There is no general agreement on the size and shape of the fibrinogen molecule. We have studied the diffusion of the fibrinogen in solution by means of dynamic light scattering, nanosecond fluorescence depolarization and analytical ultracentrifugation. The results obtained under physiological concentration, pH an ionic strength are DT = 2.0 X 10(7) cm2sec-1, DR perpendicular = 40'000 sec 1. Nanosecond fluorescence depolarization yielded DR parallel = 1.6 x 10(6) sec-1. Tentatively this value is interpreted as DR parallel, namely rotational diffusion about the major axis of the molecule. The sedimentation coefficient is 8.1 S. The hydrodynamic parameters derived from our measurements were compared with those calculated on the basis of the models proposed by Hall and Slayter, Hudry-Clergeon and Marguerie et al., Bachmann and Lederer, and Köppel. The agreement is poor even if the degree of hydration is varied within wide limits. However, satisfactory agreement can be achieved by assuming a flexible molecule of about 900 A length corresponding to two end-to-end bound trinodular structures of the Hall and Slayter type, with a nodule diameter of about 45 A. Experimental evidence indicates that the discrepancies between the different models might be due to different techniques of sample preparation leading to different conformations of the molecule.