Conformation of heparin studied with macromolecular hydrodynamic methods and X-ray scattering.

The hydrodynamic characteristics of heparin fractions in a 0.2 M NaCl solution have been determined. Experimental values varied over the following ranges: the sedimentation coefficient (at 20.0 degrees C), 1.3<s0x10(13)<3.2 s; the Gralen coefficient (sedimentation concentration-dependence parameter), 10<ks<70 cm3 g(-1); the translational diffusion coefficient, 3.9< D0x10(7)<15.4 cm2 s(-1); the intrinsic viscosity, 7.9<[eta]<40 cm3 g(-1). Combination of s0 with D0 using the Svedberg equation yielded molecular weights in the range 3.9<Mx10(-3)<37 g mol(-1). The value of the mass per unit length of the heparin molecule, ML, was determined using the theory of hydrodynamic properties of a weakly bending rod, giving ML=570 +/- 50 g nm(-1) mol(-1). The equilibrium rigidity, Kuhn segment length (A=9 +/- 2 nm) and hydrodynamic diameter (d=0.9 +/- 0.1 nm) of heparin were evaluated on the basis of the worm-like coil theory without the excluded volume effect, using the combination of hydrodynamic data obtained from fractions of different sizes. Small-angle X-ray scattering for three heparin fractions allowed an estimate for the cross-sectional radius of gyration as 0.43 nm; from the evolution with the macromolecule contour length of the radius of gyration, a value for the Kuhn segment length of 9 +/- 1 nm was obtained. A good correlation is thus observed for the conformational parameters of heparin from hydrodynamic and X-ray scattering data. These values describe heparin as a semi-rigid polymer, with an equilibrium rigidity that is essentially determined by a structural component, the electrostatic contribution being negligible in 0.2 M NaCl.