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 20.0degreesC), 1.3 < (s0) x 10(13) < 3.2 s; the Gralen coefficient (sedimentation concentration-dependence parameter), 10 < k(s) <70 cm(3) g(-1); the translational diffusion coefficient, 3.9 < D-0 x 10(7) < 15.4 cm(2) s(-1); the intrinsic viscosity, 7.9 < [η] < 40 cm(3) g(-1). Combination of s(0) with D-0 using the Svedberg equation yielded molecular weights in the range 3.9 < M x 10(-3) < 37 g mol(-1). The value of the mass per unit length of the heparin molecule, M-L, was determined using the theory of hydrodynamic properties of a weakly bending rod, giving M-L = 570 +/- 50 g nm(-1) mol(-1). The equilibrium rigidity, Kuhn segment length (A = 9 +/- 12 nm) and hydrodynamic diameter (d = 0.94 +/- 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 semirigid polymer, with an equilibrium rigidity that is essentially determined by a structural component, the electrostatic contribution being negligible in 0.2 M NaCl.