Experimental and Numerical Study of Molecular Rotational Diffusion in Gel-Like Media

The hindered rotational diffusion of biopolymers in porous or fibrous media plays a significant role in many industrial and natural processes. Nuclear magnetic relaxation experiments can be used to investigate how the gel matrix impedes the tumbling of molecules trapped in its pores. Indeed, molecular motions influence the relaxation rate by causing fluctuations of the local magnetic field experienced by the nuclear spins. Hence a measurement of relaxation times allows one to monitor the rotational diffusion of rigid molecules in gels and to detect matrix-induced anisotropic behavior. Experiments examining the 31P relaxation of cGMP trapped in polyacrylamide gels showed more than a threefold decrease in the rotational diffusion coefficient when the gel concentration was brought from zero to 30%. Similar experiments in agarose gels prove that nuclear magnetic relaxation measurements can effectively be used to extract valuable information about rotational diffusion inside gels. A Stokesian dynamics simulation of molecular diffusion in fibrous environments will complement the experimental studies. The mobility interactions between prolate spheroids in low Reynolds number linear flows have been derived. The expressions are exact at the level of forces, torques and stresslets, and the results are cast in a form suitable for numerical calculations. This extension of Stokesian dynamics to non-spherical particles forms the groundwork for computationally efficient, hydrodynamically accurate simulations of suspensions of rodlike particles.