Conformational distribution functions extracted from residual dipolar couplings: A hybrid model based on maximum entropy and molecular field theory

Abstract

This paper describes a new approach for analysis of residual dipolar couplings (RDCs). The method, which focuses on construction of the conformational distribution function, is applied to 4-n-pentyl-4′-cyanobiphenyl in the nematic phase. The RDCs are calculated from a trajectory generated in a molecular dynamics (MD) simulation, based on a realistic atom–atom interaction potential. Computer simulation is an attractive method for investigating theoretical models for partially ordered systems since the answer is provided: we know the true orientational order and molecular structure. Our new approach is based on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: the additive potential (AP) model and the maximum entropy (ME) method. These models suffer, however, from serious limitations: the AP model requires a priori knowledge of the functional form of the torsional potential, whereas the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented here (which we call APME) does not require knowledge of the functional form of the intramolecular potential and is applicable to weakly ordered systems. This makes the APME model a potentially useful tool for investigations of conformations in biomacromolecules dissolved in dilute aqueous liquid crystals. In the investigation reported in the present study, the results from the APME analysis are in excellent agreement with the true molecular structure in the MD simulation. The estimation of the validity range indicates that the APME approach is applicable to weakly ordered systems as well as to conventional nematic mesophases.