Conformational variability of helix sense reversals in poly(methyl isocyanate)

The conformations and energies of several helix sense reversal geometries in poly(methyl isocyanate) (PMIC) have been determined using the PCFF forcefield. In an extension of previous studies, a larger conformational variability for a helix sense reversal has been investigated. In addition to the reversal geometry previously detailed by several authors that results in a relatively small angle deviation from the rod-like polyisocyanate structure, we report the discovery of reversals of similar energy with much larger angle deviations from linearity. The effect of electrostatic interactions as controlled by the value of the dielectric constant, ε, on the conformation and energy of a reversal is also shown to be important. At ε=1.0 (vacuum) the conformations of the reversals with large and small angle ‘kinks’ have similar energies. However, at ε=2.0 (non-polar organic solvent) and ε=3.5 (bulk state) the reversals corresponding to the large angle kinks have lower energies.