New conformations and new types of helix sense reversals and defects in the chains of nonchiral poly(alkyl isocyanates).

Molecular mechanics studies of helix sense reversals and defects of the reg ularity in the helices of the same sense in the chains of poly(alkyl isocya nates) were performed. The effect of medium was studied by using different values of the dielectric constant. Additionally to the reversals between th e helix mirror images (MI) considered in the literature new types of revers als have been investigated. These reversals connect the helices having back bone torsion angles not only with opposite signs but also with inverse angl e values (MII). For the MI reversals we report in addition to the small ang le break conformations known from literature the discovery of new conformat ions of similar energy but with large angle breaks. Large angle reversals c an be responsible for up to 50% of the chain persistence length in non-pola r solvents. In vacuum MII reversals and defects have energies higher than M I reversals because of the strong electrostatic repulsion. At epsilon =3.5 some defects have energy even lower that that of MI reversals.