No abstract

1060PETROSELINIC ACID ones. Another important factor is the steric hindrance of the bulky carboxyl groups. The parallel orientation of the carboxyl groups is energetically disadvantageous. In the case of n-fatty acid A-super and A2 forms adopting the triclinic subcell with parallel skeletal planes (771), the carboxyl groups take two different orientations in order to reduce the steric hindrance. The formation of an O_l_-like subcell of the HM phase seems to have an analogous effect of promoting the perpendicular packing of carboxyl groups.The thermal factors of the C atoms increase gradually from the cis-olefin group as the methyl terminal is approached. This tendency has been also found in the Yl phase of erucic acid, which shows a reversible phase transition accompanied by conformational disorder at the methyl terminal (Kobayashi & Kaneko, 1990). The thermal motion of the methyl-terminal chains may be activated by the insertion of the cis-olefm group. However, the thermal factor of the methyl C atom is relatively small in comparison to that of the y~ phase. It is inferred that the thermal motion of the methyl-terminal chain is depressed by two factors, the relatively long chain length and the even number of C atoms in the methyl-terminal chain. Petroselinic acid contains three more methylene units in the methyl-terminal chains than erucic acid. The even--odd effect of the number of C atoms on the melting points and crystal structures of nalkanes and n-fatty acids is well known. This dynamical property of the HM phase probably relates to the fact that there is no order-disorder-type phase transition as found in the other cis-monoenic acids.In both molecules A and B, the carboxyl groups are located nearly coplanar to the skeletal planes of the carboxyl-terminal chains.