Structure and Crystallization Behavior of the β Phase of Oleic Acid

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Structure and Crystallization Behavior of the β Phase of Oleic Acid

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Fumitoshi Kaneko,* Kazuhiro Yamazaki, Kenji Kitagawa, Takashi Kikyo, and Masamichi Kobayashi

Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan

Yasuyuki Kitagawa and Yoshiki Matsuura

Institute for Protein Research, Osaka University, Suita, Osaka 565, Japan

Kiyotaka Sato

Laboratory of Chemical Physics, Faculty of Applied Biological Science, Hiroshima University, Higashihiroshima, Hiroshima 724, Japan

Masao Suzuki

Oleochemical Research Laboratory, NOF Corporation, Ohama, Amagasaki, Hyogo 660, Japan

J. Phys. Chem. B, 1997, 101 (10), pp 1803–1809

DOI: 10.1021/jp963400a

Publication Date (Web): March 6, 1997

Copyright © 1997 American Chemical Society

Abstract

Crystallization and crystal structure of the β phase of oleic acid (cis-9-octadecenoic acid) have been investigated with morphological observation, X-ray diffraction, and DSC (differential scanning calorimeter). The morphology of growing crystals of the β phase depends significantly on supercooling. It was found that the β phase could be classified into at least two solid modifications, β₁ (mp 16.3 °C) and β₂ (mp 16.0 °C). The crystal structure analysis of the β₁ phase has been performed. The unit cell belongs to a triclinic system of P, and the asymmetric unit contains two crystallographically independent molecules, A and B. The molecular layer is a unique interdigitated structure, where the methyl group of molecule A and the carboxyl group of molecules B (or vice versa) are located on the same plane. For the conformation around the cis-CC bond, both molecules approximate to trans−cis−trans conformation, the first case for a cis-monounsaturated fatty acid. The methyl and carboxyl side chains together form a T subcell. On the basis of its crystal structure, it was speculated that the β₁ phase has a unique surface structure at the (001) face. The factors for the characteristic properties of the β phase were also discussed.

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Introduction

Unsaturated fatty acids are attracting considerable attention from various fields of science and technology. The use of unsaturated fatty acids and their derivatives has been becoming significantly important in the field of pharmaceutical, food, and cosmetic industries.

The functional activity of biological membranes is dependent on the amount and type of unsaturated fatty acids in phospholipids. The major part of phospholipids in biomembranes contains a saturated fatty acid and a cis-unsaturated fatty acid at the first and second positions of glycerol skeletons. Recent studies on phospholipids containing one unsaturated chain clarified that unsaturated acyl chains disturb stable close-packed structures of acyl chains, and therefore, many metastable states are generated.^1-4 Through a series of polymorphic studies on cis-unsaturated fatty acids, we have clarified that cis-unsaturation brings about great diversity in molecular conformation and packing.

Oleic acid (cis-9-octadecenoic acid) is the most abundant cis-unsaturated fatty acid in nature; it is distributed in almost all organisms. Therefore, the information about its molecular level structures and properties is fundamental to understand the behavior of unsaturated chains in biological systems and industrial products. Polymorphism, a phenomenon in which a compound forms different crystal structures by changing molecular conformation or aggregation state depending on conditions, reflects sensitively the structure and dynamical property of a compound. Through structural chemical studies on each polymorphic phase, we could understand the relationship between molecular and crystal structures and physicochemical properties.

Impurities have a large influence on the polymorphic behavior of long-chain compounds. Although samples of high purity are necessary to obtain faithful results, high-purity products of cis-unsaturated fatty acids have not been obtained. About a decade ago ultrapure oleic acid sample (guaranteed more than 99.9%) was produced with modern purification technology. Using this sample, we have studied the polymorphism of oleic acid,⁵ confirming the presence of the three polymorphic phases, α, β, and γ, as shown in Figure 1. The γ and β phases correspond to the already known low-melting and high-melting phases,⁶ and the former structure has already been determined.⁷ The γ phase transforms to the α phase reversibly at −2.2 °C on heating, accompanying a selective conformational disordering in the methyl terminal chains.⁸ The γ → α solid-state phase transition has been found in other cis-monounsaturated fatty acids:  palmitoleic acid, erucic acid, gondoic acid, and asclepic acid.^9-13