Structural investigations of β′ triacylglycerols: An x‐ray diffraction and microscopic study of twinned β′ crystals

n = even) triacylglycerols which are β′-stable, Guinier X-ray powder diffraction photographs were used to determine cell parameters and space groups. The powder diffraction patterns are consistent with a pseudo-orthorhombic unit cell with space group Ic2a. Only one axis is changing as a function of chain lengthening. The experimental results were used to propose a β′-crystal packing for triacylglycerols. In contrast to earlier proposed β′-crystal structures, the acyl chains of this structure are not tilted with respect to the methyl-end group plane. Furthermore, with only one molecule in the asymmetric unit, overall orthogonal chain packing is obtained when the intramolecular acyl zigzag planes are parallel.Paper no. J8979 in JAOCS 76, 603-609 (May 1999)KEY WORDS: Crystal packing, fat, triacylglycerol, X-ray diffraction.Polymorphism of fats and triacylglycerols (TAG) is a permanent subject of research because it may have important implications for the manufacturing of many different food products such as chocolate and margarine. Pure TAG crystallize in the different polymorphic phases α, β, and/or β′ and sometimes even several different β′-and/or β-polymorphs exist (1,2). The different polymorphic phases and their variants can be distinguished by X-ray diffraction through typical reflections in the fingerprint region of the X-ray powder diagram (d-spacings from 3.0 to 6.0 Å) (3,4). Table 1 shows the characteristics of the X-ray patterns of the different phases. Depending on the TAG type the most stable phase is either β′ or β. De Jong and Van Soest (5) showed that TAG of the type C n C n+2 C n and C n C n+4 C n+2 (n = even) are normally β′-stable. Most other types are generally β-stable. TAG mixtures also show differences in phase stability. For example, milk fat is β′-stable (6) while cocoa butter has β as the most stable phase (7,8).The phase behavior of pure TAG as well as TAG mixtures is very complex. Their phase transitions are generally irreversible and depend on experimental circumstances such as temperature and time, and their thermal history as is demonstrated by memory effects (9). Knowledge of this behavior, particularly the β′ → β phase transition is very important, especially for the chocolate industry, since unwanted phase transitions of cocoa butter may cause fat bloom, a white appearance of fat crystals on the surface. Therefore, control of these transitions determine to a large extent the quality of chocolate products.Crystal structure information of the different phases is indispensable to understand the mechanism of these transitions. Unfortunately, so far only crystal structures of β-polymorphs of pure TAG have been reported (10-15), all [β-1,2,3-tridecanoyl-glycerol, or tricaprin, or C 10 C 10 C 10 (β-CCC); β-1,2,3-tridodecanoyl-glycerol, or trilaurin, or C 12 C 12 C 12 (β-LLL); β-1,2,3-trihexadecanoyl-glycerol, or tripalmitin, or C 16 C 16 C 16 (β-PPP); and β-2-11-bromoundecanoyl-1,3-didecanoyl-glycerol, or C 10 C 11 Br C 10 (β-CL Br C)] crystallizing in space group P -1. The molecules are i...

Section: Determination Of Cell Parameters and Space Groupsupporting

confidence: 88%

mentioning

confidence: 99%

Crystal packing of a homologous series β′‐stable triacylglycerols

Langevelde

Malssen

Sonneveld

et al. 1999

“…Twinned crystals of/3' LML were grown as reported earlier (1). A TopoMetrix (Darmstadt, Germany) TMX 2000 Atomic Force/Lateral Force Microscope was used to probe the c surface structure of p' LML crystal.…”

Section: Methodsmentioning

confidence: 99%

“…A crystallographic investigation of twinned crystals of the f3' modification of the triacylglycerol LML (C12C14C12) was reported recently (1). The unit cell dimensions of/T LML area--22.9A, b = 5.7A, c =66.8A, a= 90 ° ,f3= 91 ° and ), --90 °.…”

mentioning

confidence: 98%

Alkyl chain packing in a β′ triacylglycerol measured by atomic force microscopy

Birker

Blonk

1993

“…For TAG of the type p.p+2.p (p even, saturated), for which β′ is the most stable polymorph, only rough structural information is known. In 1991, Birker et al (6) published a variety of structural elements of the β′-2 structure of 12.14.12, but a final structure at the atomic level could not be obtained. A summary of their findings will be given here.…”

mentioning

confidence: 99%

Structural analogy between β′ triacylglycerols andn‐alkanes. Toward the crystal structure of β′‐2 p.p+2.p triacylglycerols

Streek

Verwer

Gelder

et al. 1999

The structural elements of the β′-2 polymorph of 1,3-dilauroyl-2-myristoylglycerol as found by Birker et al. (J. Am. Oil Chem. Soc. 68:895-906,1991) were also observed in the crystal structures of other long-chain compounds. This analogy led to the assembly of a β′-2 structure at the atomic level from known crystallographic data. The structure was optimized by molecular mechanics and was consistent with experimental data, including satisfactory reproduction of the X-ray powder pattern. To the best of our knowledge, this is the first β′ structure with a 1,2 configuration and an intramolecular orthorhombic subcell which is fully optimized by molecular mechanics to date. It shows all structural elements found earlier by Birker et al.Triacylglycerols (TAG) are esters of glycerol with three fatty acids. In the style of De Jong (1), we will denote a TAG by p.q.r where p, q, and r are the lengths of its three fatty acids, e.g., 12.14.12 for 1,3-dilauroyl-2-myristoylglycerol. TAG are important ingredients in many foods. In some foods such as margarines, where they are present in crystalline form, the particular polymorph determines the overall physical properties. Many polymorphs are known, and the stable ones are roughly classified as β and β′ according to their X-ray powder diagrams. Several structural features are recognized for these TAG polymorphs. The molecular configuration (Fig. 1) will have a direct effect on the conformation of the glycerol moiety. The arrangement of two TAG (Fig. 2), in combination with the tilt of the alkane chains with respect to the plane through the methyl end groups, determines the long-spacing reflections in the powder diagram. The subcell of the acyl chains (Fig. 3) is expected to be responsible for the characteristic short-spacing reflections in the powder diagrams: one at 4.6 Å for β, and two or more around 3.8 and 4.2 Å for β′. For pure compounds, p, q, r, their parity, and the presence of any double bonds determine which polymorph has the lowest free energy. Only the structure of the most stable polymorph of p = q = r or p.p.p (p even, saturated) is known in detail (2-5). This is a β-2 modification. For TAG of the type p.p+2.p (p even, saturated), for which β′ is the most stable polymorph, only rough structural information is known. In 1991, Birker et al. (6) published a variety of structural elements of the β′-2 structure of 12.14.12, but a final structure at the atomic level could not be obtained. A summary of their findings will be given here.From the long-spacing reflections, it was clear that the TAG molecules formed a β′-2 arrangement. The unit cell parameters for 12.14.12 were determined to be a = 22.9 Å, b = 5.7 Å, c = 66.8 Å, α = 90°, β = 91°, γ = 90°, Z = 8, ρ = 1.016 g/cm 3 . This implied that there were four acyl chains along the c-axis. Systematic absences pointed to the orthorhombic space groups Ic2a or Im2a, but since β ≠ 90°, only the monoclinic subgroups I2 or Im were possible. It was stressed that FIG. 1. Configuration of a triacylglycerol (TAG). Two of the three...