Effect of Unsaturated Acyl Chains on Structural Transformations in Triacylglycerols. Oleksandr Mykhaylyk. Chris Martin

Transcription

1 Effect of Unsaturated Acyl hains on Structural Transformations in Triacylglycerols leksandr Mykhaylyk Department of hemistry, University of Sheffield, Sheffield, S3 7HF, UK hris Martin STF Daresbury Laboratory, Warrington, heshire, WA4 4AD, UK rystallisation and Physical Properties of Fat: From Molecules to Market, June 2008, Ghent University, Belgium

2 Motivation Apart from double and triple chain type of packing common in stable crystal structures of triacylglycerols there is an evidence for another organization of molecules occurring at the very beginning of crystallization from the melt before the formation of the stable crystal structures.

3 utline Stable triacylglycerol phases bservations of transient phases of triacylglycerols Structure of the transient phases Transformation of the transient phases

4 Packing of alkane chains within a layer of triacylglycerols 2L 3L b ort b hex a hex c ort, c hex a ort H // M // T // sub-α α β β

5 P o w d e r e ll 2. 2 rystal structure and X-ray powder diffraction pattern of β-tristearin (SSS) b c a L Lamellar periodicity in the crystal structure of β-sss 4669 SSS b c a Subcell structure formed by packing of acyl chains in the crystal structure of β-sss SAXS region WAXS region Notes. Structural data were taken from the ambridge rystallographic Database, published in A. van Langevelde et al., Acta ryst. B57, 2001, p.372. omputer programme Powderell 2.4 were used for the structural presentaton and calculations of the diffraction pattern (wavelength of X-ray radiation = 1.4Å)

6 Projection of the electron density along the layer normal reconstructed from X-ray scattering patterns 2-chain packing of triacylglycerols (2L) 3-chain packing of triacylglycerols (3L) 001 α1 002 γ Intensity, a. u. SSS q, Å α1 X-ray scattering 002 pattern α1 001 γ Intensity, a. u. 003 γ 004 γ 005 γ 007 γ SLnS q, Å S(z), electrons/å z, Å Projection of the electron density 1 S( z) = L l= 1 2πlz k( l) F( l) cos L S(z), electrons/å z, Å motif of packing (atomic positions) 2L 3L

7 Time resolved SAXS of isothermal crystallization of StSt α 1 (2L) phase α2 StSt α1 StSt γ StSt Period of lamella,l 54 Å 49 Å 72 Å 002γ γ (3L) 001α 1 004γ 001α 2 001γ 002α 1 003α 1 003γ 005γ T=30 T=20 004α 2 002α q, Å -1 T=55 What is this?

8 ocoa butter phase transitions SAXS of a cocoa butter cooled to 20 with a shear rate of 3 s-1 ocoa butter composition TAG % L PL PLP P PLSt PP St StLSt PSt PPSt StSt PStSt StAr StStSt Saturated TAG Monounsaturated TAG Polyunsaturated TAG Total Form II SAXS of a cocoa butter cooled to 20 (after MacMillan S. D. et al., rystal Growth and Design, V.2, 2002) Form I T=20 T=0 T= ? q, Å 0.5 SAXS of a cocoa butter cooled to 0 and heated up to 20

9 Isothermal crystallization of triacylglycerols (time resolved SAXS ) Lard PP St q, Å q, Å q, Å ??? This phase are restricted to pure TAGs containing both saturated and unsaturated acyl chains or mixtures thereof including natural fats

10 Isothermal crystallization of triacylglycerols with 18 acyl chains (time resolved SAXS) All chains are saturated: Tristearin (StStSt) All chains are unsaturated: Triolein () α 1 -phase, hexagonal 001 α1 α 1 -phase d=4.32 Å d=4.01 Å 001 α1 100 α1 003 α1 SAXS WAXS temperature temperature 273 K 348 K q, Å -1 q, Å -1 Blends StStSt + SAXS 223 K 313 K WAXS q, Å -1 q, Å -1 Saturated and unsaturated chains: StLnSt 95 wt% StStSt 5 wt% 67 wt% StStSt 33 wt% 001 2L 002 2L 003 2L 001 2L 003 2L 001 α2 002 α2 SAXS α 2 -phase 100 α2 WAXS q, Å -1 q, Å K 353 K temperature 003 α2 004 α2 temperature 273 K 323 K q, Å -1 q, Å -1

11 Isothermal crystallization of StLnSt γ phase 001α2 002α2 100α2 001sub-α2 peak sub-α2 003 sub-α2 peak sub-α2 005 sub-α2 110 sub-α2 200 sub-α2 001γ Transient structures: sub-α 2 phase, orthorhombic α 2 phase, hexagonal temperature q, Å -1 q, Å -1 SAXS WAXS

12 Isothermal crystallization of PP 001 sub-α2 peak sub-α2 peak 2 Transient structures: sub-α 2 phase, orthorhombic 110 sub-α2 200 sub-α2 001 α2 002 α2 α 2 phase, hexagonal 100 α2 Temperature q, Å -1 q, Å -1 SAXS WAXS after Mykhaylyk et al, J. Appl. ryst., 2007, p. s297.

13 The evolution of the structural characteristics of the α 2 -phase TAG Initial stage Final stage L, Å d, Å L, Å d, Å onditions of quenching StSt , PP 52.0 n/a n/a n/a StLnSt , PLnP , St PP , after Mykhaylyk et al, J. Appl. ryst., 2007, p. s297.

14 Four groups of projections of the electron density profile on the layer normal 001 α2 002 α2 S'(z), a. u. a -1, -1, 1, -1 (1, -1, -1, -1) -1, -1, -1, -1 (1, -1, 1, -1) b -1, 1, 1, -1 (1, 1, -1, -1) -1, 1, -1, -1 (1, 1, 1, -1) y, 003 α2 004 α2 S'(z), a. u. d 25 Å -1, -1, 1, 1 (1, -1, -1, 1) -1, -1, -1, 1 (1, -1, 1, 1) c -1, 1, 1, 1 (1, 1, -1, 1) -1, 1, -1, 1 (1, 1, 1, 1) q, Å -1 SAXS pattern of α 2 -StSt z, Å z, Å S(z), electrons/å L packing z, Å

15 Two important observations! The α 2 -phase forms during quenching from the melt; therefore, the structural organization of this phase should resemble supramolecular ordering of triacylglycerols in the melt This phase is observed only in mixed saturated/unsaturated triacylglycerols

16 What do we know about liquid alkanes?

17 Alkane in gas, liquid and solid state H 4 2 H 6 4 H 10 5 H 12 8 H H 42 methane ethane butane pentane natural gas liquefied petroleum gas melting points boiling points octane Temperature, room temperature number of carbon atoms per molecular chain hexadecane petrol and diesel eicosane Alkanes are non-polar only weak intermolecular forces act between the alkane molecules

18 Folding of polymer chains Schematic drawing of a chain-folded polymer crystal (after Ungar G. & Zeng X., hemical Reviews, V. 101, 2001) Integer folded forms observed in long n-alkanes, for a given alkane more folds per molecule can be obtained with increasing supercooling. Structure of the chain fold in crystalline monoclinic cycloalkane c-(h2)34 (after Kay H. F. & Newman B. A., Acta ryst., B24, 1968)

19 Models for supramolecular ordering in liquid triacylglycerides StLnSt in a liquid state at 50 d ~ 26 Å d ~ 4.6 Å SAXS WAXS Intensity,a. u. Intensity, a. u. Smectic model by Larson, q, Å -1 q, Å -1 Discotic model by orkery et al, 2007 Nematic model by ebula et al, 1992

20 What do we know about unsaturated - bonds in alkyl chains?

21 Fatty acids Stearic acid leic acid Linoleic acid (ω-6) L L L Linoleic acid (ω-6) is one of the two essential fatty acids (the other is alpha-linolenic acid, ω-3) that humans require. They are "essential" because they can not be produced by the human body.

22 Phospholipids Phosphatidyl choline (lecithin) choline N + Polar hydrophilic head N + phosphate - P - P glycerol Unsaturated chain Saturated chain nonpolar hydrophobic tails A double bond makes an unsaturated chain The kink (or kinks) prevent close packing

23 S'(z), a. u. onsidering the group a) z, Å 20 17Å 54.3Å 0 -The position of the glycerol residues does not match the distance measured from the electron density profile; -The structure has the mass density far below the mass density of the liquid state -The mass density of the structure is very large (~1.4 g/cm 3 ); -Acyl chains are not straight; -Hexagonal packing of the acyl chains within the layer is unlikely

24 Possible dimeric units in liquid triacylglycerols Four chains interaction Two chains interaction

25 onsidering the group d) d Representative models of packing of StSt molecules in dimers (projections onto the slide plane), which is likely to be formed in the liquid state before the formation of the α 2 -phase S'(z), a. u. 25 Å -1, -1, 1, 1 (1, -1, -1, 1) -1, -1, -1, 1 (1, -1, 1, 1) z, Å 21.0 Å 3.5 Å 21.0 Å 3.4 Å 21.2 Å 20.5 Å 1 3 2

26 A model of packing of the StSt molecules in the layers of the α 2 -phase 0 electron density, electrons/å Å z, Å z, Å Å 24.6 Å S'(z), a. u. 140

27 Scattering patterns of liquid crystals K. K. han et al, Phys. Rev A, V. 34, 1986, p. 1420

28 Isothermal crystallization of StLnSt γ phase 001α2 002α2 100α2 001sub-α2 peak sub-α2 003 sub-α2 peak sub-α2 005 sub-α2 110 sub-α2 200 sub-α2 001γ Transient structures: sub-α 2 phase, orthorhombic α 2 phase, hexagonal temperature q, Å -1 q, Å -1 SAXS WAXS

29 2L+3L transient structure 002 γ 001 α1 001 γ 003 γ 005 γ 2L+3L 003 α1 001 α2+α1 Peak 1 (2L+3L) Peak 2 (2L+3L) Peak3 (2L+3L) 10 Temp 22 Intensity, a. u. 001 α2 002 α2 002 α2+α1 peak 2 peak 1 peak q, Å α α2 α2 004 α2 q, Å -1 The experimental SAXS pattern of StSt isothermal crystallization at 10 from the melt (55 ) 55 Simulated and experimental SAXS patterns of random 2L+3L StSt structure after Mykhaylyk et al, J. Appl. ryst., 2007, p. s297

30 an we make use of this finding about structural transformations in triacylglycerols? α 2 -phase is a distinctive property of mixed saturated/unsaturated triacylglycerols. It may be possible to find conditions when this property can be effectively used in fat fractionation. Transient structures of α 2 -phase can be used to control polymorphic transformations of triacylglcerols.

31 Polymorphic transformations of StSt Liquid α 2 α 1 + γ γ 002 γ Liquid α 2 sub-α 2 β + γ 002 β 003β 003 γ 005 γ 001 β β 005 β q, Å q, Å temperature of isothermal crystallisation ( ) duration of isothermal crystallisation (min) γ phase fraction in StSt crystallized at 30 after isothermal crystallization at different temperatures for a certain duration. after Mykhaylyk & Hamley, J. Phys. hem., 2004, p. 8069

32 onclusions Transient phases are identified in mixed saturated/unsaturated triacylglycerols or their mixtures. The transient phase formation is caused by a mismatch between saturated and unsaturated acyl chains followed by their separation in the melt These phases resembles smectic A 2 liquid crystal phases Reduction of the longitudinal diffusion of the molecules at low temperatures freezes the molecular motion and makes unsaturated bonds to be terminal points causing formation of interdigitated phases similar to smectic A d liquid crystal phases At certain conditions dictated by molecular structure the transient phases can develop into other structures such as 2L+3L during StSt isothermal crystallization The structural state of the transient phases effect further polymorphic transformations of triacylglycerols at high temperatures

33 Acknowledgements