R Q X HO HO OH O O 1 3 6 5 1 3 6 5 1 3 6 5 1 C O 3 6 OH 5 1 3 6 5 9 13 15 15¢ 13¢ 9¢ a b c d e 1 Zeaxanthin (R = Q = a, X = OH) 3 Capsorubin (R = Q = e) 5 Lutein epoxide (R = b, Q = d) 2 Violaxanthin (R = Q = b) 4 Capsanthin (R = a, X = OH, Q = e) 120 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 120–121 J. Chem. Research (M), 1997, 0801–0841 Kinetic Studies on the Thermal Z/E-Isomerization of C40-Carotenoids P�eter Moln�ar,*a Tam�as K�ortv�elyesi,b Zolt�an Matusa and J�ozsef Szabolcs† Department of Medical Chemistry, University Medical School of P�ecs, Szigeti �ut 12., H-7624 P�ecs, Hungary bInstitute of Physical Chemistry, J�ozsef Attila University, Szeged Rerrich B.t�er 1., H-6720 Szeged, Hungary The kinetics of thermal Z/E-isomerization among all-E-, 9(9p)Z-, 13(13p)Z- and 15Z-isomers of zeaxanthin 1, violaxanthin 2, capsorubin 3, capsanthin 4 and lutein epoxide 5 are studied at 333.4–368.4 K. The thermal isomerization of 1–5 in pyridine or benzene over the temperature range 333–368 K yielded complex equilibria25 –26 governed by first-order forward and reverse elementary steps.28–29 In compliance with the analytical approach, four models (I–IV) were used to describe the mechanisms by which the Z- and all-E-isomers were interconverted.Model I was used to describe the thermal Z/E-isomerization when the formation or the consumption of the Z-isomers with high cis-peaks (13Z, 13pZ and 15Z) was monitored by UV–VIS-spectrophotometry:1,22,25–26,28–30 k1 All-EM13[13p(15)]Z kµ1 Model II consists of two reversible concurrent reactions11,23,25,32 and is represented by the following systems, typical of a symmetrical carotenoid: kµ1 k2 13ZMAll-EM15Z k1 kµ2 and an unsymmetrical carotenoid. kµ1 k2 13ZMAll-EM13pZ k1 kµ2 The reactions leading to equilibria were followed by HPLC.Model III was used for more complex systems containing All-E-, 13Z-, 15Z-, 9Z- and di-Z-isomers:22 kµ1 k2 13ZMAll-EM15Z k1 kµ2 kµ3 k3 9Z+di-Z (mixed peak) For the evaluation of the rate constants the respective formation and consumption of the 9Z-and di-Z- isomers were considered together.Model IV was based on the most detailed analysis of the thermal isomerization mixtures. *To receive any correspondence (e-mail: molpeter@apacs. pote.hu). †Present address: Benc�es Gimn�azium, V�ar 1., H-9090 Pannonhalma, Hungary. Table 7 Arrhenius and activation parameters of the reversible isomerization of the (all-E)-1 to (13Z)-1 and (15Z)-1 using the rate constants obtained in the simulation of the reaction mechanism defined by models II and III; Tav.=357.4 Ka Rate constant log (A/sµ1) EA/kJ molµ1 DH‡/kJ molµ1 DS‡/J molµ1 Kµ1 DG‡/kJ molµ1 Model II k1 kµ1 k2 kµ2 11.53 (0.31) 10.68 (0.16) 12.85 (0.41) 13.39 (0.61) 104.6 (3.1) 97.0 (1.3) 118.6 (4.6) 115.6 (11.1) 101.6 (3.1) 94.0 (1.4) 116.9 (5.1) 112.6 (11.2) µ34.1 µ50.6 µ5.1 µ1.6 113.8 112.1 118.7 112.0 Model III k1 kµ1 k2 kµ2 k3 kµ3 11.55 (0.48) 10.60 (0.26) 12.77 (0.42) 13.22 (0.67) —— 104.9 (6.0) 96.6 (2.4) 118.0 (4.8) 114.5 (10.8) —— 102.0 (6.0) 93.6 (2.4) 115.1 (4.8) 111.6 (10.8) —— µ33.6 µ51.8 µ10.3 µ1.8 —— 114.0 112.1 118.8 118.2 —— aData in parentheses are the standard deviations.Arrhenius and activation parameters of reactions (3), (µ3) and (4), (µ4) are not given because of significant error. Model IV was used, with relatively high deviation, to calculate k3, kµ3 and K4 MJ. CHEM.RESEARCH (S), 1997 121 kµ1 k2 13ZMAll-EM15Z k1 kµ2 kµ3 k3 9Z kµ4 k4 di-Z (mixed peak) Rate constants were calculated using the program package ZITA for advanced kinetic simulations;24 that is, the ordinary differential equations (ODE) of the kinetic models were solved by the GEAR method.33 Curve fitting was carried out applying the Marquard–Levenberg method.34 The Arrhenius parameters were calculated by non-linear parameter estimation with statistical weights wi=1/k2i . Values of EA, DH‡, DS‡, DG‡ and A were obtained from the rate constants determined on the basis of Models I–IV.The Arrhenius and activation parameters for thermal interconversions among (all-E)-1, (13Z)-1 and (15Z)-1, calculated from the rate constants obtained in simulation of the different models (II–IV), are listed in Table 7. Techniques used: UV–VIS spectroscopy, HPLC, column chromatography References: 48 Tables: 12 Received, 18th November 1996; Accepted, 23rd December 1996 Paper E/6/07820F References cited in this synopsis 1 J.Szabolcs, Pure Appl. Chem., 1976, 47, 147. 11 C. A. Pesek, J. J. Warthesen and P. S. Taoukis, J. Agric. Food Chem., 1990, 38, 41. 22 B. H. Chen, T. M. Chen and J. T. Chien, J. Agric. Food Chem., 1994, 421, 2391. 23 W. von E. Doering, C. Sotiriou-Leventis and W. R. Roth, J. Am. Chem. Soc., 1995, 117, 2747. 24 G. Peintler, A Comprehensive Program Package for Fitting Parameters of Chemical Reaction Mechanisms, A. J. University, Szeged (Hungary), 1993. 25 P.Moln�ar, Structural Elucidation of Mono- and Di-cis-Carotenoids, Isolation of New Carotenoids, Kinetics of E/Z-Isomerization, Ph.D. Dissertation, Department of Medical Chemistry, University Medical School of P�ecs, Hungary, 1988. 26 L. Zechmeister, Cis-Trans Isomeric Carotenoids, Vitamins A and Arylpolyenes, Springer, Wien, 1962. 28 Z. G. Szab�o, in Comprehensive Chemical Kinetics, ed C. H. Bamford and C. F. H. Tipper, Elsevier, Amsterdam, 1969, vol. 2, ch. 1. 29 K. Schwetlick, Reakci�omechanizmusok kinetikai viszg�alata (Investigation of Kinetics and Mechanism of Reactions), Mu��szaki K�onyvkiad �o (Technological Publishing House), Budapest, 1978. 30 H. A. Franck, Ann. N.Y. Acad. Sci., 1993, 691, 1. 32 (a) G. N. Vriens, Ind. Eng. Chem., 1954, 669l; (b) N. A. Sørensen, Beitr�age zur Kinetik der Mutarotation, I Kommission Hos F. Bruns Bokhandel, Trondheim, 1937. 33 A. C. Hindmarsch, Ordinary Differential Equation Solver, UCID- 30001 rev. 3. Lawrence Livermore Laboratory, P.O. Box 808, Livermore, CA 94550, 1974. 34 Y. Bard, Nonlinear Paramet