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Hydrogenation, isomerization and migration of unsaturated acyl moieties in natural triacylglycerols
Chemistry andPhysics o f Lipids 23 (1979) 1-5
© Elsevier/North-Holland Scientific Publishers Ltd.
HYDROGENATION, ISOMERIZATION AND MIGRATION OF UNSATURATED ACYL MOIETIES IN NATURAL TRIACYLGLYCEROLS* K. ILSEMANN**, IRENE REICHWALD-HACKER and K.D. MUKHERJEE Federal Center for Lipid Research, Piusallee, D-4400 Miinster (G.F.R.)
Received March 21st, 1 9 7 8
accepted May 12th, 1978
Triacylglycerols of soybean were partially hydrogenated with a copper chromite catalyst, which reduced the octadecadienoyl and octadecatrienoyl moieties selectively to octadecenoyl moieties. Composition of the acyl moieties, including the distribution of positional isomers of cis- and transoctadecenoyl moieties, both in 1,3- and 2-positions of trlacylglycerols, was determined after hydrolysis with pancreatic lipase. The results show that the octadecadienoyl, but not the octadecatrienoyl moieties are reduced at a faster rate in the 1,3-positions than in the 2.position, whereas the accumulation of trans-octadecenoyl moieties is much higher in the 2-position than in the 1,3positions. On the other hand, the distribution of positional isomers, both in cis- and transoctadecenoyl moieties, is essentially the same in 1,3- and 2-positions. Practically no acyl migration occurs during hydrogenation under the conditions desczibed. I. Introduction
It has been believed for many years that the rates of reduction of unsaturated acyl moieties during catalytic hydrogenation ar~ independent of the position they occupy in triacylglycerols [I,2]. Yet, it has been shown recently that, with non-selective nickel [3,4] and palladium [4] catalysts, the hydrogenation of unsaturated acyl moieties is in fact faster in the 1,3-positions than in the 2-position of triacylglycerols. A study from this laboratory using pure symmetrical triacylglycerols and a palladium catalyst at ambient temperature of hydrogenation has proven unequivocally that the rate of reduction of c/s-monounsaturated acyl moieties is higher and the rate of their geometrical isomerization is lower in the 1,3-positions than in the 2-positions of the triacylglycerols [5]. The present study was designed to check the validity of the above ffmdings in the partial hydrogenation of polyunsaturated acyl moieties in naturally occurring triacylglycerols under conditions similar to those in the industrial hydrogenation of vegetable oils. The triacylglycerols of soybean were partially hydrogenated using a highly selective copper chromite catalyst, which is known to reduce the linoleoyl and linolenoyl moieties up to octadecenoyl moieties without any further reduction to stearoyl moieties [6]. * This paper is dedicated to the cherished memory of the late Professor J.G. Kane, University of Bombay, Bombay (India) with Whom one of the authors, (K.D.M.) has been associated ** Present address: Institut fflr Arterioskleroseforschung, D-4400 Mflnster (G.F.R.)
2
K. Islemann et al., Acyl moieties in triacylglycerols
II. Experimental A. Material
Refined soybean oil was purchased locally. A sample of copper chromite (Cu-1800 P, containing 51% CuO and 47% Cr203) used as catalyst for hydrogenation, was provided by Harshaw Chemie N.V., De Meem (The Netherlands). Methyl esters, width were used as reference compounds, were purchased from Nu-Check.Prep, t~lysian, MN 56028 (U.S.A.).Porcine pancreatic lipase and all reagents of analytical grade were obtained from E. Merck AG, D.6100 Darmstadt, Germany.
B. Methods
Partial hydrogenation of the soybean oil was carried out in an autoclave (Autoclave Engineers, Inc., Erie, PA U.S.A.). The oil, 500 g, containing 1% copper chromite in suspension, was heated under stirring to 180°C in an atmosphere of nitrogen. Subsequently, nitrogen was replaced by hydrogen and the reaction was carried out under stirring at 180°C and 505 kPa hydrogen pressure for 30 rain. Thereafter, the hydrogen pressure was released, the autoclave purged with nitrogen and the reaction products removed after cooling to about 40°C. Finally, the oil was separated from the catalyst by f'dtration. Aliquots of the triacylgiycerols, both from the soybean oil und the partially hydrogenated product, were isolated by preparative thin-layer chromatography [7] and analyzed according to procedures described earlier [5]. The composition of acyl moieties in the 1,3-positions of triacylglycerols as well as the distribution of positional isomers in c/s- and trans-octadecenoyl moieties located in these positions were calculated from the corresponding data for total acyl moieties and 2-acyl moieties.
III. Results and discussion The composition of the acyl moieties in the 1,3- and 2-positions of unhydrogenated and partially hydrogenated triacylglycerols of soybean is given in Table 1. It is evident that in both samples of triacylgiycerols, the palmitoyl and stearoyl moieties are located almost exclusively in the 1,3-positions and their levels in these positions are almost identical in the two samples. This indicates the virtual absence of acyl migration during hydrogenation. Furthermore, the constant level of stearoyl moieties, both in 1,3- and 2-positions of the two samples of triacylglycerols shows that the polyunsaturated acyl moieties are reduced up to octadecenoyl moieties and not further, which is a well known phenomenon with copper chromite catalysts [6]. The results given in Table 1 also show that the total reduction in the levels of
K. lslemann et al., Acyi moieties in triacylglycerols Table 1 Composition (%) of the acyl moieties in the 1,3- and 2-positions of triacylglycerols of soybean before and after partial hydrogenation Sample
Unhydrogenated triacylglycerols 1,3-Positions 2-Position
Acyl moietiesa (%)
16:0
18:0
d$18:1
fra,q$o 18:1
18:2
18:3
16.9 tt.
5.3 tr.
21.9 23.3
0.0 0.0
48.6 71A
5.8
5.3 tr.
33,0 20,1
8.7 19.6
34.2 59.1
1.1 0.5
Partially hydrogenated triacylglycerols 1,3-Positions 16.7 2-Position
tr.
4.4
a Number of carbon atoms: number of double bonds tr. = traces.
octadecadienoyl and octadecatrienoyl moieties in the 1,3.positions amounts to (48.6 + 5.8--34.2--1.1) = 19.1%, which agrees very closely with the increase in the level of octadecenoyl moieties, (33 + 8.7-21.9) = 19.8% in the 1,3-positions. Similarly, the total reduction in the levels of octadecadienoyl and octadecatrienoyl moieties in the 2-position amounts to (71.4 + 4.4-59.1-0.5) = 16.2%, which is in excellent agreement with the increase in the level of octadecenoyl moieties, (20.1 + 19.6-23.3) = 16.4% in the 2-position. These figures further support the conclusion that acyl migration does not occur during hydrogenation. They also reveal a somewhat higher overall reduction of polyunsaturated acyl moieties in the 1,3-positions than in the 2-position. Since acyl migration is precluded, it is evident from the data given in Table 1 that the proportion of octadecadienoyl moieties reduced in the 1,3-positions is at least [ ( 4 8 . 6 - 3 4 . 2 ) / 4 8 . 6 ] X 100 = approx. 30% and of those reduced in the 2-position is at least [(71.4-59.1)/71.4] X 100 = approx. 17% of the octadecadienoyl moieties originally present in each of these positions. It should be noted, that the actual proportions of octadecadienoyl moieties reduced in both 1,3- and 2-positions, with respect to the octadecadienoyl moieties originally present in each of these positions are probably somewhat higher, since small proportions of the octadecadienoyl moieties found in both 1,3- and 2-positions of the partially hydrogenated triacylglycerols might have been derived by selective reduction of octadecatrienoyl moieties. Nevertheless, the figures presented show clearly that the octadecadienoyl moieties are reduced in the 1,3-positions at a higher rate than in the 2-position. On the other hand, the proportions of octadecatrienoyl moieties reduced in the 2-position, [(4.4-0.5)/4.4] X 100 = approx. 89% are somewhat higher than in the 1,3-positions [(5.8--1.1)/5.8] × I00 = approx.' 81%. This discrepancy between octadecadienoyl and octadecatrienoyl moieties with regard to their differential rates of reduction in the 1,3- and 2-positions of triacylglycerols can not be explained. The data presented in Table 1 show a large
4
K. Islemann et al., Acyl moieties in triacylflycerois
accumulation of trans-octadecenoyl moieties in the 2-position of the partially hydrogenated triacylglycerols as compared to that in the 1,3-positions. These results are similar to findings on the geometrical isomerization of monounsaturated acyl moieties during partial hydrogenation of symmetrical triacylglycerols [5]. Composition of positional isomers of c/s- and trans-octadecenoyl moieties in the 1,3- and 2-positions of unhydrogenated and partially hydrogenated triaeylglyeerols of soybean is given in Table 2. It is interesting to note that in the unhydrogenated triacylglycerols, the 9-c/s-octadecenoyl moieties are preferentially located in the 2-position, whereas the 11-cis-octadecenoyl moieties occur preferentially in the 1,3-positions. Similarly, in the partially hydrogenated triacylglycerols, the 9-cisoctadecenoyl moieties are found in greater abundance in the 2-position than in the 1,3-positions, whereas most of the other positional isomers, such as 8-eis, 11 ~eis, 12.cis and 13-cis-octadecenoyl moieties are found predominantly in the 1,3-positions. Obviously, the double bonds in the cis-octadeeenoyl moieties are somewhat more scattered in those occurring in the 1,3-positions of the triacylglyeerols than in those at the 2-position. Interestingly, the distribution of positional isomers of trans-octadecenoyl moieties is almost identical in the 1,3- and 2-positions of the partially hydrogenated triacylglycerols, although the octadeeenoyl moieties in the 2-position contain a much higher proportion of tram-isomers than those in the 1,3-positions. The results of these f'mdings might be of significance in metabolic studies concerned with the nutritional evaluation of partially hydrogenated fats.
Table 2
(%) of positional isomers of cis- and trans-octadeeenoyl moieties in 1,3- and 2-positions of triacylglycerolsof soybean before and after partial hydrogenation
Composition
Sample
Positionalisomers (%) A4
A5
A6
/`7
/`8
/`9
/`10
All
A12 A13 A14
0.8 0.2
7.7 1A
3.6 3.7
12.9 4.3
6.0 2.9
2.8 1.1
18.6 23.8
27.3 26.5
15A 16.1
12.2 11.0
/`15
/'16
1.4 0.5
0.4 0.8
tx. tr.
7.2 6.7
4.5 4.2
1.6 0.4
Unhydrogenated triacyiglycerols
c/s-18 : 1 1,3-Positions 2-Position
tr. tr.
tr. tr.
tr. tr.
0.5 89.9 0.2 98.0
tr. tr.
Partially hydrogenated triacylglycerms cis-18 : 1 1,3-Positions 2-Position
tr. tr.
0.3 0.5 0.4 0.3
1.6 71.2 0.5 85.4
trans-18 : 1 1,3-Positions 0.4 2-Position 0.2 tz. = traces.
0.6 0.9 1.3 0.3 0.6 0.5
3.2 2.0
7.8 7.6
K. lslemann et al., Acyl moieties in triacylglycerols
5
References [1] [2] [3] [4] [5J [6] [7]
F.H. Mattson and R.A. Volpenhein, J. Am. Oil Chem. Soc. 39 (1962) 307 H.V. Turner, R.O. Feuge, T.L° Ward and E.R. Cousins, J. Am. Off Chem. Soc. 41 (1964) 413 P. Lukacs, E. Kurucz, M. Jeranek and M.P. Janoshegyi, Olaj, Szappan, Kozmet. 24 (1975) 67 B. Drozdowski, J. Am. Off Chem. Soc. 54 (1977) 600 MJvl. Paulose, K.D. Mukherjee and I. Richter, Chem. Phys. Lipids 21 (1978) 187 S. Koritala and HJ. Dutton, J. Am. Off Chem. Soc. 46 (1969) 245 H.K. Mangold, J. Am. Oil Chem. Soc. 38 (1961) 708
© Elsevier/North-Holland Scientific Publishers Ltd.
HYDROGENATION, ISOMERIZATION AND MIGRATION OF UNSATURATED ACYL MOIETIES IN NATURAL TRIACYLGLYCEROLS* K. ILSEMANN**, IRENE REICHWALD-HACKER and K.D. MUKHERJEE Federal Center for Lipid Research, Piusallee, D-4400 Miinster (G.F.R.)
Received March 21st, 1 9 7 8
accepted May 12th, 1978
Triacylglycerols of soybean were partially hydrogenated with a copper chromite catalyst, which reduced the octadecadienoyl and octadecatrienoyl moieties selectively to octadecenoyl moieties. Composition of the acyl moieties, including the distribution of positional isomers of cis- and transoctadecenoyl moieties, both in 1,3- and 2-positions of trlacylglycerols, was determined after hydrolysis with pancreatic lipase. The results show that the octadecadienoyl, but not the octadecatrienoyl moieties are reduced at a faster rate in the 1,3-positions than in the 2.position, whereas the accumulation of trans-octadecenoyl moieties is much higher in the 2-position than in the 1,3positions. On the other hand, the distribution of positional isomers, both in cis- and transoctadecenoyl moieties, is essentially the same in 1,3- and 2-positions. Practically no acyl migration occurs during hydrogenation under the conditions desczibed. I. Introduction
It has been believed for many years that the rates of reduction of unsaturated acyl moieties during catalytic hydrogenation ar~ independent of the position they occupy in triacylglycerols [I,2]. Yet, it has been shown recently that, with non-selective nickel [3,4] and palladium [4] catalysts, the hydrogenation of unsaturated acyl moieties is in fact faster in the 1,3-positions than in the 2-position of triacylglycerols. A study from this laboratory using pure symmetrical triacylglycerols and a palladium catalyst at ambient temperature of hydrogenation has proven unequivocally that the rate of reduction of c/s-monounsaturated acyl moieties is higher and the rate of their geometrical isomerization is lower in the 1,3-positions than in the 2-positions of the triacylglycerols [5]. The present study was designed to check the validity of the above ffmdings in the partial hydrogenation of polyunsaturated acyl moieties in naturally occurring triacylglycerols under conditions similar to those in the industrial hydrogenation of vegetable oils. The triacylglycerols of soybean were partially hydrogenated using a highly selective copper chromite catalyst, which is known to reduce the linoleoyl and linolenoyl moieties up to octadecenoyl moieties without any further reduction to stearoyl moieties [6]. * This paper is dedicated to the cherished memory of the late Professor J.G. Kane, University of Bombay, Bombay (India) with Whom one of the authors, (K.D.M.) has been associated ** Present address: Institut fflr Arterioskleroseforschung, D-4400 Mflnster (G.F.R.)
2
K. Islemann et al., Acyl moieties in triacylglycerols
II. Experimental A. Material
Refined soybean oil was purchased locally. A sample of copper chromite (Cu-1800 P, containing 51% CuO and 47% Cr203) used as catalyst for hydrogenation, was provided by Harshaw Chemie N.V., De Meem (The Netherlands). Methyl esters, width were used as reference compounds, were purchased from Nu-Check.Prep, t~lysian, MN 56028 (U.S.A.).Porcine pancreatic lipase and all reagents of analytical grade were obtained from E. Merck AG, D.6100 Darmstadt, Germany.
B. Methods
Partial hydrogenation of the soybean oil was carried out in an autoclave (Autoclave Engineers, Inc., Erie, PA U.S.A.). The oil, 500 g, containing 1% copper chromite in suspension, was heated under stirring to 180°C in an atmosphere of nitrogen. Subsequently, nitrogen was replaced by hydrogen and the reaction was carried out under stirring at 180°C and 505 kPa hydrogen pressure for 30 rain. Thereafter, the hydrogen pressure was released, the autoclave purged with nitrogen and the reaction products removed after cooling to about 40°C. Finally, the oil was separated from the catalyst by f'dtration. Aliquots of the triacylgiycerols, both from the soybean oil und the partially hydrogenated product, were isolated by preparative thin-layer chromatography [7] and analyzed according to procedures described earlier [5]. The composition of acyl moieties in the 1,3-positions of triacylglycerols as well as the distribution of positional isomers in c/s- and trans-octadecenoyl moieties located in these positions were calculated from the corresponding data for total acyl moieties and 2-acyl moieties.
III. Results and discussion The composition of the acyl moieties in the 1,3- and 2-positions of unhydrogenated and partially hydrogenated triacylglycerols of soybean is given in Table 1. It is evident that in both samples of triacylgiycerols, the palmitoyl and stearoyl moieties are located almost exclusively in the 1,3-positions and their levels in these positions are almost identical in the two samples. This indicates the virtual absence of acyl migration during hydrogenation. Furthermore, the constant level of stearoyl moieties, both in 1,3- and 2-positions of the two samples of triacylglycerols shows that the polyunsaturated acyl moieties are reduced up to octadecenoyl moieties and not further, which is a well known phenomenon with copper chromite catalysts [6]. The results given in Table 1 also show that the total reduction in the levels of
K. lslemann et al., Acyi moieties in triacylglycerols Table 1 Composition (%) of the acyl moieties in the 1,3- and 2-positions of triacylglycerols of soybean before and after partial hydrogenation Sample
Unhydrogenated triacylglycerols 1,3-Positions 2-Position
Acyl moietiesa (%)
16:0
18:0
d$18:1
fra,q$o 18:1
18:2
18:3
16.9 tt.
5.3 tr.
21.9 23.3
0.0 0.0
48.6 71A
5.8
5.3 tr.
33,0 20,1
8.7 19.6
34.2 59.1
1.1 0.5
Partially hydrogenated triacylglycerols 1,3-Positions 16.7 2-Position
tr.
4.4
a Number of carbon atoms: number of double bonds tr. = traces.
octadecadienoyl and octadecatrienoyl moieties in the 1,3.positions amounts to (48.6 + 5.8--34.2--1.1) = 19.1%, which agrees very closely with the increase in the level of octadecenoyl moieties, (33 + 8.7-21.9) = 19.8% in the 1,3-positions. Similarly, the total reduction in the levels of octadecadienoyl and octadecatrienoyl moieties in the 2-position amounts to (71.4 + 4.4-59.1-0.5) = 16.2%, which is in excellent agreement with the increase in the level of octadecenoyl moieties, (20.1 + 19.6-23.3) = 16.4% in the 2-position. These figures further support the conclusion that acyl migration does not occur during hydrogenation. They also reveal a somewhat higher overall reduction of polyunsaturated acyl moieties in the 1,3-positions than in the 2-position. Since acyl migration is precluded, it is evident from the data given in Table 1 that the proportion of octadecadienoyl moieties reduced in the 1,3-positions is at least [ ( 4 8 . 6 - 3 4 . 2 ) / 4 8 . 6 ] X 100 = approx. 30% and of those reduced in the 2-position is at least [(71.4-59.1)/71.4] X 100 = approx. 17% of the octadecadienoyl moieties originally present in each of these positions. It should be noted, that the actual proportions of octadecadienoyl moieties reduced in both 1,3- and 2-positions, with respect to the octadecadienoyl moieties originally present in each of these positions are probably somewhat higher, since small proportions of the octadecadienoyl moieties found in both 1,3- and 2-positions of the partially hydrogenated triacylglycerols might have been derived by selective reduction of octadecatrienoyl moieties. Nevertheless, the figures presented show clearly that the octadecadienoyl moieties are reduced in the 1,3-positions at a higher rate than in the 2-position. On the other hand, the proportions of octadecatrienoyl moieties reduced in the 2-position, [(4.4-0.5)/4.4] X 100 = approx. 89% are somewhat higher than in the 1,3-positions [(5.8--1.1)/5.8] × I00 = approx.' 81%. This discrepancy between octadecadienoyl and octadecatrienoyl moieties with regard to their differential rates of reduction in the 1,3- and 2-positions of triacylglycerols can not be explained. The data presented in Table 1 show a large
4
K. Islemann et al., Acyl moieties in triacylflycerois
accumulation of trans-octadecenoyl moieties in the 2-position of the partially hydrogenated triacylglycerols as compared to that in the 1,3-positions. These results are similar to findings on the geometrical isomerization of monounsaturated acyl moieties during partial hydrogenation of symmetrical triacylglycerols [5]. Composition of positional isomers of c/s- and trans-octadecenoyl moieties in the 1,3- and 2-positions of unhydrogenated and partially hydrogenated triaeylglyeerols of soybean is given in Table 2. It is interesting to note that in the unhydrogenated triacylglycerols, the 9-c/s-octadecenoyl moieties are preferentially located in the 2-position, whereas the 11-cis-octadecenoyl moieties occur preferentially in the 1,3-positions. Similarly, in the partially hydrogenated triacylglycerols, the 9-cisoctadecenoyl moieties are found in greater abundance in the 2-position than in the 1,3-positions, whereas most of the other positional isomers, such as 8-eis, 11 ~eis, 12.cis and 13-cis-octadecenoyl moieties are found predominantly in the 1,3-positions. Obviously, the double bonds in the cis-octadeeenoyl moieties are somewhat more scattered in those occurring in the 1,3-positions of the triacylglyeerols than in those at the 2-position. Interestingly, the distribution of positional isomers of trans-octadecenoyl moieties is almost identical in the 1,3- and 2-positions of the partially hydrogenated triacylglycerols, although the octadeeenoyl moieties in the 2-position contain a much higher proportion of tram-isomers than those in the 1,3-positions. The results of these f'mdings might be of significance in metabolic studies concerned with the nutritional evaluation of partially hydrogenated fats.
Table 2
(%) of positional isomers of cis- and trans-octadeeenoyl moieties in 1,3- and 2-positions of triacylglycerolsof soybean before and after partial hydrogenation
Composition
Sample
Positionalisomers (%) A4
A5
A6
/`7
/`8
/`9
/`10
All
A12 A13 A14
0.8 0.2
7.7 1A
3.6 3.7
12.9 4.3
6.0 2.9
2.8 1.1
18.6 23.8
27.3 26.5
15A 16.1
12.2 11.0
/`15
/'16
1.4 0.5
0.4 0.8
tx. tr.
7.2 6.7
4.5 4.2
1.6 0.4
Unhydrogenated triacyiglycerols
c/s-18 : 1 1,3-Positions 2-Position
tr. tr.
tr. tr.
tr. tr.
0.5 89.9 0.2 98.0
tr. tr.
Partially hydrogenated triacylglycerms cis-18 : 1 1,3-Positions 2-Position
tr. tr.
0.3 0.5 0.4 0.3
1.6 71.2 0.5 85.4
trans-18 : 1 1,3-Positions 0.4 2-Position 0.2 tz. = traces.
0.6 0.9 1.3 0.3 0.6 0.5
3.2 2.0
7.8 7.6
K. lslemann et al., Acyl moieties in triacylglycerols
5
References [1] [2] [3] [4] [5J [6] [7]
F.H. Mattson and R.A. Volpenhein, J. Am. Oil Chem. Soc. 39 (1962) 307 H.V. Turner, R.O. Feuge, T.L° Ward and E.R. Cousins, J. Am. Off Chem. Soc. 41 (1964) 413 P. Lukacs, E. Kurucz, M. Jeranek and M.P. Janoshegyi, Olaj, Szappan, Kozmet. 24 (1975) 67 B. Drozdowski, J. Am. Off Chem. Soc. 54 (1977) 600 MJvl. Paulose, K.D. Mukherjee and I. Richter, Chem. Phys. Lipids 21 (1978) 187 S. Koritala and HJ. Dutton, J. Am. Off Chem. Soc. 46 (1969) 245 H.K. Mangold, J. Am. Oil Chem. Soc. 38 (1961) 708