Differential scanning calorimetric studies on the thermotropic phase transitions of N-acylethanolamines of odd chainlengths

Differential scanning calorimetric studies on the thermotropic phase transitions of N-acylethanolamines of odd chainlengths

Chemistry and Physics of Lipids 94 (1998) 43 – 51 Differential scanning calorimetric studies on the thermotropic phase transitions of N-acylethanolam...

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Chemistry and Physics of Lipids 94 (1998) 43 – 51

Differential scanning calorimetric studies on the thermotropic phase transitions of N-acylethanolamines of odd chainlengths M. Ramakrishnan, Musti J. Swamy * School of Chemistry, Uni6ersity of Hyderabad, Hyderabad 500 046, India Received 9 December 1997; received in revised form 27 March 1998; accepted 31 March 1998

Abstract N-acylethanolamines (NAEs) exhibit a wide spectrum of properties that are biologically important. Recently, the authors investigated the thermotropic phase transitions of a homologous series of NAEs containing even number of C-atoms by differential scanning calorimetry (Ramakrishnan, M., Sheeba, V., Komath, S.S., Swamy, M.J. 1997. Biochim. Biophys. Acta 1329, 302–310). These studies have been extended now to the NAEs with odd number of C-atoms (n=9–19) in the acyl chain. All the NAEs show a major sharp endothermic phase transition that coincides with the melting point for the dry NAEs, while for the hydrated samples it occurs at considerably lower temperatures. The NAEs in the longer chainlength range also display an additional, minor transition before the chain-melting transition. The transition enthalpy (DHt) and transition entropy (DSt) have been found to depend linearly on the chainlength, n and a least squares analysis yielded the incremental values, DHinc and DSinc, contributed by each CH2 unit to DHt and DSt, respectively. The incremental values obtained for the dry NAEs are smaller than those obtained for the hydrated samples if only the main chain-melting phase transitions are considered, however, if the enthalpy changes of the major as well as the minor transitions are combined then the incremental values obtained in the two cases are comparable. Further, the values of DHinc and DSinc obtained for odd chainlength series are comparable to those obtained for the even chainlength series of NAEs, but are considerably higher than those obtained for the diacyl phospholipids on a per chain basis. An alternation has been observed in the DHt, DSt and transition temperatures obtained for the odd chainlength series and the even chainlength series, similar to that observed in long chain hydrocarbons and fatty acids. © 1998 Elsevier Science Ireland Ltd. All rights reserved. Keywords: N-acylethanolamine; Phase transition; Transition enthalpy; Transition entropy; Chainlength dependence; Differential scanning calorimetry; Lipid membrane

Abbre6iations: NAE, N-acylethanolamine; NAPE, N-acylphosphatidylethanolamine; DPPC, dipalmitoylphosphatidylcholine; PE, phosphatidylethanolamine; Tt, transition temperature; DHt, transition enthalpy; DSt, transition entropy; DSC, differential scanning calorimetry; DHinc, incremental value of transition enthalpy per CH2 unit; DSinc, incremental value of transition entropy per CH2 unit; DHo, end contribution of transition enthalpy; DSo, end contribution of transition entropy. * Corresponding author. Tel.: + 91 40 3010221; fax: + 91 40 3010120/0145; e-mail: [email protected] 0009-3084/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0009-3084(98)00020-6

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1. Introduction N-aclyethanolamines (NAEs) and N-acylphosphatidylethanolamines (NAPEs) are present in biological membranes under normal conditions as well as under conditions of stress such as injury in animals, or dehydration in plant seeds (Schmid et al., 1990). Both NAEs and NAPEs were found to accumulate in mammalian cells undergoing extensive degeneration involving phospholipid degradation such as infarcted canine myocardium (Epps et al., 1980, 1982a) and ischemic rat brain (Natarajan et al., 1986). NAEs are produced in vivo by the degradation of NAPEs by a phospholipase D type enzyme (Schmid et al., 1996). They exhibit diverse and interesting biological and medicinal properties such as stabilization of mitochondrial membranes against permeability dependent Ca2 + release stimulation of Ca2 + Mg2 + , ATPase activity, as well as antibacterial, antiviral and antiinflammatory activities (Epps et al., 1982a,b; Schmid et al., 1990). Recently, it has been shown that N-arachidonylethanolamine (anandamide) acts as an endogenous ligand for cannabinoid receptor (Devane et al., 1992), inhibits gap junction conductance (Venance et al., 1995) and reduces sperm fertilizing capacity by inhibiting the acrosome reaction (Schuel et al., 1994). In addition, NAEs and N-acyl PEs are potentially useful in formulating liposomal drug delivery systems because N-acyl egg-PE and N-palmitoyl DPPE have been shown to stabilize liposomes (Domingo et al., 1993; Mercadal et al., 1995), while NAEs have been shown to stabilize bilayer structure (Ambrosini et al., 1993a). In view of the foregoing, it is important to investigate the properties of NAEs and NAPEs in a systematic manner. Though considerable amount of work has been done on the biological, medicinal and pharmacological properties of NAEs (see, for reviews, Schmid et al., 1990, 1996), very few biophysical studies have been reported regarding their phase behavior and their interaction with other membrane lipids. In one study, Epps and Cardin (1987) showed that mixing Noleoylethanolamine with phosphatidylcholine vesicles decreases the phase transition temperature of DPPC whereas in other studies Ambrosini et

al. (1993a,b) showed that N-lauroylethanolamine and N-oleoylethanolamine stabilize the bilayer structure of egg PE and investigated the interaction of several NAEs with DPPC multilamellar liposomes. Apart from these studies, to the best of the authors’ knowledge, there were no other reports on the physical behavior and phase properties of NAEs. In view of this lacuna the authors have initiated systematic studies on the phase behavior of NAEs and NAPEs and investigated the phase behavior of a homologous series of NAEs containing even number of C-atoms in the acyl chains using differential scanning calorimetry (Ramakrishnan et al., 1997) and also characterized the thermodynamics of the chain-melting phase transitions of a homologous series of NAPEs with matched N- and O-acyl chains (Swamy et al., 1997). These investigations have been extended to NAEs with odd number of C-atoms in the acyl chains and the results obtained are presented in this communication.

2. Materials and methods

2.1. Materials All the fatty acids used were purchased from (Sigma, MO). Oxalyl chloride was a product of (E. Merck, Germany). All other reagents were of analytical grade and were obtained from local suppliers. Solvents were distilled and dried before use. Aqueous solutions were made with double distilled water. N-acylethanolamines containing odd number of C-atoms in the acyl chains were synthesized by the reaction of acid chlorides with 2-ethanolamine and characterized by TLC and IR spectroscopy as described earlier (Ramakrishnan et al., 1997).

2.2. Differential scanning calorimetry All the calorimetric measurements were carried out at a scan rate of 2.5°/min using a PerkinElmer DSC-4 differential scanning calorimeter equipped with a data station. The details of sample preparation have been described in an earlier publication (Ramakrishnan et al., 1997). Transi-

M. Ramakrishnan, M.J. Swamy / Chemistry and Physics of Lipids 94 (1998) 43–51

tion enthalpies (DHt) were determined by integrating the peak area and transition entropies DSt were determined from the transition enthalpy (DHt) assuming a first order transition according to the expression (Marsh, 1990): DSt =DHt/Tt

(1)

where Tt refers to the transition temperature.

3. Results and discussion

3.1. DSC studies on dry NAEs Differential scanning calorigrams of heating and cooling scans of the thermotropic phase transitions of dry N-heptadecanoylethanolamine are shown in Fig. 1 as a representative example. The compound gives two endothermic transitions in the first heating scan — a minor transition around 90°C followed by a major transition at 100.3°C (scan A). The transition observed at 100.3°C coincides with the capillary melting point of the compound and therefore it is the chain-melting phase transition. A single exothermic transition is observed in the first cooling scan (scan B). The minor peak around 90°C observed in the first heating scan is not seen in the second or subsequent scans (scan C) and is likely to be due to a solid-solid phase transition but is not further pursued here. All the NAEs containing 13 – 19 Catoms in the acyl chains displayed similar thermograms under dry condition, except N-nonadecanoylethanolamine, for which the minor transition was seen in the subsequent heating scans also. The NAEs with 9 and 11 C-atoms in the acyl chains gave only a single phase transition in all the heating cycles. The values of Tt, DHt and DSt determined from the heating scans of dry samples of NAEs are listed in Table 1.

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melting transition, the aqueous dispersions show only one sharp gel-fluid phase transition, both in the heating and in the cooling scans. The transition is centered at 76.9°C, which is considerably lower than the Tt value of 95.9°C, observed for the dry N-pentadecanoylethanolamine (Table 1). However, the DHt value obtained is comparable in magnitude to that of the major chain-melting transition of the dry sample, indicating that this transition most likely corresponds to the chainmelting transition of the fully hydrated N-pentadecanoylethanolamine. All the other NAEs with odd chainlengths in the range of 11–19 C-atoms also show a single gel-fluid transition in both the heating and cooling scans and in each case the

3.2. DSC studies on hydrated NAEs Differential scanning calorigrams of an aqueous dispersion of N-pentadecanoylethanolamine are shown in Fig. 2 as a representative example. In contrast to the dry samples of this lipid, which exhibited a minor transition and a major chain-

Fig. 1. Differential scanning calorigrams of dry N-heptadecanoylethanolamine: A, first heating scan; B, cooling scan carried out immediately after the first heating scan; and C, second heating scan recorded immediately after the cooling scan. Subsequent heating and cooling scans were identical to C and B, respectively. Upward curves are endotherms and downward curve is exotherm. Scan rate: 2.5°/min.

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Table 1 Transition temperatures (Tt), DHt and DSt for the phase transitions of N-acylethanolamines containing odd number of C-atoms in the acyl chains Chainlength (n)

9 11 13 15 17 19 a

DHt (kcal/mol)

Tt (°C)

Dry

Hydrated

68.5 81.5 88.8 95.9 100.3 103.0

55.9 68.5 76.9 82.9 87.9

a

Dry (major) 4.7 6.8 7.9 9.9 11.6 12.0

( 9 0.4) (9 0.5) (9 0.5) (9 1.4) ( 90.6) (9 0.6)

DSt (cal/mol per K)

Dry (total)

Hydrated

4.7 6.8 7.9 9.9 11.6 13.5

6.7 9.9 11.1 12.7 14.1

(9 0.4) (9 0.5) (9 0.5) (9 1.4) (90.6) (9 0.5)

a

(9 0.3) (9 0.1) (9 0.4) ( 90.4) (9 0.7)

Dry (major)

Dry (total)

13.8 19.2 21.8 26.8 31.3 31.9

13.8 19.2 21.8 26.8 31.3 35.9

(91.2) (9 1.4) (9 1.4) (9 3.8) ( 9 1.3) (9 1.3)

( 9 1.2) (9 1.4) (9 1.4) (9 3.8) ( 9 1.3) ( 9 1.5)

Hydrated a

20.4 29.0 31.7 35.7 39.1

( 90.9) ( 90.3) ( 91.1) ( 91.1) ( 91.9)

Aqueous dispersions of N-nonanoylethanolamine do not show any phase transition in the temperature range 20 – 130°C.

transition occurs at a temperature that is considerably lower than the chain-melting phase transition temperature observed for the dry sample. Hydrated dispersions of N-nonanoylethanolamine did not give any transition in the range 20 – 130°C. The values of Tt, DHt and DSt determined from the heating scans of fully hydrated dispersions of NAEs are also listed in Table 1.

Fig. 2. Differential scanning calorigrams of N-pentadecanoylethanolamine in presence of excess water: A, heating scan; and B, cooling scan. Upward curve is endotherm and downward curve is exotherm. Scan rate: 2.5°/min.

3.3. Chainlength dependence of DHt and DSt The chainlength dependence of the DHt and DSt, for the dry and hydrated samples of NAEs with odd acyl chainlengths are given in Fig. 3A,B, respectively. In the case of dry samples, the total enthalpy for the major and minor transitions has been plotted as a function of chainlength. In the chainlength range of 9–19 C-atoms for the dry samples and 11–19 C-atoms for the hydrated dispersions, a linear dependence of the calorimetric parameters on the chainlength was observed. For the dry samples, similar linear dependence was also observed when the data obtained for the major (chain-melting) transition alone were plotted against the chainlength (plot not shown). The data fit well with the expressions 2 and 3 given below (Larsson, 1986), as has been observed with the NAEs with even acyl chainlengths (Ramakrishnan et al., 1997): DHt = (n− 2)DHinc + DHo

(2)

DSt = (n− 2)DSinc + DSo

(3)

where DHo and DSo are the end contributions to the DHt and DSt, respectively, arising from the terminal methyl group and the head group region. DHinc and DSinc are the incremental values of DHt and DSt contributed by each CH2 group. A linear least square analysis of the chainlength dependent values of the DHt and DSt corresponding to the chain-melting phase transitions of different NAEs studied here yielded the incremental values (DHinc

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Fig. 3. Chain length dependence of the DHt: (A) and DSt; (B) of N-acylethanolamines of odd chainlengths. Data obtained from heating scans of dry () and hydrated (“) samples of NAEs are shown. For dry samples the combined total enthalpy and total entropy for the major and minor transitions was plotted. See Section 3 for more details.

and DSinc) and end contributions (DHo and DSo) for the dry and aqueous dispersions. The values obtained are listed in Table 2. Similar analysis has also been done on the combined total enthalpy and entropy of the major and minor transitions of the dry NAEs and the resultant incremental values and end contributions have been presented in Table 2. For such analysis, only those minor transitions that were seen in the second heating scan and subsequent heating scans were considered. Additionally, analysis of the chainlength dependence of the combined total enthalpy and entropy of the major and minor transitions of the dry NAEs of even chainlength was also done using the data from the earlier study on the chain-melting phase transitions (Ramakrishnan et al., 1997) and unpublished results on the minor transitions. The incremental values and end contributions obtained are also listed in Table 2. The incremental values (DHinc, DSinc) obtained here for the odd chainlength NAEs are compara-

ble to the values obtained earlier for the even chainlength NAEs but are considerably larger than the corresponding values obtained for different diacyl phospholipids on a per chain basis, suggesting that the acyl chains in NAEs pack more tightly than those of the diacyl phospholipids (Marsh, 1990; Swamy et al., 1994; Ramakrishnan et al., 1997). The values of DHo and DSo, both for the dry samples as well as for the hydrated dispersions are rather small and therefore contribute only marginally to the overall DHt and DSt. The linear chainlength dependence of the DHt and DSt observed here for the homologous series of NAEs with odd number of C-atoms in the acyl chains, both in the dry state and when fully hydrated, suggests that the structures of the different NAEs investigated here are rather similar both in the solid state and in the liquid state. Additionally, the structures in the gel phase and the fluid state are likely to be similar for the

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Table 2 Incremental values (DHinc, DSinc) of the chainlength dependence and end contributions (DHo, DSo) to the phase transition enthalpy DHt and entropy DSt of dry and hydrated samples of odd and even chainlength series of N-acylethanolamines Lipid NAEs NAEs NAEs NAEs NAEs NAEs

(odd) (dry, major transition) (odd) (dry, total) (odd) (hydrated) (even) (dry, major transition) (even) (dry, total) (even) (hydrated)

DHinc (kcal/mol)

DHo (kcal/mol)

DSinc (cal/mol per K) DSo (cal/mol per K)

0.769 0.06 0.869 0.03 0.889 0.10 0.829 0.02 1.029 0.06 0.959 0.06

−0.2590.72 −1.2890.35 −0.5491.32 −0.19 0.26 −1.7790.71 −0.529 0.82

1.87 90.15 2.16 90.08 2.21 90.29 2.01 9 0.06 2.50 9 0.16 2.37 9 0.17

NAEs with odd number of C-atoms in the chainlength range of 11 – 19 C-atoms. Since Nnonanoylethanolamine did not show any phase transition in the hydrated state, its structure is most probably different from the other members of the series.

Fig. 4. Chain length dependence of the transition temperatures of N-acylethanolamines of odd chainlengths. Data obtained from heating scans of dry () and hydrated (“) samples of NAEs are shown. The solid lines correspond to nonlinear least squares fits of the Tt values to Eq. (4). The fitting parameters obtained are: T =403.5 K, no − n %o = 3.69, n %o = 1.23 for t (dry) the dry samples and T = 395.2 K, no − n %o = 4.67, n %o = t (hydr) 2.39 for the hydrated samples.

1.61 91.92 −1.1590.96 2.52 9 3.82 2.129 0.71 −2.199 2.08 3.099 2.33

In an earlier study, it was observed that the values of DHinc and DSinc corresponding to the chain-melting transitions are considerably different in the absence of water and in its presence, for NAEs with even number of C-atoms in the acyl chains (Ramakrishnan et al., 1997). The data presented in Table 2 show that this is true also for

Fig. 5. Alternation in the chainlength dependence of the transition temperatures of N-acylethanolamines. Data obtained from the heating scans of dry () and hydrated (“) samples are shown.

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Fig. 6. Alternation in the chainlength dependence of the DHt (A) and DSt (B) of N-acylethanolamines. Data obtained from the heating scans of dry () and hydrated (“) samples are shown.

the NAEs with odd number of C-atoms in the acyl chains. However, when the values of DHo and DSo obtained for the major transition and the minor transition were combined and plotted as a function of the chain length, then the incremental values obtained are comparable, within the range of experimental error, to the values of DHinc and DSinc obtained for the hydrated dispersions of the NAEs, for both the even chainlength series and the odd chainlength series. While the incremental values, due to the contributions of the methylene units are comparable in the two cases, significant differences can be seen in the end effects for both the even and odd chainlength series between the dry samples and the hydrated ones. These differences most probably arise due to the differences caused by the of hydration of the head group region of the molecules.

3.4. Chainlength dependence of transition temperatures As seen from the data given in Table 1, transi-

tion temperatures of the homologous series of NAEs increase with increasing acyl chainlength, however, the increments by which the Tt increases when the chainlength is increased by the same number of C-atoms, get progressively smaller as the chainlength increases, both for the dry samples and the hydrated dispersions. This is more readily seen in Fig. 4, where the transition temperatures are plotted as a function of the acyl chainlength. Similar chainlength dependence has also been observed for the NAEs with even number of C-atoms in the acyl chains (Ramakrishnan et al., 1997). Additionally, similar trends have also been observed for long chain fatty acids and a variety of phospholipids in aqueous dispersion (Larsson, 1986; Marsh, 1990, 1991; Swamy and Marsh, 1995; Swamy et al., 1994, 1997; Ramakrishnan et al., 1997). As the chainlength increases, the contribution of the last terms in Eqs. (2) and (3) to the overall DHt and DSt will become negligible as the contributions arising from the (CH2)n portion of the molecules, i.e., (n− 2) · DHinc and (n−2) · DSinc,

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dominate the overall enthalpy and entropy of the transition. Therefore, at infinite chainlength, Eqs. (2) and (3) can be reduced to: DHt =(n− 2)DHinc

(2a)

DSt =(n− 2)DSinc

(3a)

The transition temperature at infinite chainlength will then be given by T t =DHinc/DSinc. From the data given in Table 1 the values of T for the dry and hydrated forms of NAEs t with odd number of C-atoms in the acyl chains can be estimated as 404.3 and 398.2 K, respectively. It has been shown for several diacyl phospholipids which exhibit a linear dependence of the calorimetric properties, DHt and DSt, for the chain-melting phase transitions on the chainlength, that the data can be fit to the following expression (Marsh, 1991; Swamy et al., 1994; Swamy and Marsh, 1995). This has been shown to hold good for single chain amphiphiles such as the fatty acids and NAEs of even acyl chainlengths with regard to the chain-melting phase transition in the dry state as well as the corresponding phase transitions of the fully hydrated dispersions of NAEs (Ramakrishnan et al., 1997): Tt = DHt/DSt = (DHinc/DSinc)[1 − (no −n%o/n − n%o)] (4) where no (= − DHo/DHinc) and n%o ( = −DSo/ DSinc) are the chainlengths at which the DHt and DSt, respectively, extrapolate to zero. It can be seen from Fig. 4 that the transition temperatures of NAEs with odd number of C-atoms in the acyl chains fit rather well with Eq. (4), both in the dry state as well as in the hydrated form. Moreover, from the fitting parameters, the transition temperatures at infinite chainlength can be estimated. The values estimated from the data given in the legend to Fig. 4 are: T t(dry) =403.5 K with x 2 of 0.6 K and T t(hydr) =395.2 K with x 2 of 0.1 K. These values agree rather well with the values of 404.3 and 398.2 K, respectively, for the dry and fully hydrated samples of NAEs with odd number of C-atoms in the acyl chains, predicted from the linear regression of the DHt and DSt.

3.5. Alternation in transition temperatures and calorimetric properties of NAEs with e6en and odd chainlengths When the calorimetric properties of the NAEs of even chainlength obtained in a previous study (Ramakrishnan et al., 1997) were compared with those obtained for the NAEs with odd acyl chainlength, an alternation has been observed in the transition temperatures, DHt and DSt between the two series of compounds. In general it was observed that the even chainlength series exhibit somewhat higher values of Tt, DHt and DSt than the odd chainlength series. In the case of transition temperatures, this alternation is prominently seen in the dry samples, but is rather unclear in the data obtained for the hydrated dispersions (Fig. 5). In the case of DHt and DSt the alternation is quite marked with both the dry samples and the hydrated dispersions (Fig. 6). Such alternation in transition temperatures and other physical properties has also been observed in other compound classes such as the hydrocarbons and long chain fatty acids (Larsson, 1986; Tanford, 1991). Alternation in the melting points and other physical properties in long chain molecules can be explained on the basis of the packing of the hydrocarbon chains. For example, in long chain fatty acids it has been shown by Larsson (1966) that alternation in the physical properties can be explained on the basis of differences in the packing of the terminal methyl groups between even and odd members. When the hydrocarbon chains are vertical in the layers then no alternation will be seen. However, when the hydrocarbon chains are tilted then differences in the packing of the terminal methyl groups can occur between the even and odd members, resulting in an alternation in the physical properties. The alternation seen in the present study in the melting points and calorimetric properties of NAEs therefore suggest that the acyl chains in them would be tilted with respect to the end planes.

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Acknowledgements This work was supported by a research grant (SR/OY/C-15/93) from the Department of Science and Technology, Government of India to MJS.

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