Differential scanning calorimetric studies on the thermotropic phase behavior of dry and hydrated forms of N-acyltyramines

Article abstract of DOI:10.1016/j.tca.2014.03.030

In this paper, a homologous series of N-acyltyramine (NATA) - which is biosynthetic precursor for neuroactive lipids, N-acyldopamine - have been synthesized and their thermotropic phase transitions were characterized by differential scanning calorimetry (

Full text of DOI:10.1016/j.tca.2014.03.030

Thermochimica Acta 586 (2014) 25–29
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Differential scanning calorimetric studies on the thermotropic phase
behavior of dry and hydrated forms of N-acyltyramines
a
b
a,∗
Stevenson Priscilla , D. Sivaramakrishna , Veerappan Anbazhagan
a
Department of Chemistry, School of Chemical and Biotechnology, SASTRA University, Thanjavur, Tamil Nadu, India
School of Chemistry, University of Hyderabad, Hyderabad, Andhra Pradesh, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a homologous series of N-acyltyramine (NATA) – which is biosynthetic precursor for neu-
roactive lipids, N-acyldopamine – have been synthesized and their thermotropic phase transitions were
characterized by differential scanning calorimetry (DSC). NATA undergoes a major sharp endothermic
transition with the melting point of the hydrated samples occurs considerably at lower temperature com-
pared to the dry samples. Thermodynamic parameters, transition enthalpy (ꢀHt) and transition entropy
Received 8 January 2014
Received in revised form 19 March 2014
Accepted 22 March 2014
Available online 18 April 2014
(
ꢀSt), associated with the chain-melting phase transitions depends linearly on the number of carbon
Keywords:
N-Acyltyramine
Fatty acid amides
Differential scanning calorimetry
Phase transition
atoms. The contributions of each methylene unit to the transition enthalpy and entropy and the end
contributions arising from the head group and the terminal group was determined and reported. These
results are very important to understand the thermodynamics basis of NATA phase properties in other
membrane lipids.
Chain length dependence
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Recent advancement in the field of lipodomics identified N-
easily undergoes oxidation by monophenol monoxygenase to
NADA, whereas N-acyltyrosine undergoes hydrolysis under this
condition [4]. The possibility of NADA bio-synthesis from cor-
responding NATA was further evidenced from the presence of
N-acyltyramines in tissues where N-acyldopamines are located [5].
However, the bio-synthesis and the structure-activity relationship
of these NATA in tissues are yet to be elucidated.
acyl derivatives of dopamine in the mammalian tissues. This
belongs to an important class of endogenous signaling molecules,
formed by an amide linkage with long chain fatty acids and cat-
echolamines [1,2]. Although there are strong evidences for the
existence of diverse group of fatty acid amides (FAAs) in the brain
and other tissues, only a few have been identified to date [3]. The
complete functions of these identified FAA are yet to be eluci-
dated. Biochemical studies have reveled that FAA have numerous
physiological functions in mammalian tissues. For example, N-
arachidonoyldopamine and N-oleoyldopamine binds to transient
receptor potential vanilloid 1 (TRPV1) with a potency similar to
that of capsaicin, and produce thermal hyperalgesia [1,2]. The
bio-synthesis of N-acyldopamine (NADA), a new family of multi-
function lipid effectors of cannabinoid-vanilloid system, remains
enigmatic. A proposed bio-synthesis involves several closely conju-
gated enzymatic stages proceed via condensation of free fatty acids
with tyrosine and the subsequent conversion of N-acyltyrosine
to N-acyldopamine [4]. An analogous pathway involves acyla-
tion of tyramine by an unknown enzyme. N-acyltyramine (NATA)
While significant amount of work has been contacted on N-
acylethanolamine [6], N-acylamino acids [6,7], N-acyldopamines
[8,9], there are no report on physiological roles and pharmaco-
logical potential of N-acyltyramines. N-Acetyltyramine has been
identified in mammalian urine [10] and tyramine N-acylation activ-
ities were heterogeneously distributed throughout the brain [11].
Enzymes acetyl-CoA, arylamine N-acetyltransferase have been
identified for the N-acetylation of the tyramines [11]. In mice
model it has been demonstrated that N-acetyltyramine reduces
the locomotors activity and prolonged the duration of barbitu-
rate anesthesia [12]. Tyramines are primarily metabolized by an
enzyme, monoamine oxidase (MAO) [13], whereas the correspond-
ing N-acetyltyramines are not metabolized by MAO, suggesting
different physiological role for N-acetyltyramines [11]. NATA are
found to be better substrates for tyrosinase, an enzyme responsi-
ble for the production of melanin [14]. In view of their presence in
mammalian system, and considered to be biosynthetic precursor
for N-acyldopamine, it is crucial to carry out systematic studies on
the properties of NATA. The knowledge of their phase behavior in
aqueous dispersion in pure state as well as in mixtures with other
∗ Corresponding author. Tel.: +91 4362 264101 657; fax: +91 4362 264120.
E-mail addresses: anbazhagan@scbt.sastra.edu, anbugv@gmail.com
(
V. Anbazhagan).
http://dx.doi.org/10.1016/j.tca.2014.03.030
040-6031/© 2014 Elsevier B.V. All rights reserved.
0

2
6
S. Priscilla et al. / Thermochimica Acta 586 (2014) 25–29
membrane lipids is very important to delineate how they function
in vivo. As the first step in this direction, a homologues series of N-
acyltyramines with saturated acyl chains of even number of carbon
atoms has been synthesized and studied by differential scanning
calorimetry (DSC). The thermotropic phase behavior of dry and
hydrated samples of NATA are obtained, analyzed and reported.
For studies with both dry and hydrated samples, transition
enthalpy (ꢀHt) for each transition was determined by integrating
the area under the transition. In all cases only the first heating scan
is considered for further analysis. Transition entropies (ꢀSt) were
determined from the transition enthalpies assuming a first order
transition according to the equation [15]:
ꢀHt = Tt · ꢀSt
(1)
2
. Materials and methods
where Tt is the transition temperature determined from the peak of
the thermogram. ꢀHt values are taken at this temperature in order
to calculate the corresponding ꢀSt values.
2.1. Materials
Tyramine, decanoyl chloride, lauroyl chloride, myristoyl chlo-
3
. Results and discussion
ride and palmitoyl chloride were obtained from Sigma–Aldrich,
USA. All other reagents were of analytical grade obtained from local
suppliers. All solvents were distilled and dried prior to use.
3.1. Synthesis and characterization of N-acyltyramines
N-Acyltyramines of even acyl chainlengths (n = 10–16) have
2
.2. Synthesis of N-acyltyramines
been synthesized in fair yields by the reaction of the corresponding
acid chlorides with tyramine. NATA were purified by column chro-
N-Acyltyramines were synthesized by simple condensation
◦
matography and recrystallization (from DCM/acetone at −20 C),
reaction between fatty acid chloride and tyramine. Briefly, 200 mg
of tyramine were dissolved in 5 ml of dichloromethane and tetrahy-
drofuran (2:1, v/v). Approximately, 1 mole equivalent of the acid
chloride and triethylamine were then added in an ice-bath, under
constant stirring. After all the reagent was added, the reaction
was allowed to continue for 24 h at room temperature. The com-
pletion of the reaction was judged by thin layer chromatography
on silica gel (solvent system: hexane/ethylacetate 70/30, v/v).
Then, the solution was washed successively with double distilled
water, dilute hydrochloric acid and dilute bicarbonate solution.
The organic layer was collected and evaporated to dryness under
reduced pressure. The resultant residue was purified by flash chro-
matography (eluent: hexane/ethylacetate 70/30, v/v). The obtained
yields ranged between 55 and 60% for different NATA. The NATAs
were found to be pure by thin layer chromatography. The IR spec-
tra of the purified NATA shows an absorption band corresponding
−1
to the amide I and amide II at 1637 and 1548 cm , respectively
(
Fig. S1). Further the structures of the NATA were characterized by
1
13
H NMR (Fig. S2) and C NMR (Fig. S3) spectroscopy. In addition,
purity of the NATA was analyzed by LC–MS (Fig. S4) and elemental
analysis. The results obtained from these methods are fully con-
sistent with structures of NATA and suggest that they are of high
purity.
3
.2. Differential scanning calorimetry
Fig. 1 shows a differential scanning calorigrams of saturated
NATA of even acyl chainlengths. The heating thermograms pre-
sented in Fig. 1 shows endothermic solid to fluid phase transition.
Except N-decanoyltyramine and N-dodecanoyltyramine, all other
NATA shows two peaks with major transition matching the cap-
illary melting point of the compound. The minor transitions are
reproducible for samples obtained from different batches, indi-
cating the presence of solid-solid phase transition with possible
polymorphism. However, the cooling scans gave broad peaks, with
the mid-point of the transition centered at several degrees lower as
compared to the heating thermograms (Fig. S5). This suggests that
during cooling, some of the NATA does not return back to its orig-
inal structural form. The reason for this not known with certainty,
but could be related to difficulty in effectively packing the N-acyl
chain in the solid phase. However, upon immediate second heat-
ing, the major chain-melting transition reappears with an enthalpy
identical to that observed in the first heating (Fig. S4). Nevertheless
there was a small decrease in the transition enthalpies of minor
peaks. These calorimetric data provide strong evidence for the pres-
ence of a metastable state [16]. Therefore, in all cases first heating
scan is consider for further analysis, and the total area under the
major transitions was integrated to get the transition enthalpies.
The phase transition temperatures, Tt, were estimated from the
peak positions at highest the maximal heat flow, whereas the tran-
sition entropy, ꢀSt were calculated from the transition enthalpy
using Eq. (1). The transition temperatures (Tt), transition enthalpies
(ꢀHt) and transition entropies (ꢀSt) obtained from the first heating
thermograms presented in Table 1.
1
obtained were characterized by TLC, melting point, FTIR, H NMR,
13
C NMR, elemental analysis and LC–MS (details in supporting
information).
2.3. Differential scanning calorimetry
DSC studies were carried out on a Perkin-Elmer PYRIS Diamond
differential scanning calorimeter on NATA that were obtained by
◦
recrystallization from dichloromethane/acetone mixture at −20 C.
About 1–3 mg of dry NATA were weighed accurately into aluminum
sample pans, covered with an aluminum lid and sealed by crimping.
Reference pans were prepared similarly, but without any sample in
them. Heating and cooling scans were performed from room tem-
◦
◦
◦
perature (ca. 25 C) to about 110 C at a scan rate of 2.0 C/min, and
each sample was subjected to three heating scans and two cooling
scan.
DSC studies of hydrated NATAs were carried out on a VP DSC
microcalorimeter from MicroCal (Northampton, MA). Samples of
NATAs (4–5 mg) were weighed accurately in aluminum pans and
transferred into clean, dried glass test tubes. Each sample was dis-
solved in ∼300 ␮L of chloroform, and a thin film of the lipid was
obtained by blowing a gentle stream of dry nitrogen gas over the
solution. Final traces of the solvent were removed by vacuum des-
iccation for about 4–5 h. Then the thin film was then hydrated with
1
.0 mL of double distilled water and the hydrated sample was sub-
jected to 5–6 cycles of freeze–thawing with occasional vortexing
in order to obtain a homogeneous suspension, which was used for
DSC studies. Each sample was subjected to three heating scans and
two cooling scans. While the lower end of the scan was set as 10 C
the upper end was set between 85 and 110 C depending on the
DSC of NATA in the presence of excess of water shows
an endothermic solid to fluid phase transition. Except N-
decanoyltyramine and N-dodecanoyltyramine, all NATA shows
two peaks with major transition lower than the dry sample,
indicating that the hydration results in reducing the chain melt-
ing transition temperature. In general, the hydration of polar
◦
◦
main, chain-melting phase transition temperature of the sample.
All scans were performed at a scan rate of 1 /min.
◦

S. Priscilla et al. / Thermochimica Acta 586 (2014) 25–29
27
Fig. 1. Differential scanning calorigrams of N-acyltyramines (A) dry and (B) hydrated states. The number of carbon atoms in the acyl chain is indicated against each thermogram.
◦
Scan rate was 1.0 C/min.
Table 1
Transition temperatures (Tt), transition enthalpies (ꢀHt), and transition entropies (ꢀSt) of N-acyltyramines in dry and hydrated states.
Acyl chain length (n)
Dry NATA
Hydrated NATA
◦
−1
−1 −1
◦
−1
−1 −1
ꢀSt (cal mol K )
Tt ( C)
ꢀHt (kcal mol
)
ꢀSt (cal mol
K
)
Tt ( C)
ꢀHt (kcal mol
)
10
12
14
16
81.47
91.39
98.05
5.08
6.69
10.43
11.85
14.32
18.38
28.13
31.46
68.49
80.78
89.95
96.01
1.55
4.46
6.08
8.25
4.55
12.60
16.77
21.65
103.76
head group reduces the chain melting transition temperature.
Similar observations were reported for the aqueous disper-
sion of phosphotidylcholine [17], phosphotidylethanolamine [17]
N-biotinylphosphotidylethanolamine [18], N-acylethanolamines
chains in the solid phase and aqueous dispersion of NATA are more
ordered than the respective solid phase and aqueous dispersion of
NAE. This difference in the ꢀHinc of NATA and NAE possibly arises
from the hydrophobic effect and ␲-stacking of the aromatic rings
of the phenolic moiety. The observed negative end contributions of
enthalpy, ꢀH , and entropy, ꢀS , is an indicative of the hydropho-
[
[
19], N-acylphosphotidylethanolamine [20] and N-acyldopamines
8].
0
0
bic effect playing a role in packing the NATAs. The more negative
ꢀS0 for hydrated samples as compared to the dry samples indi-
cate that the water molecules around the phenolic moiety plays
a significant role in packing the aqueous dispersion of NATAs. On
3
.3. Chain length dependence of transition enthalpy and
transition entropy
The chain length dependence of transition enthalpy and transi-
tion entropy for the chain-melting phase transitions of the various
NATAs investigated in this study, both for the dry samples and the
hydrated dispersions are reported in Table 1. Fig. 2 shows that both
transition enthalpy and transition entropy are approximately lin-
ear with respect to chainlength (n). The data could be fit well to
expressions 2 and 3 given below [21]:
ꢀ
Ht = (n − 2)ꢀHinc + ꢀH0
St = (n − 2)ꢀSinc + ꢀS0
(2)
(3)
ꢀ
where ꢀH0 and ꢀS0 are the end contributions to the transition
enthalpy and transition entropy, respectively, arising from the ter-
minal methyl group and the polar head group region of NATA. ꢀHinc
and ꢀSinc are the incremental values of transition enthalpy and
transition entropy per CH group. Theoretically, ꢀHinc gives a mea-
2
sure of cohesiveness of acyl chain packing. The linear chain length
dependence of transition enthalpy and transition entropy suggests
that the structure, molecular packing and intramolecular interac-
tion of the different NATAs in the solid state are likely to be very
similar. From the linear least square analysis, the incremental val-
ues and end contributions of transition enthalpy and entropy were
determined and reported in Table 2. For comparison, data for the
corresponding NAEs [22] and NADAs [8] are reported in Table 2. The
values of ꢀHinc for solid-to-liquid phase transition of NATA in the
dry state are higher than the ꢀHinc in the hydrated state. Similar
observation was found in NAE [20]. Nevertheless, ꢀHinc found in
the case of NATA, both in dry state and hydrated state, are signifi-
cantly higher than those obtained for NAE. It is likely that the acyl
Fig. 2. Chain length dependence of transition enthalpies (A) and transition entropies
(
B) of N-acyltyramines. Data obtained from heating scans of dry (᭹) and hydrated
ꢀ) samples of NATA are shown. Solid line represents linear least-squares fit of the
(
data.

2
8
S. Priscilla et al. / Thermochimica Acta 586 (2014) 25–29
Table 2
Incremental values (ꢀHinc, ꢀSinc) of chainlength dependence and end contributions (ꢀH0, ꢀS0) to phase transition enthalpy and entropy of N-acyltryamines.
Thermodynamic parameters
Dry fatty acid amides
aNAEs
Hydrated fatty acid amides
NATAs
bNADAs
aNAEs
NATAs
bNADAs
ꢀ
ꢀ
ꢀ
ꢀ
Hinc (kcal mol−1)
H0 (kcal mol 1)
0.82
−0.10
2.01
1.21
−4.72
3.05
−10.57
0.95
1.01
2.27
7.22
0.95
−0.52
2.37
1.09
−6.86
2.77
−16.62
1.42
−10.55
3.76
−26.99
−
−
1
−1
Sinc (cal mol
K )
−
1
−1
)
S0 (cal mol
K
2.12
3.1
a
For comparison thermodynamic values are adopted from ref. [20].
For comparison thermodynamic values are adopted from ref. [9].
b
the other hand, the end contribution from enthalpy and entropy of
the aqueous dispersion of NATAs is less negative compared to N-
acydopamine [see Table 2]. This could arise due to the presence of
catechol moiety in NADA, which are expected to form water medi-
ated hydrogen bond between the catechol moieties (2H-bond). It is
−
1
well known that, each hydrogen bond contributes 5–6 kcal mol
for the enthalpy of stabilization [22]. The additional stabilization
from H-bond, might impose geometrical constrain in ␲-stacking
the catechol moieties, results in more negative end contribution
from the enthalpy and entropy. Whereas NATA differs from NADA
by one OH group in the ortho-position. NATA contains one OH
group, therefore, it is expected to have only one H-bond between
the phenolic moieties of NATAs. Therefore, the imposed geomet-
rical constrain in forming ␲-stacking in the NATAs might be less
compared to the NADA. As a result ꢀH , and ꢀS0 is less nega-
0
tive for NATA. Furthermore, the gel-to-fluid transition temperature
Fig. 3. Chain length dependence of phase transition temperatures of NATA in the
dry (᭹) and hydrated (ꢀ). Solid line corresponds to non-linear least-squares fit of
each data set to Eq. (6).
◦
decreases upon hydration of NATAs was >8 , whereas the cor-
◦
◦
responding transition for NADA was ∼5 and for NAE was >15 .
Collectively, these observation clearly suggest that the acyl chains
are less ordered in the aqueous dispersion of NATA than NADA,
whereas the acyl chains of NATAs are more tightly packed than the
NAE, and the most likely factor responsible for this are hydropho-
bic effect and ␲-stacking. Based on the above analysis, we suggest
that the chain packing in the fatty acid amides might be in the fol-
lowing order, NAE < NATA < NADA. The results also illustrate that
how slight modification in the polar head group region of FAAs can
significantly affect principles underlying the molecular packing in
FAAs.
It has been shown for a number of diacyl lipids as well as for
single chain amphiphiles, the chain length dependence of transition
temperature could be fitted with Eq. (6), provided the transition
enthalpy and transition entropy follows linear dependence [6,23].
ꢂ
ꢃ
ꢀ
ꢁ
ꢁ
ꢀ
Ht
ꢀHinc
ꢀSinc
(n − n )
0
ꢀTt =
=
St
1 −
(6)
ꢁ
n − n
0
ꢀ
ꢁ
where n = (−ꢀH /ꢀHinc) and n0 = (−ꢀS /ꢀSinc) are the values
0
0
0
of chain length (n) at which the transition enthalpy and transition
entropy extrapolate to zero. It can be seen from Fig. 3, that the
transition temperature of the entire series of both dry and hydrated
NATAs could be satisfactorily fit by non-linear least squares method
to Eq. (6). Additionally, from the fitting parameters, the transition
temperature at infinite chainlength of the dry and hydrated sam-
3.4. Chain length dependence of transition temperature
The transition temperature Tt, determined from the DSC stud-
ies, both for the dry samples of NATA as well as their dispersion
in water are listed in Table 1. It can be seen that transition
temperatures of the NATA increase with increasing chain length.
However, the magnitude of the change in transition of both dry
and hydrated samples decreases steadily with increase in the chain
length (Fig. 3). Similar trends were observed for the aqueous dis-
persion of phosphotidylcholine, phosphotidylethanolamine [17],
N-biotinyl phosphotidylethanolamine [18], N-acylethanolamines
∞
ples of N-acyltyramines, Tt , have been estimated as 400.7 and
3
88.9 K, respectively. The values estimated from the fitting param-
eters for the dry and hydrated NAEs are in reasonable agreement
∞
with the Tt values predicted from the linear regression of the
calorimetric data.
4. Conclusions
[
[
(
19], N-acylphosphotidylethanolamine [20] and N-acyldopamines
8]. As the acyl chain length increases, the total contribution from
In this paper, a homologous series of biological important N-
CH )n dominates the transition enthalpy and transition entropy,
2
acyl tyramines have been synthesized and their thermotropic phase
behavior of dry and hydrated states was studied by differential
scanning calorimetry. A linear dependence has been observed in
the thermodynamic parameters, ꢀHt and, ꢀSt, associated with the
chain-melting phase transitions in the dry state as well as in the
hydrated state. The incremental transition enthalpies, ꢀHinc, and
and results in negligible end contribution from transition enthalpy
and transition entropy. Therefore, at infinite acyl chain length, Eqs.
(
2) and (3) can be reduced to [21]:
ꢀ
Ht = (n − 2)ꢀHinc
St = (n − 2)ꢀSinc
(4)
(5)
entropies, ꢀSinc, per CH group are more than the NAEs, indicating
ꢀ
2
that chain packing in the gel and/or fluid phase differs between N-
acyltyramines and NAEs. This also implies that the acyl chains are
tightly packed in the NATAs, presumably due to the hydrophobic/␲-
stacking interactions from the non-polar moieties in the head group
Then the transition temperature for infinite chain length will be
∞
∞
given by Tt = ꢀHinc/ꢀSinc. From Table 2, the values of Tt for dry
and hydrated samples have been estimated as 394.3 and 391.6 K,
respectively.
region of NATAs. The end contribution of entropy, ꢀS , is also
0

S. Priscilla et al. / Thermochimica Acta 586 (2014) 25–29
29
negative for dry and hydrated samples, suggesting that hydropho-
bic effect playing a role in the packing of NATAs. However, the
values of ꢀHinc and ꢀSinc of the hydrated dispersion of NATAs
are considerably less than the aqueous dispersion of NADAs, sug-
gesting that the acyl chains are tightly packed in the aqueous
dispersion of NADAs. These results suggest that the head groups
metabolism of N-acyldopamines in rat tissues, Russ. J. Bioorg. Chem. 33 (2007)
02–606.
6
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[
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(
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N-acylphosphatidylethanolamines, Chem. Phys. Lipids 163 (2010) 266–279.
in determining the chain packing. In summary, our study shows
that how a slight modification in the polar head group region of
FAAs can significantly affect principles underlying the molecular
packing in FAAs. Currently, we are investigating the miscibility of
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This work was supported by research grants from Department of
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and Prof. T.R. Rajagopalan research fund, SASTRA University to VA.
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