Thermophysical study of several α- And β-amino acid derivatives by differential scanning calorimetry (DSC)

Article abstract of DOI:10.1021/je200292z

The present study reports a differential scanning calorimetry (DSC) study of the amino acids sarcosine [CAS Registry No. 107-97-1], α-alanine (dl) [CAS Registry No. 302-72-7], β-alanine [CAS Registry No. 107-95-9], N-benzyl-α-alanine (dl) [CAS Registry No

Full text of DOI:10.1021/je200292z

ARTICLE
pubs.acs.org/jced
Thermophysical Study of Several α- and β-Amino Acid Derivatives
by Differential Scanning Calorimetry (DSC)
,†
†
‡
§
§
María Victoria Roux,* Rafael Notario, Marta Segura, Ram ꢀo n Guzm ꢀa n-Mejía, Eusebio Juaristi, and
||
James S. Chickos
†
Instituto de Química Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain
PerkinElmer Espa ~n a S.L., Ronda de Poniente 19, 28760 Tres Cantos, Madrid
‡
§
Departamento de Química, Centro de Investigaci ꢀo n y de Estudios Avanzados del IPN, Apartado Postal 14-740,
7000 M ꢀe xico D.F., M ꢀe xico
0
Department of Chemistry and Biochemistry, University of Missouri-St. Louis, One University Boulevard, St. Louis,
Missouri 63121-4499, United States
ABSTRACT: The present study reports a differential scanning calorimetry (DSC) study of the amino acids sarcosine [CAS Registry
No. 107-97-1], α-alanine (DL) [CAS Registry No. 302-72-7], β-alanine [CAS Registry No. 107-95-9], N-benzyl-α-alanine (DL) [CAS
Registry No. 40297-69-6], and N-benzyl-β-alanine [CAS Registry No. 5426-62-0] in the temperature interval from T = 268 K to
their respective melting/decomposition temperatures. Temperatures and heat capacities as a function of temperature of all solids
and the enthalpies and entropies of fusion of two of the amino acids, sarcosine and N-benzyl-β-alanine, are reported.
’
INTRODUCTION
neglect all next-to-nearest neighbor interactions because of the
limited accuracy of the available experimental heat capacity data.
Although current improvements in instrumentation and proto-
col allow the experimental determination of heat capacities
by differential scanning calorimetry (DSC) within an error of
Amino acids are among the most important building blocks of
life and are also believed to play key roles in interstellar chemistry
1,2
as well as in the origin of life on Earth. They are the building
blocks of peptides and the backbone of proteins, and for this
reason, they have been extensively studied.
21
0
.15 %, the actual experimental uncertainty of the measured heat
capacity is often larger due to the presence of impurities in the
samples under investigation. Special care must be taken to
remove or reduce the water content, especially for liquid samples,
since the heat capacity of water departs considerably from that of
the majority of organic compounds. Estimations of heat capacity
of solids are more problematic than their liquid counterparts.
This is due in part to the lack of data but also due to the
anisotropic nature of the solid state. Phase transitions in solids
can affect their heat capacities near these transitions. Solids that
form liquid crystals, for example, appear to have larger heat
capacities in certain temperature regions, and total phase change
entropies seem to be attenuated in comparison to systems that
In addition to their fundamental biological importance, amino
acids present rather interesting physical properties; for example,
in solution or in the crystalline state they exist as zwitterions that
3
are stabilized by intermolecular electrostatic polarization, as well
4,5
as hydrogen-bonding interactions with their environment. By
contrast, in the gaseous state, or when isolated in a noble gas
4,5
matrix at low temperature, the neutral forms are more stable.
The knowledge of heat capacity as a function of temperature is
an essential property for the calculation of thermodynamic
properties such as ΔH, ΔS, and ΔG at different temperatures,
and this property plays an important role in identifying and
understanding transitions occurring in the solid state of crystals
and in liquid crystals. Heat capacities at T = 298.15 K have proven
quite useful in adjusting vaporization, sublimation, and fusion
enthalpies with temperature. Equations for doing this have
22
melt directly to isotropic liquids. Group values for estimating
the heat capacity of crystalline solids have been reported, but the
estimations in many cases have been hampered by the lack of
sufficient reference data.
6
ꢀ8
recently been reported by Chickos and co-workers.
are several compilations of critically evaluated calorimetrically
measured heat capacities,
There
During the past few years, we have been involved in the experi-
mental determination of enthalpies of fusion and heat capacities,
as well as the study of polymorphism of pure crystalline organic
9
ꢀ13
but new and reliable data on the
heat capacity of important families of compounds are still
2
3ꢀ31
1
4,15
compounds.
acids, reliable experimental thermochemical studies are scarce. Very
Nevertheless, despite the importance of amino
needed,
particularly for crystalline solids. There has been
an effort to develop reliable and accurate group contribution schemes
to improve the estimation and compensate for the scarcity of this
data. The simplest schemes are based on first-order additivity
32,33
recently we have carried out
a thermochemical study of α-alanine
(
DL), β-alanine, N-benzyl-α-alanine (DL), and N-benzyl-β-alanine,
16,17
and only consider the constituent groups of the molecule.
Other methods use a second-order additivity scheme that takes
Received: May 11, 2011
into account nearest-neighbor interactions in the definition of
Accepted: September 8, 2011
Published: September 26, 2011
18ꢀ20
the structural units of molecules.
These schemes normally
r 2011 American Chemical Society
3807
dx.doi.org/10.1021/je200292z
|J. Chem. Eng. Data 2011, 56, 3807–3812

Journal of Chemical & Engineering Data
ARTICLE
2
4 h at 313 K before removal of the catalyst by filtration over
Celite. The solvent was removed at reduced pressure in a rotary
evaporator, and the crude product was purified by flash column
chromatography (gradient EtOAc to EtOAcꢀMeOH, 70:30).
The expected product (3.0 g, 50 % yield) was obtained as a white
41
1
solid, mp (528 to 529) K (lit. mp 526 K). H NMR (D O, 270
2
MHz) δ: 1.46 (d, 3H, J = 7.2 Hz), 3.66 (q, 1H, J = 7.2 Hz), 4.17
13
(
AB, 2H, J = 12.8 Hz), 7.44 (s, 5H). C NMR (D O, 67.5 MHz)
2
δ: 15.4, 49.9, 57.5, 129.4, 129.7, 130.0, 131.0, 174.9.
N-Benzyl-β-alanine [CAS Registry No. 5426-62-0] was pre-
pared in a 500 mL hydrogenation flask in which was placed 10.0 g
Figure 1. Molecular structures of the amino acids studied in the present
work: sarcosine (1), α-alanine (2), β-alanine (3), N-benzyl-α-alanine
(
112.2 mmol) of β-alanine in 150 mL of MeOH/H O (80:20),
2
(
4), and N-benzyl-β-alanine (5).
and to this was added 11.4 mL (133.7 mmol) of benzaldehyde
and 1.0 g of 10 % Pd/C catalyst. The resulting mixture was
pressurized to 60 psi with hydrogen and shaken for 24 h before
removal of the catalyst by filtration over Celite. The solvent was
removed at reduced pressure in a rotary evaporator, and the
crude product was purified by flash column chromatography
(gradient EtOAc to EtOAcꢀMeOH, 70:30). The expected
product (13.0 g, 65 % yield) was obtained as a white solid, mp
reporting their molar enthalpies of formation in the condensed
and gas states.
To our knowledge, since 1937, there are four data points in the
literature for heat capacity at T = 298.15 K for α-alanine (DL):
ꢀ
1
ꢀ1
34 35 36
(
121.7, 121.6, 113.8, and 114.0) J K mol , according to
3
3
Huffman and Ellis, Spink and Wads €o , Kulikov et al., and
Badelin et al., respectively. It can be appreciated that these
3
7
42
1
(
466 to 467) K (lit. mp 469 K). H NMR (D O, 270 MHz) δ:
2
ꢀ
1
ꢀ1
values are spread over a range of ca. 8 J K mol
.
3
3
2.43 (t, 2H, J = 20.0 Hz), 3.08 (t, 2H, J = 18.9 Hz), 4.10 (s, 2H),
For β-alanine, there are four published values for the heat capacity
13
7
.37 (s, 5H). C NMR (D O, 67.5 MHz) δ: 32.5, 43.8, 50.8,
2
ꢀ
1
ꢀ1
37
at T = 298.15 K: (116.4, 109.3, 109.0, and 116.2) J K mol ,
reported by Skoulika and Sabbah, Kulikov et al., Badelin et al.,
and most recently by Paukov et al., respectively. These values are
also spread over a range of ca. 8 J K mol .
For sarcosine we have found a single report from Sabbah and
Laffitte disclosing its heat capacity, Cp,m(cr, 298.15 K) = 128.9
J K mol . There are no data in the literature for the heat
3
3
129.4, 129.7, 129.8, 130.9, 178.0.
38
36
The standards used for DSC calibration were hexafluoroben-
zene, 99.9 % purity, supplied by Aldrich; benzoic acid, National
Institute of Standards and Technology (NIST) standard refer-
ence sample 39j; and high-purity indium (mass fraction: >
39
ꢀ
1
ꢀ1
3
3
4
0
0
.99999), tin (mass fraction: > 0.99999), and synthetic sapphire,
ꢀ
1
ꢀ1
3
3
supplied by Perkin-Elmer.
capacities of N-benzyl-α-alanine (DL) and N-benzyl-β-alanine.
The present report discloses the temperature, enthalpy and
entropy of fusion, and heat capacities of several amino acids
measured by DSC. The target compounds (see Figure 1) are
sarcosine (1), α-alanine (DL) (2), β-alanine (3), N-benzyl-α-
alanine (DL) (4), and N-benzyl-β-alanine (5). An objective of this
work was to expand the database of available experimental heat
capacities of amino acids and to provide data to adjust and refine
group contribution schemes for the estimation of this property
for compounds that have not yet been investigated.
Purity Control. All samples were carefully dried under vacuum.
The purity and composition of the samples were checked by
DSC and C, H, and N microanalysis. Calculated for C H NO ,
3
7
2
2
2
2
1
0 w(C) = 0.4044, 10 w(H) = 0.0792, 10 w(N) = 0.1572;
2 2
found 10 w(C) = 0.4052 ( 0.012, 10 w(H) = 0.0843 ( 0.0041,
2
2
2
2
2
1
0 w(N) = 0.1596 ( 0.0036; 10 w(C) = 0.4051 ( 0.0026, 10
2
w(H) = 0.0797 ( 0.0023, 10 w(N) = 0.1552 ( 0.0023; 10
2
w(C) = 0.4054 ( 0.0019, 10 w(H) = 0.0775 ( 0.0006, 10
w(N) = 0.1575 ( 0.0007; for sarcosine, α-alanine (DL), and
2
β-alanine, respectively. Calculated for C H NO , 10 w(C) =
12
13
2
2
2
2
0
=
0
.6702, 10 w(H) = 0.0731, 10 w(N) = 0.0782; found 10 w(C)
2 2
0.6537 ( 0.0001, 10 w(H) = 0.0723 ( 0.0004, 10 w(N) =
2 2
’
EXPERIMENTAL PROCEDURES
.0761 ( 0.0006 and 10 w(C) = 0.6644 ( 0.0011, 10 w(H) =
2
Materials. A commercial sample of sarcosine (Aldrich, cat. no.
0.0710 ( 0.0001, 10 w(N) = 0.0791 ( 0.0003 for N-benzyl-α-
alanine (DL) and N-benzyl-β-alanine, respectively. For the de-
termination of purity by DSC a heating rate of 0.04 K s was
used, and five to eight samples weighing (1 to 2) mg were
recorded. A fresh sample was used for each run. The determina-
1
31776, mp 481 K, decomp.) was used without further purification.
Commercial samples of α-alanine (DL) [CAS Registry No.
02-72-7] (15.0 g, 168.4 mmol, Aldrich, cat. no. 13,522-4, mp
62 K, decomp.) and β-alanine [CAS Registry No. 107-95-9]
ꢀ
1
3
3
5
43
(
15.0 g, 168.4 mmol, Aldrich, cat. no. 14,606-4, mp 475 K,
decomp.) were placed in sublimators, which were evacuated to
.5 mmHg and heated in an oil bath to (483 to 493) K and (433
tion of purities, using the fractional fusion technique, indicated
that the purities of sarcosine and N-benzyl-β-alanine were 99.0 (
0.03 and 98.4 ( 0.02, respectively. It was not possible to
determine the purity of α-alanine (DL), β-alanine, and N-ben-
zyl-α-alanine because of decomposition of the compounds
during the fusion process. No solidꢀsolid phase transitions were
observed in the compounds over the temperature interval where
the heat capacity measurements were measured.
Apparatus and Procedure. The behavior of the samples as a
function of temperature was studied by DSC. A Pyris 1 instru-
ment from Perkin-Elmer equipped with an intracooler unit was
used to monitor purity, to study the fusion process, to establish or
rule out the possible existence of phase transitions in the solid
samples, and to determine heat capacities as a function of
0
to 443) K, respectively, to afford 11.8 g (79 % yield) of the sublimed
α-alanine (DL), mp (563 to 564) K (decomp.), and 6.0 g (40 %
yield) of the sublimed β-alanine, mp (477 to 478) K (decomp.).
The purity of the samples was checked by NMR and C, H, and N
microanalysis: NMR analysis suggested at least 99.5 % purity.
N-Benzyl-α-alanine (DL) [CAS Registry No. 40297-69-6] was
obtained as follows: In a 300 mL hydrogenation flask was placed
3
.0 g (33.7 mmol) of DL-alanine in 150 mL of MeOH/H O
2
(60:40), and to this was added 3.42 mL (33.7 mmol) of
benzaldehyde and 0.30 g of 10 % Pd/C catalyst. The resulting
mixture was pressurized to 200 psi with hydrogen and shaken for
3
808
dx.doi.org/10.1021/je200292z |J. Chem. Eng. Data 2011, 56, 3807–3812

Journal of Chemical & Engineering Data
ARTICLE
ꢀ
6
temperature. The apparatus was previously calibrated in tem-
perature and energy with reference materials. Temperature and
a detection limit of 1 10 g before and after the experiments to
3
confirm that no product had volatilized.
44ꢀ46
power scales were calibrated
at heating rates of (0.04 and
After calibration, several runs with high-purity benzoic acid
and indium were performed under the same conditions as the
experimental determinations. The accuracies associated with
measurements of temperature and enthalpy of fusion were
calculated as the percentage deviation of the experimental data
with respect to the values given in the literature; in all cases the
deviations were lower than 0.2 % and 2.0 % for temperature and
enthalpy determinations, respectively.
For determinations of purity, temperature, and enthalpy of
fusion, a heating rate of 0.04 K s was used, and five to eight
ꢀ
1
0
.17) K s . The temperature scales were calibrated by the
3
melting temperature of the high-purity reference materials,
47
hexafluorobenzene, tin, and indium. The power scales were
calibrated with high-purity indium.
47
47
Thermograms of samples hermetically sealed in aluminum
pans were recorded in a nitrogen atmosphere. All of the pans with
the samples were weighed on a Mettler AT21 microbalance with
27
ꢀ
1
3
Table 1. Group Values Used to Estimate Cp,m(cr)(298 K) and
ΔtpceS(Tfus)
samples weighing (1 to 2) mg were recorded. A fresh sample was
used for each run. All compounds for which fusion enthalpies are
reported showed thermal stability during the fusion process.
Cp,m(cr) (298 K)
Δ
tpceS(Tfus
)
ꢀ
1
Different scans at heating rates of (0.04 and 0.17) K s were
3
ꢀ1
ꢀ1
performed to determine the possible existence of phase transi-
tions in the samples over the temperature range from T = 268 K
to their melting or decomposition temperature.
J mol
K
3
3
groups (R)
Γ
R
Γ
R
Heat capacities were determined by the “scanning method”
CH
3
ꢀ
36.6
26.9
9
17.6
7.1
48ꢀ50
following the experimental methodology previously described
with synthetic sapphire (α-aluminum oxide) as reference material.
ꢀCH
ꢀCH(C
ꢀCHd aromatic
ꢀCd aromatic adj to an sp C
ꢀCO
ꢀNH
ꢀNHꢀ (acyclic)
2
ꢀ
47ꢀ50
2
)
ꢀ16.4
7.4
DSC is a commonly accepted method for the quantitative
determination of heat capacities, and it has been proven as a
suitable technique for obtaining reliable and accurate thermo-
17.5
8.5
3
ꢀ9.6
16.2
21.4
ꢀ5.3
51,52
2
H
53.1
21.6
ꢀ0.3
chemical data.
As a check of the reliability of the experimental
2
(acyclic)
method, control heat capacity experiments were performed with
2
7
benzoic acid in the temperature interval T = (268 to 360) K.
ꢀ
1
ꢀ1
Table 2. Mean Experimental Cp,m(cr) Values (in J mol
K )
3
3
ꢀ1
ꢀ1
ꢀ1 ꢀ1
T
C
p,m(cr)/J mol
K
compound
T
C
p,m(cr)/J mol
K
compound
3
3
3
3
K
1
2
3
4
5
K
1
2
3
4
5
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
63.15
65.15
70.15
73.51
75.15
80.15
85.15
90.15
95.15
98.15
00.15
05.15
10.15
15.15
20.15
25.15
30.15
35.15
40.15
45.15
50.15
55.15
60.15
65.15
70.15
75.15
107.3
108.3
109.9
110.8
111.5
112.8
114.2
116.0
117.4
118.2
119.0
120.5
122.1
123.5
125.1
126.4
128.1
129.8
131.4
133.2
134.9
136.4
138.5
140.6
142.3
143.9
107.6
108.4
110.2
111.3
111.9
113.4
115.0
116.9
118.9
119.9
120.8
122.7
124.7
126.2
127.9
128.4
130.6
132.2
134.3
136.1
138.2
140.3
142.4
144.1
145.9
146.8
105.2
105.8
107.3
108.2
108.8
110.3
111.7
113.1
114.5
115.2
115.9
117.3
118.8
119.8
121.3
122.2
123.8
125.4
126.9
128.4
130.0
131.7
133.5
135.0
136.6
137.7
199.3
200.7
204.2
206.2
207.8
211.1
213.9
217.2
220.4
222.4
224.0
227.2
233.2
232.3
235.6
237.2
240.5
244.6
248.2
250.9
253.6
256.3
258.4
257.7
261.6
261.0
193.4
194.4
196.9
198.2
199.0
201.4
204.2
207.1
210.5
221.7
214.3
217.8
221.7
227.0
230.7
234.4
237.6
241.6
245.2
248.3
251.0
254.3
257.8
262.7
266.3
270.9
380.15
385.15
390.15
395.15
400.15
405.15
410.15
415.15
420.15
425.15
430.15
435.15
440.15
445.15
450.15
455.15
460.15
465.15
470.15
475.15
480.15
485.15
490.15
495.15
500.15
145.4
147.0
148.8
150.5
152.2
154.0
155.6
148.3
149.8
151.3
153.0
154.6
156.3
158.1
159.7
161.5
163.0
164.9
167.0
168.0
170.1
172.0
173.8
175.4
177.0
178.7
178.8
180.7
183.4
184.9
187.5
190.1
139.1
140.7
142.4
144.0
145.7
147.4
148.9
150.0
152.0
152.7
154.4
156.2
158.0
159.6
161.4
263.6
266.4
269.8
273.0
276.8
280.6
283.5
285.5
288.7
293.1
295.7
274.1
277.6
281.6
285.3
289.2
292.9
296.6
300.7
305.1
3
809
dx.doi.org/10.1021/je200292z |J. Chem. Eng. Data 2011, 56, 3807–3812

Journal of Chemical & Engineering Data
ARTICLE
Table 3. Coefficients of the Fitted Curves
2
4
6
a
T
A
B 10
C 10
D 10
rmsd
3
3
3
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1 ꢀ1
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
K
J mol
K
J mol
K
J mol
K
J mol
K
R
J mol
K
3
3
3
3
3
3
3
3
3
3
1
2
3
4
5
263ꢀ410
263ꢀ500
263ꢀ450
263ꢀ430
263ꢀ420
118.16 ( 0.03
119.00 ( 0.06
115.09 ( 0.04
223.41 ( 0.17
214.02 ( 0.14
30.81 ( 0.09
35.8 ( 0.2
27.91 ( 0.13
62.7 ( 0.6
68.1 ( 0.5
4.2 ( 0.3
ꢀ2.2 ( 0.3
2.2 ( 0.3
ꢀ1.6 ( 0.3
0.69 ( 0.13
ꢀ0.41 ( 0.17
14.5 ( 1.0
0.9998
0.9995
0.9997
0.9978
0.9992
0.20
0.52
0.30
1.31
0.99
ꢀ25.1 ( 1.6
10.3 ( 1.4
ꢀ4.5 ( 1.0
a
Root mean square deviation.
The relative percentage error of our measurements in comparison
27
with those reported in the literature was less than 2 %.
The mass of sapphire used in each run was 0.030345 g. For
heat capacity determinations, three to six fresh samples weighing
(
10 to 25) mg were scanned for each solid compound in the
temperature range from T = 268 K to its melting or decomposi-
tion temperature at 0.17 K s . The complete temperature
ꢀ1
3
range for determination of heat capacity was divided in intervals
of approximately 40 K, overlapping by 15 K from one interval to
another. For the calculations, an overlap of only 5 K was used.
The initial 10 K allowed the instrument's electronics to establish
a linear baseline. The molecular weights used to convert the specific
heat capacities measured to their molar values were calculated
from the atomic weights recommended by the International
53
Union of Pure and Applied Chemistry (IUPAC) in 2009.
Estimations of Cp,m(cr) and ΔtpceS(Tfus). The group values
16
listed in column 2 of Table 1 were used to estimate heat capacities,
and those provided in the third column were used to estimate total
54
phase change entropy, ΔtpceS(T ), from T = (0 to T /Tdecom)
fus
fus
K of the amino acids studied. The protocol used to estimate
Cp,m(cr)(298 K) is a straightforward additive group method and
requires little clarification. Since group values for carboxyl and
ammonium salts are not currently available, the group values for a
primary and secondary aliphatic amine and a carboxylic acid were
used as substitutes. As an example, using the group values in Table 1,
the heat capacity for N-benzyl-α-alanine would be evaluated as:
Figure 2. A comparison of the results of this work (lines) calculated
using eq 1 with previous experimental literature values (symbols). Top
3
4
curve: the heat capacity of DL-alanine (for purposes of separating the
two curves, a value of 20 was arbitrarily added to each experimental C
p,m
39
(cr) value). Bottom curve: the heat capacity of β-alanine.
ꢀ1
ꢀ1
5
(17.5) + 8.5 + 26.9 ꢀ 0.3 + 9 + 36.6 + 53.1= 221.3 J mol K .
3
3 3
in Table 2 are averages of three to six independent runs. The
standard deviation of all of the data associated with multiple
A similar situation regarding the absence of appropriate group
values for ammonium and carboxylate salts exists for the estima-
ꢀ
1
ꢀ1
measurements is less than 2 J mol
K .
3
3
tion of ΔtpceS(T ). Therefore, the same substitutions were
fus
The experimental results for the compounds were fit to a
employed for these estimations. In these cases, it should be
third-order polynomial in temperature of the type:
pointed out that the group value for a carboxylic acid, 13.4
ꢀ
1
ꢀ1
J mol
K
, depends on the number of functional groups
2
3
3
Cp, mðcrÞ ¼ A þ BðT=K ꢀ 298:15Þ þ CðT=K ꢀ 298:15Þ
5
4
ꢀ1
ꢀ1
present. The value of 16.2 J mol
reflects the contribution of a carboxylic acid in the presence of
K
listed in Table 1
3
3
3
þ DðT=K ꢀ 298:15Þ
ð1Þ
two functional groups (13.4 1.21). Finally, the contribution of a
3
The range studied for each compound, the coefficients of
the fitted third-order equation in temperature, and the root-
mean-square deviation for all the compounds are collected in
Table 3.
methylene group, whenever the number of consecutive ꢀCH ꢀ
2
groups equals or exceeds the sum total of the remaining groups, is
equal to the 1.31 times the value listed in column 3 of the table.
With the amino acids of the present study, this condition applies
only to β-alanine: 21.4 + 2 7.1 1.3 + 16.2 = 56.2 J mol
For substances containing only aromatic and aliphatic groups,
ꢀ1
ꢀ1
As noted previously, the heat capacity of DL-alanine was
K .
3
3
3
3
34
measured in 1937 by Huffman and Ellis and β-alanine most
39
recently in 2009 by Paukov et al., both by adiabatic calorimetry.
Figure 2 illustrates the previous results with those obtained by
DSC in this study. Over the temperature interval in which both
previous studies overlap with the current results, the results
appear in good agreement. The percent error for DL-alanine
varies from (2.6 to 3.1) % and from (0.03 to 1.1) % for the most
recent detailed study of β-alanine.
the protocol used in estimating ΔtpceS(T ) is otherwise similar
to the estimation of Cp,m(cr) (298 K).
fus
’
RESULTS AND DISCUSSION
The measured molar heat capacities as a function of tempera-
ture for all compounds are collected in Table 2. The values given
3
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dx.doi.org/10.1021/je200292z |J. Chem. Eng. Data 2011, 56, 3807–3812

Journal of Chemical & Engineering Data
ARTICLE
Table 4. Temperatures, Enthalpies, and Entropies of Fusion and Heat Capacities at T = 298.15 K for the Compounds Studied in
This Work
ꢀ1
ꢀ1
ꢀ1 ꢀ1
T
fus
Δ
fusH(Tfus
)
Δ
fusS(Tfus)/(J mol
K
)
C
p,m(cr)(298.15 K)/(J mol
K
)
3
3
3
3
ꢀ
1
a
b
c
K
kJ mol
exp.
calcd
exp.
calcd
3
sarcosine
481.6 ( 1.4
20.6 ( 0.7
42.8 ( 1.6
35.6
38.8
56.2
46.6
59.6
118.2
119.9
115.2
222.4
221.7
116.3
120.3
128.5
221.3
229.5
α-alanine
β-alanine
N-benzyl-α-alanine
N-benzyl-β-alanine
457.0 ( 0.5
42.7 ( 0.7
69.0 ( 0.5
a
b
Values calculated according to ref 54; they represent the sum of all transition enthalpies from T = (0 ꢀ Tfus/Tdecomp) K. Experimental values
c
at T = 298.15 K from Table 2. Values calculated according to ref 16.
Experimental temperatures, enthalpies, and entropies of fusion of
the compounds and heat capacities at T = 298.15 K measured and
estimated in this work are summarized in Table 4. The uncer-
tainties were taken as the standard deviation of the mean value.
Tfus values are reported as DSC onset temperatures. No solidꢀ
solid phase transitions were observed over the temperature interval
from T = 268 K to the corresponding melting or decomposition
temperatures of any of the compounds.
(2) Brack, A., Ed. The Molecular Origins of Life; Cambridge Uni-
versity Press: Cambridge, U.K., 1998.
(3) Allen, F. H. The Cambridge Structural Database: A Quarter of a
Million Crystal Structures and Rising. Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem. 2002, 58, 380–388.
(
4) Levy, H. A.; Corey, R. B. The Crystal Structure of DL-Alanine.
J. Am. Chem. Soc. 1941, 63, 2095–2108.
5) Donohue, J. The Crystal Structure of DL-Alanine. II. Revision of
Parameters by Three-Dimensional Fourier Analysis. J. Am. Chem. Soc.
(
Also included in column 5 of the Table 4 is the estimated total
1
950, 72, 949–953.
(6) Chickos, J. S. A Protocol for Correcting Experimental Fusion
54
phase change entropy, ΔtpceS(T ), for all compounds. This
fus
term includes the total phase change entropy associated in going
Enthalpies to 298.15 K and Its Application in Indirect Measurements of
Sublimation Enthalpy at 298.15 K. Thermochim. Acta 1998, 313, 19–26.
(7) Chickos, J. S.; Hesse, D. G.; Hosseini, S.; Nichols, G.; Webb, P.
Sublimation Enthalpies at 298.15 K Using Correlation Gas Chromatog-
raphy and Differential Scanning Calorimetry Measurements. Thermo-
chim. Acta 1998, 313, 101–111.
from T = 0 K to the liquid at T = T . For compounds without
fus
any other phase transitions, this entropy change is identical to the
fusion entropy. The estimation of total phase change entropies,
ΔtpceS(T ), and heat capacity, Cp,m(cr) of the amino acids,
fus
because of their zwitterionic nature, would require group values
for various ammonium (primary, secondary, etc.) and carbox-
ylate ions. However, because of the paucity of such data, these
particular group parameters are not available. Both sets of
estimations were therefore conducted using the group values
for the parent amino acids as found in their gas phase structures.
The average error in the total phase change entropy estimation
(8) Chickos, J. S.; Hosseini, S.; Hesse, D. G.; Liebman, J. F. Heat
Capacity Corrections to a Standard State: A Comparison of New and
Some Literature Methods for Organic Liquids and Solids. Struct. Chem.
1
993, 4, 271–278.
ꢁ
(9) Z ꢀa bransk ꢀy , M.; Kolsk ꢀa , Z.; Ru ꢁz i ꢁc ka, V., Jr.; Domalski, E. S. Heat
Capacity of Liquids: Critical Review and Recommended Values. Supple-
ment II. J. Phys. Chem. Ref. Data 2010, 39, 013103.
ꢀ
1
ꢀ1
ꢁ
reported in Table 4 is about 8 J mol
K
. Similarly for the
(10) Z ꢀa bransk ꢀy , M.; Ru ꢁz i ꢁc ka, V., Jr.; Domalski, E. S. Heat Capacity of
3
3
estimated heat capacities at T = 298.15 K, the use of existing
group values for a carboxylic acid and primary and secondary
amine appear to reproduce the experimental heat capacities
within a standard deviation of ( 6.6 J mol
the limited statistics, both sets of estimation are well within the
Liquids: Critical Review and Recommended Values. Supplement I.
J. Phys. Chem. Ref. Data 2001, 30, 1199–1689.
ꢁ
(
11) Z ꢀa bransk ꢀy , M.; Ru ꢁz i ꢁc ka, V., Jr.; Majer, V.; Domalski, E. S. Heat
ꢀ
1
ꢀ1
Capacity of Liquids: Volume 1 - Critical Review and Recommended
Values. J. Phys. Chem. Ref. Data 1996, Monograph 6.
K . Despite
3
3
(12) Palczewska-Tulinska, M.; Wyrzykowska-Stankiewicz, D.;
uncertainty associated with similar estimations on nonzwitter-
Szafranski, A. M.; Cholinski, J. Solid and Liquid Heat Capacity Data
Collection, Part 1: C1-C6; Dechema, Chem. Technik und Biotechno-
logie e.V.: Frankfurt am Main, 1997.
16
ionic solids, reaffirming an earlier suggestion that new group
values for the ionic functional groups in amino acids may not be
8
necessary.
(13) Palczewska-Tulinska, M.; Wyrzykowska-Stankiewicz, D.;
Szafranski, A. M.; Cholinski, J. Solid and Liquid Heat Capacity Data
Collection, Part 2: C6-C33; Dechema, Chem. Technik und Biotech-
nologie e.V.: Frankfurt am Main, 1997.
’
AUTHOR INFORMATION
ꢁ
Corresponding Author
(14) Z ꢀa bransk ꢀy , M.; Ru ꢁz i ꢁc ka, V. Heat Capacity of Liquids: A Survey
*
E-mail: victoriaroux@iqfr.csic.es.
and Data Needs. Fluid Phase Equilib. 2002, 194, 817–824.
(15) Becker, L.; Gmehling, J. Measurement of Heat Capacities for 12
Funding Sources
Organic Substances by Tian-Calvet Calorimetry. J. Chem. Eng. Data
2001, 46, 1638–1642.
(16) Chickos, J. S.; Hesse, D. G.; Liebman, J. F. A Group Additivity
Approach for the Estimation of Heat Capacities of Organic Liquids and
Solids at 298 K. Struct. Chem. 1993, 4, 261–269.
The support of the Spanish Ministry of Science and Innovation
under Projects CTQ2007-60895/BQU and CTQ2010-16402 is
gratefully acknowledged. E.J. is also indebted to CONACYT,
Mexico, for financial support via Grant 60366.
(17) Goodman, B. T.; Wilding, W. V.; Oscarson, J. L.; Rowley, R. L.
Use of the DIPPR Database for Development of Quantitative Structure-
Property Relationship Correlations: Heat Capacity of Solid Organic
Compounds. J. Chem. Eng. Data 2004, 49, 24–31.
(18) Ru ꢁz i ꢁc ka, V., Jr.; Domalski, E. S. Estimation of the Heat-
Capacities of Organic Liquids as a Function of Temperature Using
’
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