J. Phys. Chem. 1995, 99, 12063-12064
12063
COMMENTS Partial Molar Isothermal and Isentropic Compressibilities of Glycine, Alanine, and Glycylglycine in Aqueous Solution at 25 "C Gavin R. Hedwig Department of Chemistry and Biochemistry, Massey University, Palmerston North, New Zealand Received: December 12, 1994; In Final Form: May 6, 1995
The standard-state partial molar isothermal compressibility of a solute in solution is defined as
TABLE 1: Coefficients a and b in Eq 3
solute glycine" alanine" glycylglycineb
G,*
b/cm3
temp
mol-' K-2
range/'C 15-55 15-55 15-45
-0.0009
* 0.0001
-0.0010 f 0.0002 -0.0006 rlr 0.0001
data from ref 8. The uncertainties for used in the weighted least-squares analysis were obtained from ref 9. The data at 25, 35, and 45 "C were from ref 10; at 15 "C from ref 11.
TABLE 2: Comparison of the E; and q,2 Data at 25 "Cwith Some Literature Data solute
glycine where q is the partial molar volume of the solute at infinite dilution. The property is a sensitive measure of solutesolvent interactions and, as such, can be used to monitor solute hydration in aqueous solution.'.2 It is not an easy task to determine isothermal compressibilities directly.' However, through precise speed of sound measurements that can be determined using modem instrumentation: the partial molar isentropic compressibility of a solute at infinite dilution, q,2, can be readily determined.' This isentropic compressibility can then be converted to the isothermal compressibility using the relation4
a/cm3
mol-] K-I 0.067 f 0.003 0.063 k 0.005 0.093 & 0.001
alanine glycylglycine
,5'/cm3 mol-' K-'
C;,2/J K-I mol-'
0.067 i 0.003" 0.10 f 0.01' 0.068 iz O.Old 0.069 f 0.009' 0.063 f 0.005" 0.088 f 0.01' 0.05 1 f 0.003' 0.093 f 0.001"
37.6 f 0.7".b 39.2 f 0.41
*
141.2 0.6",b 141.4 iz 0.2f 100.8 f 0.3",5
*
105.0 0.41 99.3 f 1.0h a Used in this work. From ref 8. From ref 7. From ref 12. These values were determined using apparent molar volume data in the concentration range 1-3 mg mL-l. e Determined using apparent molar volume data at a concentration of 1 mg mL-' taken from ref 13. f From ref 14. 5 From ref 15. From ref 16. 0.103 4~0.003' 0.115 3~ 0.015d
the temperature range 15-55 "C. These data were analyzedherein using a polynomial in temperature, t, of the form
q2
where q and are respectively the partial molar expansibility (q= (aq/EUJ,) and heat capacity of the solute at infinite dilution, a: and a :, are respectively the coefficient of thermal expansion (a: = (aq/aT')Jq) and the volumetric heat capacity of the pure solvent, and Sp is the difference between the isothermal coefficient of compressibility & ($0, = -(a q / a p ) ~ / V pand ) the isentropic coefficient of compressibility pi = -(aq/ap)s/Vp) for the pure solvent (Sp = pl; - pi). In a recent communication in this j ~ u m a lYayanos ,~ reported G,2values at 25 "C for the amino acids glycine and alanine, their corresponding uncharged isomers glycolamide and lactamide, and the dipeptide glycylglycine in aqueous solution. These results were obtained from an analysis of apparent molar volume data at pressures to 1000 bar that the author had published in a previous paper.6 For glycine, alanine, and glycylglycine the G,2 values were compared with selected literature results7 that were derived from q,2 values using eq 2. Although there was fair agreement between the G,2 values for the two amino acids, there was a large difference between the G,2 values for glycylglycine. As G,*values for these solutes have been determined in numerous studies, it is worthwhile to explore whether the literature results chosen for the comparison are indeed the most reliable available. As Yayanos quite correctly points literature q,* values for amino acids and peptides are scarce. It is important therefore to carefully scrutinize those that are available. Accurate expansibilities are required in order to obtain reliable G,*values using eq 2.'.4 Hakin et aL8 have reported recently precise data for the amino acids glycine and alanine over
=q(25)
+ a(t - 25) + b(t - 25)2
(3)
where q ( 2 5 ) is the standard-state partial molar volume of the amino acid at 25 "C, and a and b are the fitted coefficients. For glycylglycine, q data9.l0over the range 15-45 "C were also analyzed using eq 3. The polynomial coefficients and their standard deviations obtained from these analyses are given in Table 1. At 25 "C the standard-state partial molar expansibility of the solute is simply the coefficient a . The values for the amino acids and glycylglycine are compared with some literature data in Table 2. For glycine and alanine, the q values obtained in this work differ significantly from those reported by Cabani et aL7 The values given in Table 2, along with the most reliable G.2and results taken from the literature (see Tables 2 and 3), were u d to determine values for at 25 "C using eq 2. The solvent parameters a: and p0, used were those given by Ke11,22the value of was calculated using the speed of sound data given by Del Grosso and Mader23and the value of a: was derived from that heat capacity data given by Stim~on.'~The q,2 results obtained, together with those reported by Yaya n ~ sare , ~ given in Table 3. The errors in the K& values were estimated from the combined uncertainties in the q,g,2, and values. The uncertainties in the solvent parameters used in eq 2 make a negligible contribution to the uncertainty in the value of q32. In contrast to the results given by Y a y a n o ~the , ~ agreement between the q,? value determined using eq 2 and that obtained from an analysis of the pressure dependence of the apparent
q,2
H
q2
0022-3654/95/2099-12063$09.00/0 0 1995 American Chemical Society
G$,
12064 J. Phys. Chem., Vol. 99, No. 31, 1995
Comments
TABLE 3: Standard-State Partial Molar Isothermal and Isentropic Comm?ssibuties at 25 "C ~
~
~~~~~
1O4G,$m3 mol-' bar-' 104G,?/cm3mol-' bar-' this work
ref 5"
thisb work
g1ycine
-24.6 f 0.4
-20.8 f 0.8
-27.0 f 0.4'
alanine
-22.9 f 0.2
-20.2 f 0.8
-25.0 f 0.1'
glycylglycine
-36.9 f 0.1
-37.2 f 0.8
-40.2 i 0.1"
solute
literature values -27.2 & 0.4d -25.0 f 0.6f -24.7 & 0.2d -25.1 f 0.29 -40.8 f 0.3' -35.91 i 0.09'
-26.6 f 0.2' -26.6 & 0.29 -21.6 f 0.9 -35.5 & 0 . 9 -41.5 i 0.9
" Calculated using apparent molar volume data as a function of pressure. Values used in eq 2. From ref 17. From ref 18. e From ref 12. The value is the average apparent molar quantity over the concentration range 1-3 mg mL-'. fFrom ref 7. 8 From ref 19. The value is the apparent molar property at a concentration of 0.3 wt %. * From ref 20. From ref 21. From ref 1. J
molar volume5 is excellent for glycylglycine but is poor for both References and Notes glycine and alanine. This contrasting comparison arises because (1) Hoiland, H. In Thermodynamic Data f o r Biochemistry and Eiothe q,2 and q values used in this workdiffer significantly technology; Hinz, H.-J., Ed.; Springer-Verlag: Berlin 1986; Chapter 4. (2) Mathieson, J. G.; Conway, B. E. J. Solurion Chem. 1974, 3, 455. from those determined by Cabani et ale7which were used in (3) Chalikian, T. V.; Sarvazyan, A. P.: Breslauer, K. J. Eiophys. Chem. the paper by Y a y a n o ~ . ~ 1994, 51, 89. The results in Table 3 show that for each solute, the (4) Desnoyers, J. E.; Philip, P. R. Can. J. Chem. 1972, 50, 1094. (5) Yayanos, A. A. J. Phys. Chem. 1993, 97, 13027. G*2value determined in this work is less negative (by about (6) Yayanos, A. A. J. Phys. Chem. 1972, 76, 1783. 8-9%) than the corresponding q,2 value. This result differs (7) Cabani, S.; Conti, G . ; Matteoli, E.; Tint, M. R. J. Chem. Soc., from that reported by Y a y a n o ~wherein ,~ for glycine and alanine Faraday Trans. I 1981, 77, 2385. (8) Hakin, A. W.; Duke, M. M.; Klassen, S. A.; McKay, R. M.; Preuss, the G,2values were around 25% less negative than the q,2 K. E. Can. J. Chem. 1994, 72, 362. values but for glycylglycine the value of G,2was slightly (9) Hakin, A. W., personal communication. more negative than the ZC2value. (10) Reading, J. F.; Watson, I. D.; Hedwig, G.R. J . Chem. Thermodyn. 1990, 22, 159. There does not appear to be a satisfactory explanation for (11) Hedwig, G. R.; Hoiland, H., unpublished results. the differences between the q,* values for glycine and alanine (12) Chalikian, T. V.; Sarvazyan, A. P.; Funck, T.; Breslauer, K. J. determined in this work and those calculated by Y a y a n o ~In .~ Biopolymers 1994, 34, 541. (13) Kharakoz, D. P. Eiophys. Chem. 1989, 34, 115. a recent study25 using ultrasonic velocity measurements to (14) Jolicoeur, C; Boileau, J. Can. J. Chem. 1978, 56, 2707. pressures of 1000 bar, the apparent molar isentropic and (15) Reading, J. F.; Hedwig, G.R. J. Solution Chem. 1989, 18, 159. isothermal compressibilities were determined for glycine and (16) Bhat, R.: Ahluwalia, J. C. J. Phys. Chem. 1985, 89, 1099. (17) Millero, F. J.; Lo Surdo, A.; Shin, C. J. Phys. Chem. 1978, 82, alanine in aqueous solution. The results showed that both these 784. thermodynamic quantities were not independent of pressure over (18) Ogawa, T.; Yasuda, M.; Mizutani, K. Bull. Chem. Soc. Jpn 1984, the range 1-500 bar. This result does not support the original 57, 662. (19) Kharakoz, D. P. J . Phys. Chem. 1991, 95, 5634. work by Yayanos6 in which the apparent molar volumes for glycine and alanine were found to vary linearly with pressure (20) Hedwig, G.R.; Hoiland, H. J. Chem. Thermodyn. 1991.23, 1029. over the range 1-500 bar. Maybe the precision of these molar (21) Iqbal, M.; Verrall, R. E. J. Phys. Chem. 1987, 91, 967. volume measurements was too low for any nonlinearity to be (22) Kell, G.S. J . Chem. Eng. Data 1975, 20, 97. discerned. (23) Del Grosso, V. A,; Mader, C. W. J. Acousr. Soc. Am. 1972, 52, Provided accurate q values are available, eq 2 remains a 1442. useful means for the determination of the thermodynamic (24) Stimson, H. F. Am. J. Phys. 1955, 23, 614. property G,2.The G32 results obtained in this work are (25) Chalikian, T. V.; Sarvazyan, A. P.; Funck, T.; Cain, C. A,; considered to be the most reliable yet determined for the Breslauer, K. J. J. Phys. Chem. 1994, 98, 321. biologically important solutes glycine, alanine, and glycylglycine in aqueous solution. JP9432962
