J. Phys. Chem. 1993, 97, 13027-13028
13027
COMMENTS Partial Molal Isothermal Compressibility of Glycine, DL-a-Alanine, Glycylglycine, Glycolamide, and Lactamide
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A. Aristides Yayanos
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Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0202
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Received: July 16, 1993
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The partial molal isothermal compressibility &,T, for a solute in binary solution is defined' as
= -(dv2/dP)T (1) where 2 2 . T is the partial molal isothermal compressibility of the solute r2 is its partial molal volume, P is the pressure, and T is the temperature. Values of GZp the partial molal isothermal compressibility at infinite dilution, of amino acids are used in computations of the effect of pressure on proteim2 A paper3 recently came to my attention wherein an isentropic partial molal compressibility, G3,of glycylglycine at infinite dilution was cited as originating from data in a paper of mine4 and was said to have a value of -32 X 10-4 cm-3 mol-' b a r ' . Although I did not report isentropicor isothermal compressibilities in that paper, the isothermal values can be calculated from the data. Some other authors5 have also used those data to derive a value for the isothermal partial molal compressibility of -32 X 10-4~ m mol-' - ~ atm-' for glycylglycine. I here show that the %,T of glycylglycine is (-37.3 f 0.8) X 10-4 cm-3 mol-' b a r ' . Cabani et aL5 made a similar calculation for glycine and alanine using my previously reported data to find values of -19 X 10-4 and -21 X 10-4 cm-3 mol-' bar1 for the G,Tof glycine and DL-a-alanine, respectively. Again, I calculate somewhat different values from my data and show agreement with their experimental values for glycine and for DL-alanine that is better than they had indicated. Since there are not many values of isothermal compressibility for amino acids in solution in the literature, it seems reasonable to get as right as possible the few values that are available. Values of the apparent molal volume,' @v, of glycine, DLalanine, glycylglycine, glycolamide, and lactamide are available4 to pressures of around 1000 bar. These published %values were based onvalues6for the specificvolumes of water at high pressures that were subsequently shown to be in error.'J Recalculation of the @v values with the now accepted values for water d e n ~ i t y ~ . ~ does not result in any substantially different values of @v in the pressure range up to 500 bar. Above 500 bar, the currently accepted water densities have a noticeable affect. The highpressure water densities that I determined along with the solution densities are in good agreement with the presently accepted values of water density and were used to calculate 9" values for this paper. The specific volume of water was calculated from the following equation where u is the specific volume in cm3/g and P is the pressure in atmospheres:
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K2,T
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v = 1.002961 - (0.46068 X 104)(P- 1.0) (0.722391 X lO")(P- 1.0)2- (0.107207 X lO-")(P- l.0)3 (2) The @v values a t a given concentration varied linearly with pressure from atmospheric pressure to somewhat beyond 500 bar. The slope of the linear regression line is the negative of the isothermal apparent molal compressibility,l @K. For these five compounds, the @K values were nearly constant up to pressures of 500 bar or so.
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MOLALITY Figure 1. Values of the apparent molal compressibility,3 ~plotted , against the molality in aqueous solution for: (0)glycine, (m) glycolamide, ( 0 ) DL-a-alanine,(0)lactamide,and (A)glycylglycine. The infinitedilution and are given in Table I. values are those for q,T
TABLE I: Isothermal (pT) and Isentro ic ( p )Partial
Molal Compressibilities (cm-3 mol-' barq) at &finite Dilution ( X 104)
amino acid glycine
PT(own)
PT(lit)
-20.8 f 0.8
-21.4 f 0.7 (5)'
glycolamide m-a-alanine
+4.2 f 0.8 -20.2 f 0.8
-18.5 f 0.6 (5)
lactamide glycylglycine
+2.7 f 0.8 -37.2 f 0.8
-31.9 f 0.6 (5)
(lit) -27 f 0.13 (9) -25.0 f 0.6 (5) -27.19 (10) +2.74 (10) -25.03 f 0.13 (9) -21.6 h 0.5 (5) -26.8 (10) +4.32 (10) -35.5 (5) -35.91 (3)
Number in parentheses is a reference. The isothermal apparent molal compressibility of these compounds in solution changed with concentration, as seen in Figure 1. I assumed a linear dependence of BK on concentration. The value of @K extrapolated to zero solute concentration gave the isothermal partial molal compressibility at infinite dilution, since
(3) The values I calculate for the partial molal isothermal compressibility a t infinite dilution, %,-,_are shown in Table I along with the literature values for both G,,and Glycine and DL-a-alanine have a value of G,Tthat is 25% smaller than the one for Glycylglycine has a value of G,Tthat is slightly larger than its g3 value. Glycolamide and lactamide, uncharged molecules, have positive G,Tvalues that are nearly indistinguishable from each other within experimental error and similarly from their corresponding values.
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References and Notes (1) Harned, H.S.;Owen, B. B. The Physical Chemistry of Electrolyte Solutiom; Reinhold Book Corp.: New York, 1958; pp 1-803. (2) Gekko, K.;Hasegawa, Y . Biochemistry 1986, 25, 65636571.
0 1993 American Chemical Society
13028 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 (3) Iqbal, M.; Verrall, R.E.1. Phys. Chem. 1987, 91, 967-971. (4) Yayanos, A. A. 1. Phys. Chem. 1972, 76, 1783-1792. (5) Cabani,S.;Conti,G.;Matteoli,E.;Tint,M.R.J.Chem.Soc.,Fur~uy Trans. 1 1981, 77,2385-2394. (6) Kell, G. S.;Whallcy, E. Proc. R. Soc. London A 1965,258,565-614. (7) Fine, R. A.; Millero, F. J. 1. Chem. Phys. 1973, 59, 5529-5536.
Comments (8) Kell, G. S.; Whalley, E. J. Chem. Phys. 1975, 62, 34963503. (9) Millero, F. J.; Lo Sudo, A,; Shin, C . J. Phys. Chem. 1978,82,784792. (10) Gucker, F.T.,Jr.; Lamb. F. W.; Marsh, G. A.; Haag, R. M. 1.Am. Chem. Soc. 1950, 72,310-317.
