J. W.
2074
Larson, W. J. Plymale, and A. F. Joseph
The Enthalpy of Interaction between Various Amino Acids and Sodium Chloride John W. Larson," W. Joseph Plymale, and Albert F. Joseph Department of Chemistry, Marshall University, Huntington, West Virginia 25701 (Received June 2, 1977)
The enthalpies of interaction between aqueous NaCl and aqueous solutions of seven amino acids have been determined calorimetrically. These enthalpies AH,", are for the reaction amino acid*(aq) = amino acid*(ideal, 1 M NaC1). The values obtained for A H S O are -211 cal/mol for glycine, 3 cal/mol for a-alanine, 112 cal/mol for valine, -120 cal/mol for serine, -78 cal/mol for &alanine, -16 cal/mol for y-aminobutyric acid, and 161 cal/mol for t-aminocaproic acid. These results are discussed in terms of compensation and the influence of the amino acids on the structure making-breaking of water.
Introduction Biological systems consist largely of highly polar molecules (water, amino acids, etc.) and of ions carrying a net charge. The chemistry of these systems cannot be properly understood without an understanding of the physical interactions in these systems. In a previous paper,l data were presented on the enthalpy of interaction between glycine and NaCl in water. These results indicated that even though electrostatic theories account nicely for the free energy of interaction between dipolar ions and electrolytes, they do not even qualitatively account for the enthalpy of interaction. In this paper, data are presented on two series of amino acids in order to provide information to answer two fundamental questions: How do the electrostatic theories account for free energy data but not enthalpy data? What information can be obtained from enthalpy data? In the first series of amino acids studied, the dipole moment of the amino acid increased from 13.5 D for glycine to 24.1 D for t-aminocaproic acid. Comparable free energy data are also available for this series.2 The second series consisted of glycine, alanine, valine, and serine in which the dipole moment remained approximately constant and the side group varied.
the aqueous amino acid solution, AHl(a) values were obtained for the reaction NaCl( aq) = NaCl(a M amino acid)
(1)
Small corrections for the heat of dilution from the final concentration of the measurements (0.05 M) to infinite dilution were made to both measured heats of solution. Literature values for the heats of dilution in water4 were used to estimate these corrections. The AHl(a)values were then fit by linear least squares to A H , ( a ) / a = AH," t ba (2) The details of these calculations as applied to glycine are reported in ref 1. The resulting enthalpy of interaction, AH3", corresponds to the enthalpy of the reaction NaCI( aq) = NaCI( ideal, 1 M amino acid)
(3)
Because of the limited solubility of a-alanine and valine, method 1 was used for these amino acids. The experimental procedure and calculations of method 2 closely follow those of method 1. The enthalpy of solution of a concentrated amino acid solution into aqueous NaCl solutions is measured. The corrections for the heats of dilution in this method were very small ( f 2 cal/mol), but an additional correction had to be applied to account for the effect of the NaCl on the intermolecular proton Experimental Section transfer reaction. Details of this procedure and calculations are also reported in ref 1. The calorimeter has been described previ~usly.'~~ Equations analogous to eq 1-3 may be written as follows: Certified ACS sodium chloride was obtained from the Fisher Scientific Co. It was used without further puriamino acid'(aq) = amino acid'(a M NaC1) (4) fication. 0-Alanine (J. T. Baker Co., mp 199 "C), yA H,(a)/a = A H6" + ba (5) aminobutyric acid (Nutritional Biochemical Corp., mp 194 "C), t-aminocaproic acid (Nutritional Biochemical Corp., amino acid*(aq) = amino acid'(idea1 1 M NaCl) (6) mp 203 "C), dl-a-alanine (Matheson Coleman and Bell, In these equations a refers to the concentration of NaCl mp 293 "C), dl-valine (Eastman Kodak Co., mp 296 "C), in the solvent system. and dl-serine (Nutritional Biochemicals Corp., mp 233 "C) Both methods should give the same results for the were used in the measurements. The melting points of all enthalpy of interaction if the amino acid and the elecof these were in good agreement with literature values and trolyte are the same (the Gibbs-Duhem relationship must were used without further purification. 0-Alanine was are therefore be satisfied5). The resulting AH3" and AHH6" recrystallized from a water-ethanol mixture and dried referred to as AH,", regardless of the method used. Exbefore using. perimental values of AH,"are reported in Table I. The uncertainties are the standard deviations in the fit to eq Procedure and Calculations 2 or 5. The enthalpy of interaction between the amino acids and In addition to the enthalpy data, Robinson and Schrier2 NaCl was determined by one of two methods. In method have determined values of the interaction coefficient, k,, 1,a concentrated aqueous solution of NaCl was made up for reactions 2 or 5. Their values of k, for glycine, 0by weight. Samples of about 5 g were taken from this alanine, y-aminobutyric acid, and t-caproic acid are given solution and weighed accurately into thin glass bulbs of in Table I1 along with the values of the free energy and 6 1 2 mL capacity. Sample entry into aqueous amino acid entropy of interaction calculated from solutions of varying concentrations was effected by breaking the bulb. AG,"= -2.3Q3RTks (7) By subtracting the heat of solution of the concentrated AGso= A y " - TA8" (8) solution of NaCl into water from its heat of solution into The Journal of Physical Chemistry, Vol. 81, No. 22, 7977
Interaction Enthalpy between Amino Acids and NaCl
2075
TABLE I: Enthalpy of Interaction between Amino Acids and NaCl Amino acid Glycine Glycine a-Alanine Valine Serine p -Alanine 7-Aminobutyric acid €-Aminocaproic acid
AH,",
Concn No. of range, M Method runs
cal/mol
0-1.0 0-1.0 0-1.0 0-0.5 0-1.0 0-1.0
1 2 1 1 2 2
11 11 10 4 12 10
-212i -210 i t3i +112i -120 i -78+
0-1.0
2
10
-16
0-1.0
2
10
i
7 3 28 10 22 5 6
+ 1 6 1 i 10
TABLE 11: Thermodynamics of Interaction between Amino Acids and NaCl
AG,",
AH,",
Amino acid
k
cal/mol
cal/mol
AS,", eu
Glycine p- Alanine 7-Aminobutyric acid €-Aminocaproic acid
0.127 0.170
-173 -232
-211 -78
-0.13 0.52
0.213
-291
-16
0.92
0.213
-291
+161
1.51
Discussion Because of the large dipole moment of dipolar ions, the dominant term in the descriptions of the interaction between amino acid and electrolytes was assumed to be the electrostatic term.1)2i6The Kirkwood equation (eq 9) k, =
2n2z2e2
(9)
has successfully predicted the variation of k, with the dielectric constant of the solvent; correlated k, values with reasonable estimates of and a; and with an additional term to account for the salting out of the nonpolar parts of the dipolar ion, quantitatively accounted for the k, of glycine, P-alanine, y-aminobutyric acid, and e-aminocaproic acid.2 The corresponding thermodynamics of interaction can be arrived at from eq 9 and result in P2 a
AG," = 2.303RTkS= 7.47-
+ 6.36a:(p)V.
(10)
6.36The AHsovalues calculated from eq 11 are not even in qualitative agreement with the experimental values (see Table 111). If an additional term is added to account for the salting out effect of the nonpolar CH2 groups, the
disagreement becomes worse. We estimate this term to be about +175 cal/mol per CH2;lwhereas the experimental difference is negative and not proportional to the number of CH2 groups. The origin of this anomaly we believe lies in the unusual properties of liquid water that results from its highly structured nature. Solutes have a highly specific effect on this structure that is reflected in solution thermodynamics, viscosity, etc.&1° The magnitude of these effects may be estimated by assuming each of the thermodynamic functions may be written as the sum of three terms: its value in a constant structure water, its additional value from long-range structure making, and its value from shortrrange structure breaking. (Not included is Frank's very short range structure making region that results from the hydration of the solutes.) The first two terms constitute Frank's bulk water region and the values are calculated from the Kirkwood equation. The values calculated for the glycine-NaC1 system are reported in Table IV. In the first row, the AG, AH, and A S values are calculated from eq 10 and 11assuming the structure of water does not change. This is done by assuming the dielectric constant of water can be calculated from the Kirkwood-Frohlich equation in which the structural parameter, g, is assumed constant.'lJ2 This results in d In D / d T -1/T and a very small enthalpy contribution. The overlapping of regions of ordered water dipoles results in an appreciable entropy increase. The second row of values are the additional contributions due to electrostatic structure making in the outer region and are calculated using the experimental value of d In D/dT for water.13 Whereas the free energy is unchanged, the positive enthalpy and entropy changes result from the overlap of the structured regions of water around the ions and dipolar ions. The third row values of Table IV (and the last column of Table 111) are the additional contributions necessary in order to obtain the experimental values. These values reflect the highly specific structural changes in the water close to the ion and dipolar ions. The dominant term for these charged solutes is the overlap of structure breaking regions of water in the high fields close to the solutes. Hydrophobic structure making (and primary hydration structure making) would show up in this term as a reduction of the amount of structure breaking. The addition of hydrophobic groups, that do not change the dipole moment, as in a-alanine and valine, greatly reduces this term. Our conclusions are as follows: (1)The free energy of interaction is determined predominantly by and yields information about the eiectrostatic interactions of the solute species. (2) Linear compensation14J5between the enthalpy and entropy changes that occur as the result of the solutes effect on the structure of water leads to an almost unchanged free energy term.
-
TABLE 111: Enthalpies of Interaction
----________~ Amino acid Glycine 0-Alanine y- Aminobutyric acid €-Aminocaproic acid a-Alanine Valine Serine
P, D
v,
A H," 7
AH,", cal/mol
A 4.2 4.45
cm3/mol
4P)
13.5 16.7
57 73.3
1.30 1.32
cal/mol -211 -78
Eq 195 290
- 406 - 368
19.9
4.68
92.3
1.34
- 16
400
-416
24.1 13.7 14.2 14
5.01 4.45 4.85 4.45
122.2 73.3 105.9 73.7
1.36 1.32 1.34 1.32
161 3 112 - 120
551 186 134 186
-394 - 183 - 22 - 306
-__.____-
a,
Diff
The Journal of Physical Chemistry, Vol. 81,
No. 22, 1977
George D. Halsey
2076
TABLE IV: C o n t r i b u t i o n s to t h e T h e r m o d y n a m i c s of I n t e r a c t i o n of G l y c i n e and N a C l A 8,
-
~
-
-
Constant structure water Long-range structure making Shorter-range structure breaking
AG, AH, calimol cal/mol cal/mol K
--
- 213
+5
0.72
0
t190
0.65
t40
-406
-1.50
(3) The enthalpy of interaction is determined predominantly by yields information about the structure making-breaking of the solutes. An exothermic enthalpy of interaction indicates the amino acid is a net structure breaker while an endothermic value indicates a net structure maker. (4) The difference between the experimental enthalpies of interaction and those calculated by eq 11 indicates the effect of the amino acid on the structure of water close to the amino acid. Though differing in detail, the above interpretation is consistent with the conclusions reached by other workers examining the enthalpy of interaction data and heats of dilution.’6-18 Acknowledgment. We thank the Union Carbide Corp.
for scholarship support (A.F.J.) and the Marshall University Foundation for financial support. References and Notes J. W. Larson and D. G. Morrison, J. Phys. Chem., 80, 1449 (1976). E. E. Schrier and R. A. Robinson, J. Biol. Chem., 248, 287 (1971). R. N. Goldberg and L. G. Hepler, J. Phys. Chem., 72, 4654 (1968). V. B. Parker, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 2, (1965). (5) J. H. Stern, J. Lazartic, and D. Fost, J. Phys. Chem., 72, 3053 (1968). (6) J. T. Edsall and J. Wyman, “Biophysical Chemistry”, Vol. 1, Academic Press. New York. N.Y.. 1958. (7) E. J. dohn and J. T. EdGll, Ed., “Proteins, Amino Acids and Peptides”, Reinhold, New York, N.Y., 1943. (8) H. S. Frank and M. W. Evans, J. Chem. Phys., 13, 507 (1945). (9) J. L. Kavanau, “Water and Solute-Water Interactions”. Holden-Dav. San Francisco, Calif., 1964. (10) R. A. Horne, Ed., “Water and Aqueous Solutions”, Wiley-Interscience, New York, N.Y., 1971. (1 1) J. B. Hasted, “Aqueous Dielectrics”, Chapman and Hall, London, 1973. (12) H. Frohlich, “Theory of Dielectrics”, Oxford University Press, London, 1949. (13) H. S. Harned and B. B. Owens, “The Physical Chemistry of Electrolyte Solutions”, Reinhold, New York, N.Y., 1958. (14) R. Lumbry and S. Rajender, Biopolymers, 9, 1125 (1970). (15) L. G. Hepler, J. Am. Chem. SOC., 85, 3089 (1963). (16) G. C. Kresheck and L. Benjamin, J. Phys. Chem., 88, 2476 (1964). (17) R. H. Wood, H. L. Anderson, J. D. Beck, J. R. France, W. E. deVry, and L. J. Soltzberg, J. Phys. Chem., 71, 2149 (1967). (18) S. Lindenbaum, J. Phys. Chem., 75, 3733 (1971). (19) J. H. Stern and J. T. Swearingen, J . Phys. Chem., 74, 167 (1970). (20) J. H. Stern and J. D. Kulluk, J . Phys. Chem., 73, 2795 (1969). (1) (2) (3) (4)
Disregistry Transition in the Krypton Monolayer on Graphite George D. Halsey Department of Chemistry, University of Washington,Seattle, Washington 98 195 (Received Juqe 16, 1977) Publication costs assisted by the University of Washington
An earlier two-structuremodel for transition between two Langmuir isotherms with different adsorption maxima is applied to the transition of an adsorbed layer from a substrate-registeredlattice structure into a more dense self-determinedlattice. This treatment locates the transition but neglects the transition region structure. The latter is described in terms of the interaction of clustered, lattice-dislocated atoms viewed as a large mobile molecule. The Hill-van der Waals isotherm is used to calculate an adequate shape for the transition region. The Gibbs-Eriksson theory of surface tension is used to predict the behavior of the spreading pressure and surface tension during the transition. Thomy, Regnier, and Duvall have presented detailed isotherms for krypton adsorbed on graphite. Similar isotherms have been found by Putnam and Fort.2 Each of these isotherm show a series of steps which can be identified with “events” on the surface. In particular the AF-B1 or last such “substep” in the monolayer region has been identified with a transition from a triangular lattice of krypton in registry with the graphite surface, to a similar structure out of registry with the substrate. This interpretation has been confirmed by LEED measurements which produce similar steps when the lattice parameter is plotted against pre~sure.~ It should be noted that these steps are not vertical, but appear rounded in shape, especially at the upper edge. They amount to about 0.1 monolayer in height, although, because of the general upward slope of the isotherm,l it is difficult to estimate the exact extent of the transition itself, as distinguished from vacancy filling and second layer formation.
I. Langmuir-Langmuir Transition Two-Structure Models. In several earlier the author has presented a treatment of the transition between The Journal of Physical Chemistry, Vol. 81, No. 22, 1977
two distinct surface structures, such as the Langmuir and Volmer models of lattice and random structure, respectively. Similar but more complex models have been examined by Price and Venables.6 Since these theories do not include intermediate structures, they are strongly predisposed to predict stepwise or first-order transitions. If the adsorption maxima differ for the two structures, they may be otherwise the same and a transition will still be possible. Such a model was proposed7 to describe the transition between substrate fit to closest packing, or the in registry-out of registry transition, that was suggested by the observations of Landera and the discussion that followed. The model of Price and Venables gives a first-order disregistry transition for essentially the same reason. High-Coverage Approximation. Since the initial and final states of the transition are at high coverage and of the same triangular structure, they can both be approximated by the Langmuir model. The lower regions of each isotherm will be represented by slightly different Langmuir equations, rather than ones involving lateral interaction. The situation is shown in Figure 1. A t the transition,
