THE BEHAVIOR OF THE PROLAMINS IN MIXED SOLVENTS. II. (Received for publication, December 20, 1926.)

Transcription

1 THE BEHAVIOR OF THE PROLAMINS IN MIXED SOLVENTS. II. BY D. B. DILL.* (From the Department of Physical Chemistry in the Laboratories of Physiology, Harvard Medical School, Boston.) (Received for publication, December 20, 1926.) A quantitative description of the solubility behavior of gliadin in mixtures of water with certain alcohols has been given recently by Dill and Alsberg (1). There now has been completed a similar study of zein, a companion protein. In addition to mixtures of water with methyl, with ethyl, and with npropyl alcohol, mixtures of water with tertiary butyl alcohol have been employed as solvents. Also, observation has been made of the behavior of gliadin in mixtures of water with tertiary butyl alcohol. The zein preparation used is the one of Professor Osborne used by Cohn, Berggren, and Hendry (2). The gliadin was prepared by the author according to the method of Dill and Alsberg (1). Their experimental procedure for determining turbidity temperatures was followed without modification. It had been found that there is no well defined upper limit to the solubility of gliadin in certain alcoholwater mixtures and within certain temperature limits. The results of two preliminary experiments on zein, detailed in Table I, indicate that this prolamin may be characterized similarly. These results, together with a former similar experiment with gliadin, are illustrated in Fig. 1. In the indicated mixtures of alcohol and water, the solubility of gliadin and of zein increased rapidly with the temperature until a concentration of about 3 gm. per 100 gm. of solution was reached. At this point no further increase in temperature was required to dissolve a great deal more protein. That this curious phenomenon is not explained by supersaturation was proved by observing both the appearance of turbidity on * National Research Fellow in Chemistry. 239

2 Prolamins in l&fixed Solvents. II cooling and its disappearance on warming. In a given solution, turbidity appeared and disappeared at about the same temperature. It is difficult on account of increasing viscosity to prepare in the usual manner zein solutions more concentrated than 25 per cent. However, by evaporation at 50 C. of concentrated solutions of zein or of gliadin in 70 per cent npropyl alcohol, a more and more TABLE I. Turbidity Temperatures of Solutions of Zein in AlcoholWater. Zein. Temperature at which twbidity appeoas$m coobng A. 70 per cent EtOH by volume in the solvent. Temperature at which turbidity disappeared cm warming from 0. gm. per 100 gm. solution OC. C B. 58 per cent EtOH by volume in the solvent concentrated solution is obtained without loss of apparent homogeneity. Finally, a perfectly transparent and dry film is obtained. The statement is justified, then, that there is no well defined upper limit to the solubility of gliadin or of zein in suitable alcoholwater mixtures within certain temperature limits. Therefore, the phenomenon may be described appropriately by the term peptization. The determination of the solubility of zein by Galeotti and Giampalmo (3), quoted by Seidell(4), was in reality a determination of

3 D. B. Dill 241 the relative rates of peptization in each of the various solvent mixtures used. There was no evidence presented that saturation was attained and we are safe in concluding that it was not attained. Two series of solutions of zein in the various solvents were then prepared. One series contained approximately 4 per cent, and the 40 J of Proteln per 100 Grams of Solution L Grams FIQ. 1. Solubility of zein and of gliadin in certain EtOHHI0 mixtures. TABLE Turbidity Temperatures of.j Per Cent and 8 Per Cent Solutions of Zein in Methyl AlcoholWater Solvents. vols. per cent Methyl alcohol. mol fraction II. Turbidity 4 p*r cent zein solution. temperature. 8 per cent z&n eolution. YT. v other 8 per cent zein by weight. The turbidity temperatures for zein in methyl alcoholwater mixtures are given in Table II; in ethyl alcoholwater, Table III; in npropyl alcoholwater in Table IV. The turbidity temperatures for 4 per cent zein solutions and

4 242 Prolamins in Mixed Solvents. II TABLE Turbidity Temperatures of 4 Per Cent and 8 Per Cent Solutions of Zein in Ethyl AlcoholWater Solvents. III. Ethyl alcohol. 001s. per cent mol fraction C TABLE Turbidity temperature. 4 per cent eein solution. T IV.. 8 per cent, zein solution. C Turbidity Temperatures of.j Per Cent and 8 Per Cent Solutions of Zein in npropyl AlcoholWater Solvents. npropyl alcohol. vols. per Cent mol fraction C Turbidity 4 per cent sein eolution. temperature. 8 per cent zein solution. C

5 D. B. Dill 243 for 4 per cent gliadin solutions in tertiary butyl alcoholwater are shown in Table V. In these tables, the concentrations of the three primary alcohols are shown both in terms of volumes per cent and mol fraction. Since the latter is the more fundamental unit, it is used in plotting the zein results which are shown graphically in Fig 2. The new gliadin results, together with the earlier observations, are illustrated graphically in Fig. 3. Here, too, the concentrations are in terms of mol fraction. TABLE V. Turbidity Temperatures of Q Per Cent Solutions of Zein and of Gliadin in Tertiary Butyl AlcoholWater Solvents. Tertiary butyl alcohol. mol fraction I 4 per cent zein solution. C Turbidity temperature. i 4 per cent gliadin soluticn. C The solubility behavior of nitro cotton is not unlike that of gliadin and of zein. It dissolves in many pairs of mixed solvents, even though it may be insoluble in one of the components. There is no well defined upper limit to its solubility. Evaporation of its solutions may yield a transparent film. A theory explaining its solubility behavior has been advanced by McBain, Harvey, and Smith (5). The significant portion of their theory is as follows: The fundamental facts operating in the behavior of solvents and nitro cotton is direct combination between solvent and suitable complementary chemical groups in the nitro cotton. The apparent viscosity of the solutions is almost entirely due to the presence of loose ramifying aggre

6 244 Prolamins in Mixed Solvents. II gates of colloidal particles united by local and specific bonds of residual afsnity of different kinds and degree. The best solvents are those which most effectively combine with these bonds.... and dismember the aggregates. Hence, the best solvents yield solutions of the lowest ap MaI Fraction FIQ. 2. Critical peptization temperatures of zein in MeOHH20, EtOH HzO, nprohhzo, and tertiary BuOHHzO. parent viscosities and mixtures of solvents and nonsolvents affording a variety of suitable chemical groups are much more efficient than a single solvent. If this theory is to be applied to prolamin solutions in mixed solvents, it must be in accord with the following observations.

7 D. B. Dill For a given alcoholwater mixture, there is a critical peptization temperature. Below this temperature, the solubility rapidly decreases. At, or above this temperature, there is no well defined upper limit to the solubility. It may be added as a corollary that the position and shape of the curves are independent of the protein concentration when this exceeds a minimum of 2 to 3 per cent al x, P 2 20 P e I I Akohol water flol Fraction FIQ. 3. Critical peptization temperatures of gliadin in MeOHH20, EtOHH20, nprohh20, and tertiary BuOHH In the case of the three primary alcohols, the relative position of the three curves for each protein bears a qualitative relationship to the molecular weights of the alcohols. The position of a given curve appears to be a function of the molecular weight of the alcohol.

8 246 Prolamins in Mixed Solvents. II 3. For a given alcohol, the gliadin and zein curves are quite different, and it follows that the position and shape of the curve is a function of the character of the protein. There is a suggestive correlation between the properties of these two proteins and their behavior in these two mixed solvents. Cohn and his associates (2,6) have shown that zein is one of the most inert of proteins. It has a very large molecule and it is not amphoteric inasmumh as it does not combine with acid. Gliadin is amphoteric, is less inert, and has a smaller molecule. The difference in the solubility behavior of these two proteins accords with these properties. In the case of the primary alcohols, zein is dissolved in a narrow range of methylalcoholwater mixtures and in a very wide range of n propyl alcohol and water, with ethyl alcohol and water occupying an intermediate position. A reciprocal relationship holds for gliadin in these mixed solvents. Of these three alcohols, methyl alcohol is most polar and npropyl alcohol is least polar. Their dielectric constants are MeOH, 35; EtOH, 27; nproh, 22. That of tertiary BuOH is 11. There is a similar contrast in the behavior of these proteins in mixtures of water with the relatively nonpolar tertiary butyl alcohol. At 50, the alcohol concentration range for the gliadin solvent is from a mol fraction of 0.02 to one of 0.12, while the range for the zein solvent at this temperature is from 0.09 to The theory of McBain et al. accounts for all these phenomena except the sudden transition from turbidity to homogeneity, independently of protein concentration. The suggestion is advanced that this is a freezing point phenomenon; that a given alcoholwater mixture dissolves the protein with the formation of an alcoholwaterprotein complex. Above the critical peptization temperature, this complex is liquid and miscible in all proportions with this alcoholwater mixture. At, and below this temperature, it is solid and has definite and limited solubility. SUMMARY. A qualitative description has been given of the solubility behavior of zein in mixtures of water with methyl alcohol, with ethyl alcohol, and with npropyl alcohol. The solubility behavior of both zein and gliadin in mixtures of water with tertiary butyl alcohol has been studied.

9 D. B. Dill BIBLIOGRAPHY. 1. Dill, D. B., and Alsberg, C. L., J. Biol. Chem., 1925, lxv, Cohn, E. J., Berggren, R. E. L., and Hendry, J. L., J. Gen. Physiol., 1924, vii, Galeotti, G., and Giampalmo, G., Z. Chem. Ind. Kolloide, 1908, iii, Seidell, A., Solubilities of inorganic and organic compounds, New York, 2nd edition, 1919, McBain, J. W., Harvey, C. E., and Smith, L. E., J. Physic. Chem., 1926, xxx, Cohn, E. J., and Conant, J. B., Proc. Nat. Acad. XC., 1926, xii, 433.

10 THE BEHAVIOR OF THE PROLAMINS IN MIXED SOLVENTS. II D. B. Dill J. Biol. Chem. 1927, 72: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at ml#reflist1