COMPLEX SALTS OF AMINO ACIDS AND PEPTIDES

COMPLEX SALTS OF AMINO ACIDS AND PEPTIDES II. DETERMINATION OF Z-PROLINE WITH THE AID OF RHODAN- ILIC ACID. THE STRUCTURE OF GELATIN BY MAX BERGMANN (From the Laboratories of The Rockefeller Institute for Medical Research, New York) (Received for publication, May 24, 1935) The first step in determining the structure of a protein is the quantitative study of the constituent amino acids. The ratios of such amino acids, resulting from the classic analytical methods of Kossel, Fischer, Dakin, and Van Slyke, indicate a very complex structure for nearly all proteins investigated. It seemed possible to simplify this picture by improving the methods of determining certain amino acids. The use of complex salts for the selective precipitation of amino acids and peptides has been proposed in Paper I (1). Here the complex salt method is further developed. Gelatin will be discussed as an example. Through Dakin s analysis it has become one of the most completely known proteins. Of the fifteen amino acids found in gelatin hydrolysates, glycine (25.5 per cent), hydroxyproline (14.1 per cent), and proline (9.5 per cent) are most abundant. A simple calculation by Atkin (2), in which Dakin s results (3) are used, shows that glycine and hydroxyproline account for about one-third and one-ninth, respectively, of the total amino acid residues in gelatin. On the same basis, Astbury (4) has recently pointed to the possibility that every third amino acid in gelatin might be glycine and every ninth, hydroxyproline. Making use of the known x-ray photographs of gelatin and collagen, this author suggests that the amino acid residues are somehow grouped in sets of three. However, by an indirect method, Waldschmidt-Leitz and Akabori (5) have recently found the content of hydroxyproline to be 8.9 to 9.4 per cent, and have claimed Dakin s figure to be erroneous. On the other hand, Patton (6), using a calorimetric method, has recently found the glycine content of gelatin to be 22 per cent. 471

472 Amino Acids and Peptides. II In view of these contradictions it was necessary to reinvestigate the content of these amino acids in gelatin. The first step was the development of a new complex for the determination of proline. The new reagent is a variation of the Reinecke salt complex. Kapfhammer and Eck (7) in 1927 used Reinecke salt, [Cr(CNS)k- (NH&]*NHI, for the simultaneous precipitation of proline and hydroxyproline from gelatin hydrolysates. The two amino acids were regenerated by a tedious procedure from the Reineckates and separated from each other by extraction with alcohol. Thus, they obtained 8 per cent free hydroxyl-l-proline and 4.4. per cent i- proline as its cadmium chloride compound. The transformation into this double salt was necessary to liberate the proline completely from hydroxyproline. The liberation of the prolines from the Reineckates can be considerably simplified according to Dakin by decomposing the Reineckates with dimethylaniline. It has been possible to change the behavior of the Reineckate complex towards amino acids by introducing into the complex amines other than ammonia. With two aniline groups an excellent means for the determination of proline was obtained. This new complex acid is tetrathiocyanato-dianilidochromiato acid, [Cr(CNS)4(C6H5. NH&]H. It is referred to in this paper as rhodanilic acid and its salts are called rhodanilates. Rhodanilic acid is formed from chromic sulfate or chrome alum, potassium thiocyanate, and aniline in the manner described in the experimental section. The following reactions take place. [Cr,(SO&] + 12KCNS = 2[Cr(CNS)e]Ks + 3K,S04 [Cr(CNS)6]K3 + 4CsH,.NH, + CH&OOH = SKCNS + C&COOK + [Cr(CNS)&XLNH&l (CsH5.NH,).(CsHa.NH2) An aniline rhodanilate containing an extra molecule of aniline is obtained. This salt may easily be converted into the ammonium salt with ammonium hydroxide. Rhodanilic acid forms rose-colored, well crystallized salts with basic nitrogen compounds, and in particular with alkaloids and with amino acids. Although rhodanilic acid lacks definite specificity, the various rhodanilates differ greatly in their solubilities, crystalline form, and rate of crystallization. It is therefore often I Personal communication. This procedure suggested the pyridine method for obtaining proline and hydroxyproline, which will be described later in this paper.

M. Bergmann 473 possible to separate from mixtures of amines, amino acids, or peptides single homogeneous products by fractional precipitation with rhodanilic acid. In cases where several rhodanilates form simultaneously, a separation by fractional crystallization is often possible. Of the rhodanilates of amino acids, that of proline distinguishes itself by slight solubility and rapid crystallization. With the aid of the rhodanilate method a determination of proline in gelatin was performed. From a hydrochloric acid hydrolysate of gelatin, arginine was first removed by flavianic acid. Then ammonium rhodanilate was added to the mother liquor as long as a uniform precipitation of the proline salt took place. The quantity necessary was determined by examining the precipitate under the microscope in the course of successive additions. The proline rhodanilate was analyzed and converted into free l-proline. In order to obtain the free amino acid from the proline rhodanilate, advantage was taken of the fact that pyridine rhodanilate is very difficultly soluble in water. It is therefore sufficient to suspend the solid proline rhodanilate in water and to add a little pyridine in order to precipitate almost instantaneously the entire rhodanilic acid as pyridine salt; on filtration a faintly colored aqueous solution of I-proline is obtained. Pure I-proline of [c&, = -85.6 crystallizes out on evaporation. These preparations contain no dl-proline or hydroxyproline. From 118 gm. of Gold Label gelatin (i.e., 100 gm. of water-free gelatin), 97 gm. of pure I-proline rhodanilate were obtained. This corresponds to 18.5 gm. of I-proline. Similarly, from 100 gm. of water-free Bacto-Gelatine 100 gm. of pure I-proline rhodanilate, corresponding to 19.0 gm. of I-proline, were obtained. In the precipitation of the proline rhodanilate there remains dissolved in the mother liquor a fraction which we have determined by special crystallization experiments with pure material to be approximately 5 gm. Accounting for this quantity, the proline content of the water-free gelatin is 19.4 per cent (Gold Label) and 20.0 per cent (Bacto-Gelatine). In decomposing the rhodanilate of 100 gm. of water-free gelatin, 15 gm. of pure I-proline were obtained. Pyridine rhodanilate is formed as a by-product in the preparation of proline. It may easily be reconverted into ammonium rhodanilate with ammonia and so recovered for further use.

474 Amino Acids and Peptides. II Thus, in the course of the rhodanilate method for proline, rhodanilic acid passes stepwise through the combination with aniline, ammonia, proline, pyridine, and finally again ammonia. In each step of the method two amines compehe for the rhodanilic acid, so that the conditions of the experiment determine t.he outcome. With this method, proline has become easily accessible. In order to obtain it from gelatin hydrolysate, arginine need not be removed first; however, one may work in such a manner as to combine the preparation of arginine, proline, and hydroxyproline as flavianate, rhodanilate, and Reineckate, respectively. Likewise, the quantitative determination of proline may or may not be combined with the determination of arginine. It has been found possible to estimate hydroxyproline as Reineckate after the determination of arginine and proline. From the Reineckate the hydroxyproline could be regenerated with pyridine. In this way from 100 gm. of water-free anhydrous gelatin (Gold Label) or Bacto-Gelatine, 10.5 gm. of I-hydroxyproline were obtained. By correcting for the solubility of hydroxyproline Reineckate in the mother liquor,2 the yield becomes 14.4 gm. The earlier results of Dakin are thus confirmed. For the estimation of the third important amino acid, glycine, the complex method, with potassium trioxalatochromiate, has again been employed. The use of this reagent has been described previously (1). The glycine, in the hydrolysate of Gold Label gelatin, was precipitated and the nitrogen content of the precipitate determined. The exact method will be described later. In this way, 25.7 per cent glycine was found. This value agrees exactly with that of Dakin. The amino acids now known to be formed in gelatin hydrolysates represent 103 per cent of the weight of gelatin. 119 per cent may be expected. For those three amino acids constituting half of the gelatin-glycine, proline, hydroxyproline-there are now reliable quantitative figures. In combining these figures with the respective molecular weights, the number of gm. molecules of each formed in the hydrolysis of 100 gm. of gelatin is obtained. The results are summarized in Table I. It may be seen that the molecular ratios for glycine, proline, and 2 See experimental section.

M. Bergmann 475 hydroxyproline bear the simple relation, 6: 3:2. The number of gm. molecules of these three amino acids-glycine, proline, and hydroxyproline-in gelatin may be calculated, assuming that they represent one-third, one-sixth, and one-ninth, respectively, of all the amino acids and that the average molecular weight of all the amino acids is 111. These calculat,ed values are included in Table I. The close agreement between these two sets of figures and the simple ratio of these to the number of average amino acids (Column 5) support the suggestion that these three amino acids occupy a definite periodic arrangement within the protein. If this were true, every third amino acid in the chain could be glycine, every sixth proline, and every ninth hydroxyproline. Such an arrangement could be represented by either of the two TABLE Amino Acid Contertt of Gelatin after Hydrolysis I Gm. molecules Weight YF?- Ratio weight Found %x (1) (2) (3) (4) (5) per cent Glycine... 25.5 75 0.34 0.36 6.0 I-Proline. 19.7 115 0.17 0.18 3.0 I-Hydroxyproline 14.4 131 0.11 0.12 2.0 Average amino acid.. 119.0 111 1.07 18.8 diagrams below, in which the glycyl residue is represented by G, the prolyl by P, and the other amino acids by X. -G.P.X.G.X.X.G.P.X.G.X.X- -G.X.P.G.X.X.G.X.P,G.X.X- The best proof for this hypothesis would be the isolation of large quantities of dipeptides, such as prolylglycine (PG) or glycylproline (GP), or of larger peptides from partial hydrolysates of gelatin. Work with this in view is now in progress in this laboratory. Grateful acknowiedgment is hereby made to Dr. William F. Ross for assistance in preparing the manuscript, and to Mr. Samuel C. Carpenter for technical assistance.

476 Amino Acids and Peptides. II EXPERIMENTAL Aniline Rhodadate-500 gm. of chrome alum, 600 gm. of potassium thiocyanate, and 0.5 liter of water were heated for 4 hours on the steam bath. The solution was cooled, and after addition of 0.5 liter of aniline was warmed for 3 hours with stirring in a bath of 60. It was then cooled again, and a mixture of 6 liters of water and 600 cc. of glacial acetic acid added. After some hours the precipitate was filtered by suction, extracted with cold methyl alcohol (3.5 to 2 liters), filtered, and 6 liters of water added with stirring. This occasioned the formation of a rather thick pulp of violet crystals. After another recrystallization the yield amounted to 330 gm. The preparation contained aniline of crystallization. [ClsH*4N~S4Cr].(CsHsN).(CsH?N). Calculated. C 51.1, H 4.4, N 17.1 Found. 51.1, 4.5, 17.0 All of the nitrogen determinations, except that for the pyridine rhodanilate, were made by the Kjeldahl method. The Pregl- Dumas results were 0.8 to 1.0 per cent too low. The pyridine salt by the Kjeldahl method gave only 6 of its 7 nitrogen atoms. Thus, it had to be analyzed by the Pregl-Dumas method in spite of the low results. Ammonium Rhodanilate-400 gm. of the aniline salt were treated with 600 cc. of methanol and 300 cc. of a concentrated aqueous solution of ammonia. Then 3 liters of water were slowly added to the ice-cold solution. The precipitate, consisting of crystalline plates, was treated again in the same manner with methanol, ammonia, and water. Yield, about 200 gm. [ClsHlrNeSaCr].(NH~).ltHzO. Calculated. C 37.3, H 4.1, N 18.7 Found. 37.2, I 4.3, I 18.7 Z-Proline Rho&nil&e-The solution of 1.5 gm. of ammonium rhodanilate in 20 cc. of methyl alcohol was added to a solution of 0.35 gm. of Z-proline in 15 cc. of 0.25 N hydrochloric acid. The red prisms which separated out were again dissolved in methyl alcohol and precipitated with very dilute hydrochloric acid. Melting point, 133-134 (decomposition). lc16h14n6s4cr].(c6h1002n).h10. Calculated. C 41.7, H 4.3, N 16.2 Found. 41.7, 4.5, 16.4

M. Bergmann 477 Pyridinium Rhodanilate-It is obtained quantitatively from either of the previously described salts with pyridine in acid salt solution. Red rhomboids were obtained. [C18H,,N&Crl(CGH~N). Calculated. C 45.8, H 3.6, N 17.8 Found. I 45.7, 3.7, 17.4 The nitrogen was determined with the micro-dumas method. Determination of I-Proline and I-Hydroxyproline in Gelatin- 500 gm. of commercial gelatin (Gold Label), containing 15 per cent of water, were hydrolyzed with 3 times the theoretical quantity of concentrated hydrochloric acid for 8 hours. The solution was evaporated under diminished pressure, redissolved, and evaporated again three times from water. Arginine was then removed as flavianate in the usual manner. The filtrate, including the solution used for washing, had a volume of 4.3 liters. 1 liter of this solution was added to the filtered solution of 95 gm. of ammonium rhodanilate in 600 cc. of methyl alcohol. Proline rhodanilate separated out immediately. The thick pulp was cooled for 2 hours on ice, filtered in a cold room by suction, and washed with 200 cc. of ice-cold water. The purple crystalline mass, after drying on air, weighed 97 gm. (100 gm. when Bacto- Gelatine was used). The salt had the correct composition. Calculated. C 41.7, H 4.3, N 16.2 Found. 41.7, 4.5, 16.2 In order to estimate the minimum amount of proline rhodanilate remaining in the mother liquor in the above described crystallization, 20.0 gm. of the purified solid were dissolved in 150 cc. of methyl alcohol and reprecipitated by the addition of 250 cc. of 0.5 N hydrochloric acid. It was filtered by suction after having been cooled for 2 hours, and was dried in air. 19.0 gm. were regained. The loss was therefore about 5 per cent. In the preparation above, at least 5 gm. of proline rhodanilate are therefore lost in the mother liquor. For Gold Label gelatin we may therefore assume that 102 gm. of proline rhodanilate are formed, and for Bacto-Gelatine 105 gm. This corresponds to 19.4 per cent and 20.0 per cent proline, respectively. In order to obtain the free proline, 49 gm. of crude proline rhodanilate were recrystallized from methyl alcohol with 0.5

478 Amino Acids and Peptides. II N hydrochloric acid. The purified and dried product was mixed thoroughly with 10 cc. of pyridine in 300 cc. of water and filtered by suction after standing. To the faintly red filtrate a few drops of acetic acid were added, and the small amount of precipitate was filtered off. The solution was then evaporated in VUCUO, the residue redissolved twice in water and once in alcohol, and the solvents evaporated. Pure I-proline could be obtained as a colorless, crystalline mass from absolute alcohol and ether. Yield 7.5 gm., or 15 gm. for 100 gm. of gelatin. [al: = -85.6 (in water) C~H~O~N~. Calculated. C 52.2, H 7.9, N 12.1 Found. 52.3, I 7.9, 12.1 After 1 liter of gelatin hydrolysate had been freed from most of its proline, the hydroxyproline was precipitated with Reinecke salt. As is well known, this precipitation is not quantitative. Therefore, we carried out a number of experiments to improve the yield as much as possible. In our first experiments the Reineckate was added in small portions and the mixture stirred vigorously for an hour, after which the precipitate was filtered off. Later it was found simpler to add the ammonium Reineckate at once and to add simultaneously some pyridine. A double salt containing pyridine is thought to be formed. For the separation of hydroxyproline 125 gm. of ammonium Reineckate and 7.5 cc. of pyridine were therefore added; the mixture was stirred for 2 hours, cooled in ice water, filtered in the cold, and the product dried on clay. It was then washed with 500 cc. of water and decomposed with 50 cc. of pyridine. The precipitate was filtered after 1 hour and washed thoroughly. The filtrate containing hydroxyproline besides a large number of other substances was still colored dark red. Glacial acetic acid was therefore added drop by drop, as long as a precipitate formed, and the precipitate was then removed. The now light brownish yellow filtrate was evaporated in vacua; the residue was dissolved twice in water and twice in methyl alcohol, followed each time by evaporation of the solvent. Most of the hydroxyproline had now crystallized together with ammonium salts from the syrupy residue. The whole substance was thoroughly shaken with 75 cc. of anhydrous methyl alcohol, the crystals filtered off, dried, powdered, and allowed to stand with

M. Bergmann 479 50 cc. of 95 per cent methyl alcohol for 2 hours at a temperature of f5. Then they were filtered by suction in the cold and dried in vacua over phosphorus pentoxide. In this state the hydroxyproline is still colored yellow-brown, but is already fairly pure. CcHgOaN. Calculated. C 45.8, H 6.9, N 10.5 Found. 45.4, 6.9, 10.5 The average yield of this substance in several experiments with Gold Label gelatin and with Bacto-Gelatine was 10.6 and 10.8 gm., respectively. This quantity, however, does not contain the entire original hydroxyproline of the gelatin. During the process of isolation considerable losses take place in at least two instances, the principal loss being in the precipitation with ammonium Reineckate and pyridine, and a smaller one occurring during the treatment of the crude hydroxyproline with 95 per cent methyl alcohol. In order to estimate these losses, 14 gm. of hydroxyproline of the above quality were dissolved together with 25 gm. of glycine in a mixture of 1 liter of water and 300 cc. of methyl alcohol. The solution was made acid to Congo red with hydrochloric acid. This solution was now treated with Reinecke salt and pyridine, and the precipitate formed was decomposed and worked up as described above. All details were carried out exactly as with the gelatin hydrolysate. After the treatment with 95 per cent methyl alcohol, 10.5 gm. of pure hydroxyproline were obtained. Since this yield is almost the same (10.1) as that from 1 liter of gelatin hydrolysate or 99 gm. of anhydrous gelatin, one may assume that in the hydrolysis of 100 gm. of gelatin (Gold Label or Bacto-Gelatine) at least 14.4 gm. of hydroxyproline are formed. BIBLIOGRAPHY 1. Bergmann, M., and Fox, S. W., J. Biol. Chem., 109, 317 (1935). 2. Atkin, W. R., J. Internat. Sot. Leather Trades Chem., 17, 575 (1933). 3. Dakin, H. D., J. Biol. Chem., 44, 499 (1920). 4. Astbury, W. T., in Cold Spring Harbor symposia on quantitative biology, Cold Spring Harbor, 2,15 (1934). 5. Waldschmidt-Leitz, E., and Akabori, S., 2. physiol. Chem., 224, 187 (1934). 6. Patton, A. R., J. Biol. Chem., 163, 267 (1935). 7. Kapfhammer, J., and Eck, R., 2. physiol. them., 170, 294 (1927).

COMPLEX SALTS OF AMINO ACIDS AND PEPTIDES: II. DETERMINATION OF l-proline WITH THE AID OF RHODANILIC ACID. THE STRUCTURE OF GELATIN Max Bergmann J. Biol. Chem. 1935, 110:471-479. Access the most updated version of this article at http://www.jbc.org/content/110/2/471.citation Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's e-mail alerts This article cites 0 references, 0 of which can be accessed free at http://www.jbc.org/content/110/2/471.citation.full.h tml#ref-list-1