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stereospecific h y d r ~ l y s i s . ~In a variety of these substrates, of the asymmetric type Cabde and of the symmetric type Cabdd, the absolute steric sense of the hydrolysis, where determined, was2 L. We are studying the effect of the alpha and beta acetoxyl substituent in place of the acetamido group in this reaction, and wish to report an inversion of antipodal reactivity in the hydrolysis of ethyl a-acetoxypropionate, CH&H(OCOCHa)-
THE
1-01,s1
EDITOR
mixed 1n.p. with an authentic sample, 136-147", [ aJ% 48.(i0,2.22% in chloroform. We suggest that ethyl a-acetoxypropionate rrlity associate as an extended tetrahedron with achymotrypsin in two conformations. In both, the a-hydrogen assumes its normal orientation, presumably fitting into a restricted space, determining the sense of approach of the other three groups to the enzyme, E. In one conformation, which is preferred, I-L and I-D, the acetoxyl group (Ao), lacking the polar N-H of a typical acetamido substrate, associates with the non-polar site (a)
+
COzCzHs. Ethyl dl-a-acetoxypropionate was subjected to the action of a-chymotrypsin, 12 mg./ml., a t pH Ao a Ao a 7.8 in a pH-stat for 12 hours, hydrolysis stopping after about 50% reaction. Unhydrolyzed ester was recovered iii S:S% yield, a o b s d -2.32', [ C Y ] * ~ D -22', 5.3% in chloroform. Negative rotation also was observed in acetone, ethyl acetate and in I- L I-I3 ethyl dl-a-acetoxypropionate. Since the L-ester C & a has negative r ~ t a t i o n ,[~a ] % -48', this indicates more rapid hydrolysis of the D-enantiomorph than the L from the racemate, and by a ratio of about Ao COzEt am 2.7 to 1. The product of hydrolysis, a-acetoxypropionic acid, was isolated from the hydrolysate 11-L in S5% yield aobsd 1.77', [ a I z 2 ~ 23.3', 3.8% of the enzyme a t which the @-aryl groups of the in chloroform; i t was characterized as the substi- natural substrates7 normally associate. The Ltuted ureide from 1,3-bis-(p-dimethylaminopheny1)- enantiomorph does this more effectively than the carbodiimide, m.p. 149-151°, [ a ] 2 2 D - 17'. Anal. D, but only with the D enantiomorph does this asCalcd. for C22H2804N4: C, 64.06; H, 6.84; N, sociation place the ester group near the nucleo13.58. Found: C, 63.94; H, 6.95; N, 13.62. philic site (n) which leads to hydrolysis. In the D-a-ACetOxyprOpiOniC acid has positive rotationj5 second, somewhat less favored mode of association, [ C ~ ] ~ * D 49', and this confirms the more rapid hy11-L,the acetoxyl group associates with the acyldrolysis of the D-enantiomorph by a ratio of about amido site (am) and this leads to hydrolysis of the 2.5 to 1. L enantiomorph, but not of the D. A similar analySince such experiments with racemates may lead sis of the inversion of antipodal reactivity in the to results different from those found in study of the hydrolysis of 1-keto-3-carbomethoxytetrahydroisoquinoline,s indicates that the benzamido moiety individual enantiomorphs6asbthe D( +) and L( -) ethyl a-acetoxypropionates were prepared and in that substrate may be associating with ahydrolyzed separately by a-chymotrypsin, 5 mg./ chymotrypsin a t the @-arylsite, the phenyl group ml. a t pH 7.2 in 0.1 N NaC1. The initial zero being dominant in effecting association, leading order rates of hydrolysis were determined a t several to a rotation of 120' and a situation similar to that concentrations, the first numbers in each set being of I-D,as has been proposed by Hein and Niemann." the concentration, the second the rate: L : 2.84 ( 7 ) H. Neurath and G. W. Schwert, C h e m . Rcu., 46, 69 (19,50). X -11,0.646 X lo-' mole/l./sec.; 4.30, (8) G. E. Hein, R. B. McGriff and C. S i e m a n n , J . A m . C h e m Soc., 0.800; 6.67, 0.969. D : 2.99 X M , 1.08 X 82, 1830 (19GO). (9) G. E. Hein and C. Niemann, Pvoc. N d . A c a d . Sci.,47, 1341 IO-' mole/l./sec.; 5.26, 1.27; 6.19, 1.76; 7.34, (1961). 2.38. The separate enantiomorphs also lead to SAUL G. COHEN OF CHEMISTRY JOHX CROSSLEY more rapid hydrolysis of the D compound, with the DEPARTMEXT EZRAKHEDOCRI ratio in rates approaching a value in excess of 2 BRASDEISUXIVERSITY ,54, MASSACHUSETTS ROBERTZAND with increasing concentration of substrate, con- WALTHAM RECEIVED JUSE 8, 1962 sistent with the results of the isolation experiments, which had been carried out on saturated solutions RADON FLUORIDE' of the racemate. The data indicate that the LSir: enantiomorph has a more favorable K , and a less Shortly after Bartlett? reported the reaction of favorable k 3 ; the absolute values of these kinetic parameters will require more extensive kinetic xenon with platinum hexafluoride, Claassen, Selig experiments. Enzymatic hydrolysis of the ethyl and Malm3 prepared xenon tetrafluoride by direct L-a-acetoxypropionate in a preparative experi- combination of the elements. l y e have studied ment led to L-a-acetoxypropionic acid, character- the reaction of trace amounts of radon with fluorine ized as its ureide derivative from 1,3-bis-(p- and found that radon forms a stable fluoride which dimethylaminopheny1)-carbodiimide, m.p. and is less volatile than XeF4. Gaseous radon (RnZZ2), collected from an aque( 3 ) S. G. Cohen, Y.Sprinzak and E. Khedouri, J . A m . Chem. Soc., ous solution of radium chloride, was passed through 83, 4225 (1961).
j-
+
+
+
(4) J. Kenyon. H. Phillips and H. G. Turley, J . Chem. SOL.,127, 399 1925). ( 5 ) C. If,Bean, J. Kenyon and H. Phillips, i b i d . , 303 (1936). (G) (a) P. Rona and R. Ammon, Biochem. Z., 181, 49 (1927) ; ( b ) P. Rona a n d E . Chain, ibid., 258, 4800 (1933).
(1) Based on work performed under t h e auspices of the U. S. Atomic Energy Commission. (2) N. Bartlett, Proc. Chem. SOL,218 (1962). (3) H. H. Claassen, H. Selig and J. G. Malm, J . A m . Cheni. SOL., 84, 3693 (1902).
Nov. 5 , 1902
C O M ~ I U N I C A T I O N STO T H E
a drying tube filled with magnesium perchlorate, then into a prefluorinated metal vacuum line, and frozen into a cold trap at - 195'. When the trap was warmed to -78', it was demonstrated that the radon moved readily under vacuum into other parts of the line which were cooled to - 195'. The position of the radon was determined by counting the 1.8 1Zev. gamma activity of the daughter Bi214. The Bi?14 (TI,? = 19.7 min.), following Pb2I4 (Tli2= 26 8 min.), grew into equilibrium with the radon wherever it appeared and decaycd where i t disappeared, within several hours. The measurements were made with a 400-channel pulse height analyzer and two sodium iodide scintillation detectors, shielded by lead bricks. In the first experiment in which the fluoride was prepared, a 5.1 microcurie amount of radon was condensed into a 3 cc. nickel reaction tube, fluorine was added to a pressure of 300 mm. and the mixture was heated to 400° for 30 minutes. The tube was cooled to - 78', and the excess fluorine was pumped off through a trap a t -193'. It was found that a niarked reduction in the volatility of the radon had occurred. The radon remained fixed in the tube, in a vacuum of 2 X 10F6mm., when warmed in slow stages to 150'. At 230 to 230°, part of the radon moved out of the tube and condensed in the exit valve, which was a t approximately 100'. To ascertain whether the behavior of the trace radon was the same in the presence of macroscopic amounts of carrier, a mixture oi xenon and radon next was fluorinated. X new 7-cc. nickel vessel, provided with a capillary inlet tube, was charged with 110 mm. partial pressure of xenon 87 microcuries of radon, and 1200 mm. partial pressure of fluorine and heated to 400' for 23 minutes. The vessel was cooled to -78' while the excess fluorine was pumped off, then was warmed to 50° to allow the xenon fluoride formed to sublime under vacuum into a trap a t - 195'. The less volatile radon fluoride remained behind. SVhen the vessel was heated gradually to 250', the radon fluoride moved into the cooler section of capillary tubing. lire have found that radon alone, when heated to 400' in a nickel vessel, shows no evidence of reaction with the walls. byhen the vessel is cooled to room temperature or to -T8', the radon can be In this distilled as usual into a trap at -193'. respect our results are in agreement with the very early work of Rutherford and Soddy4 and of Ramsay and Soddp,j who demonstrated that radon does not react with metals and a large number of other reagents. The composition of the radon fluoride has not yet been determined. Attempts are being made to introduce samples of the fluoride into a time-offlight mass spectrometer for stoichiometric analysis. The fluoride can be reduced with hydrogen to quantitatively recover elemental radon. At 200' the compound appears to be stable in hydrogen, but a t 300" and a hydrogen pressure of 800 mm. i t is completely reduced within 15 minutes. The tracer quantities of radon fluoride prepared thus far have shown no evidence of radiation de(4) E. Rutherford and F Soddy, Phti. M a g , [6] 4, 580 (1902).
( 5 ) W. R a m s a p and F Soddy, Pioc Roy Soc (1903).
(London), 74, 204
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composition from alpha particle emission. Samples have been stored for sex era1 days a t room tcmperature without evolving any measurable amounts of elemental radon. The compound has been present in such dilute form on the inner surfaces of the container vessels that most of the energy of the alpha particles has been absorbed by the metal walls rather than by the compound. The radiation decomposition may be significant when larger amounts of the compound are prepared. IS-e wiqh to express our deep appreciation to Dr. \I-.11. Manning, Director of the Chemistry Division, for encouraging us to undertake this research and for his continued enthusiastic support. IVe also wish t o thank Dr. J. E. Cindler for supplying the radium chloride solution. CHEMISTRY DIVISIOS
AkRGOXSE SATIONAL LABORATORY
PAULI
