Oct. 20, 1957
55S1
COXMUNICATIONS TO THE EDITOR
Since the product is a mono CoA ester of HMG, the problem arises whether the thioester bond of Xc COX or of AcAc CoA is hydrolyzed during the condensation reaction. This problem was resolved in the following manner: H M G CoA enzymatically formed by the condensing enzyme from acetyl-l-C1* COX and AcAcCoA was mixed with carrier H M G CoA9 and cleaved by the H M G Co-1 cleavage enzyme of Bachhawat, et a1.,I0 to acetoacetic acid and -4cCo=l according t o reaction
T H E SCAVENGER EFFECT I N SOLID ETHYL BROMIDE
Sir:
During an investigation of the Szilard-Chalmers effect in solid ethyl bromide, i t was found possible to produce a solid phase containing variable proportions of elementary bromine. This technique could prove useful in the study of the rather cumplicated reactions following (n,7)processes in solid organic halides. The ethyl bromide and bromine used were puri3 -. fied as described elsewhere2 and mixtures of these CHiCOHCHZCOOH --+ CHaCOCH2COOH f were frozen using liquid nitrogen. Twenty-five I CH,CO--S-CO*X cc. of the mixture was contained in a narrow necked CHsCO-S-Co.2 (2) glass vessel into which a quartz tube was fitted. Decarboxylation of the acetoacetic acid to acetone Two methods of freezing were used. I n the first, and CO? showed the radioactivity t o reside solely liquid nitrogen was poured into the central quartz in the CO,. These results show that the free car- tube and complete freezing occurred in about one boxyl group of HRIGCoA contained C14. Since hour; using the second method, the freezing time the C14was originally in the carboxyl position of ace- was reduced to 5 minutes by also immersing the tyl-l-C14Co-4 i t may be concluded that the CoA set vessel in liquid nitrogen. I n the latter case, care free during the condensation reaction came from was taken t o transform any transparent glass proAcCoA according to reaction l . l l duced on the walls of the vessel into opaque crysSince H M G CoA is the product of the condensa- talsla b y leaving i t at room temperature for a tion reaction, we would redesignate the enzyme as minute before irradiation, The sealed vessel conthe HAIG COX condensing enzyme. This enzyme taining the frozen mixture was placed in a Dewar resembles the citrate condensing enzyme,12 in that fitting in a paraffin moderator castle. A 500 mc. (1) ilc CoA condenses with the carbonyl group of polonium-beryllium source was lowered into the the other substrate, and (2) a net hydrolysis of ilc center of the quartz tube which was kept full of COB results. The enzyme appears t o be specific liquid nitrogen during irradiation (50 minutes). for thioesters of COX, since the corresponding After irradiation, the solid was left to melt a t room thioesters of pantetheine and glutathione were in- temperature and the liquid was extracted with an active. Efforts to reverse the condensation have aqueous sodium sulfite solution. The activities been unsuccessful thus far. due to BrS0 (18 min. half-life) in the unextracted At the present time two ways are known in and extracted specimen were determined, the acwhich H X G Co.1 can be formed, the first via the tivity of the other bromine isotopes being negligicondensation reaction described above, and the bie.3 other the CO, fixation reaction of Bachhawat The results obtained are shown in Fig. 1 curve et aZ.I3 The obvious relationship between HIJIG (a), where the retention? has been plotted against COX and mevalonic acid, a compound efficiently the molar fraction of the elementary bromine presconverted to cholesterol, l 4 is under investigation. ent. The speed of freezing did not influence the DEPT.OF RIOCHEMISTRP retention of pure ethyl bromide. When bromine SCHOOL OF MEDICINE HARRYRCDYEY15 was present, an appreciable drop in retention was VESTE ERN RESERVE UXIVERSITY only observed when the faster freezing method was CLEVELaSD 6, OHIO JAMES J. FERGUSON, JR.'~ RECEIVED AUGUST5, 1957 used (see Table I). (9) Prepared from HMG anhydride by t h e method of E. J . Simon and D. Shemin, THISJOURGAL, 75, 2520 (1953). ( l o ) B. K. Bachhawat, W.G . Robinson and M. J. Coon, J . Bid. Chenz., 219, 530 (19,5c). \Ye are grateful t o Dr. M. J. Coon for a sample of the H X G CoA cleavage enzyme. (11) This result would also h e obtained if a n intramolecular transfer of CoA occurred from one carboxyl group of HMG CoA t o the other. This possibility appears t o he eliminated b y experiments (H. Rudney and T. G. Farkas, Fedevaiioa Proc., 14, 757 (19.53); RI. J. Coon, J . Biol. Chem., 187, 71 (1950); G. W, E. Plaut and H. -4. Lardy, ibid., 186, 705 (1950)) which demonstrated t h a t when C ' 4 0 ~was fixed into acetoacetate by liver homogenates, assuredly aia H M G CoA formation and subsequent cleavage t o acetoacetate and Ac CoA, isotope appeared almost excluiively in t h e carboxyl group of acetoacetate. If intramolecular C o h transfer occurred between the carboxyls of H M G CoA. then after cleavage the thiolase present in t h e homogenates would have randomized t h e label between t h e carboxyl and carbonyl carbons of acetoacetate. (12) J. R . Stern. S. Ochoa and F. Lynen, J . Bioi. C h e m . , 198, 313 (1952) (13) B. K. Bachhawat, \&'. G. Robinson and M. J. Coon, ibid., 216, 727 (1955). (14) P. A. Tavorniina, H. >.I.Gibbs and J. W Huff, THISJ O U R N A L , 78, 4498 (195(i). (15) Scholar in Cancer Research of t h e American Cancer Society. (16) Post Doctoral Research Fellow, h-ational Institute of Arthritis and Metabolic Diseases.
TABLE I Mol fract. of Br
Freezing time
Retention of BrBo, %
1 hour 83.0 f 2 . 6 5 min. 82lf26 5 min. 82 0 2 5' 1 hour 782f23 5 rnin. 69 5 + 2 2 This specimen was allowed to melt in the presence of 3.8 cc. of a solution of bromine (0.38 molar fraction) in ethyl bromide. 0 0 0 5,7.10- * 5,7.
+
(1) (a) F. S.Rowland and W. F. Libby, J . C h e i n . Phys., 2 1 , 1495 (1953); (b) L . Friedman and W. F . Libby, i b i d . , 17, 647 (1949); (c) S.Goldhaber, R. S. H. Chiang and J. E. Willard, THISJ O U R N A L , 73, 2271 (1951); (d) G. Levey and J . E. Willard, i b i d . , 74, 6161 (1952). ( 2 ) Miriam Milman and P. F. D. Shaw, J . Chem. SOC.,1303 (1957). (3) Details of precautions taken, counting technique and corrections applied have been previously reported 2 (4) T h e retention is defined as the fraction of t h e total activity present in organic combinations.
5582
COBIMUNICATIOXS TO THE EDITOR
Vol. 79
is represented: namely, the atomic arrangement based upon a fifty-atom tetragonal unit earlier described.2 That there exists, nevertheless, a second. and probably more cornmo~ilyobtained niodificntion of boron has seemed certain from published powder diffraction data. For example, the interplanar spacings listed4 for prominent powder lines given by a sample of better than 99% purity (unspecified method of preparation) are wholly incompatible with our tetragonal cell. Recently we 40 have obtained from independent sources two preparations of boron briefly characterized as follows: the stated purities are 99.4 and 99.570 B but the preparative chemical methods are withheld; both are aggregates of small single crystals resulting from crystallization of the melt, and both afford specimens suitable for single crystal study; and they give a common powder diagram which in0.2 0.4 0.6 08 1.0 cludes the lines listed by Godfrey and I I - a r r e r ~ . ~ 1I.F. Dr. S. Geller of the Bell Telephone Laboratories Fig. 1.-Retention of solid ethyl bromide (curve “a”) and provided us with Preparation I. Amorphous liquid ethyl bromide (curve “ b ” ) as a function of the niolar boron, supplied by Cooper Netallurgical Associates, fraction of bromine. was melted partially in a “Heliarc” furnace and alThe purpose of experiment (a) was to determine lowed to crystallize in a helium atmosphere by E. whether free organic radicals were trapped in the Corenzwit of the Bell Laboratories. Preparation solid. The constancy of the retention indicates 11, received within the fortnight, came to us from either t h a t such radicals do not exist or, t h a t they the TJiiited States Borax & Chemical Corporation react before mixing with the scavenger solution. through the good offices of Professor X.IT.LauThe reproducibility of the results obtained using bengayer. the second method of freezing bromine-ethyl broOscillation, Weissenberg, and precession photomide mixtures was checked by repeating a number graphs of single crystals-from Preparation I Desof experiments a t the same bromine concentration. tablish a rhombohedral lattice having a = 10.12A, cy S o variations beyond statistical limits were ob- G3’2S‘ ; the associated triply primitive hexagonal served, even though the freezing time varied from cell has A = 10.95, C = 23.73 A,accurate to 0.2%. 3 to 5 minutes. These preliminary results show that the drop in The experimental density, 2.35 =k 0.01 g. cc., is retention caused by the addition of small quantities equally consistent with 107 or 10s atoms within of bromine is less sharp than that observed in the rhombohedral unit. The diffraction symliquid ethyl bromide (Fig. Ib). T o clarify the metry and lack of glide plane vanishings indicate nature of the processes involved we are now carry- R s n , R32, or RXm as the space group. Spectroing out experiments t o identify the radioactive metrically measured intensity data (using both products obtained and to determine the compara- Cu Koi and X o Kcr radiations) have been obtained tive behavior of the bromine isotopes produced by for two zones, Le., with the hexagonal A and rhombohedral a as zone axes. The statistical distri(n, y) processes. bution of intensities is centrosymmetric5 for the The author wishes to thank Professor H. Halban (Paris) for encouragement and support, t o Pro- first zone, hypercentric6 for the second, thus supfessor J. JVillard (Wisconsin) and Dr. P. F. D. Shaw porting the unique choice of R j m as the space group. 1X-e note that boron carbide’ also crystol(Oxford) for helpful discussions. lizes in R b , with a = 5.19 A, N = 63’1s’ SO that ECOLE SORMALE SUP~RIEURE LABORaTOIRE D E PHYSIQUE MIRIAMMILMAN all lattice translations are approximately half the 24, RUE LHOMOND, PARISYo, FRAKCE corresponding values in rhombohedral boron. RECEIVED AUGUST8, 1957 However, there is no suggestion in the boron data of a pseudo-unit comparable in volunle or obviously related in structure t o the boron carbide unit. XIRHOMBOHEDRAL ELEMENTAL BORON though undoubtedly belonging t o a more complex Sir: structural type, our rhombohedral boron affords The simultaneous occurrence of two widely dis- intensity data showing little or no evidence for the similar external forms for single crystals of pure presence of the large and variable concentrations of short-range defects which seem to characterize3 boron growing1 on a hot filament was for a time to reflect true structural polymorphism, all actual specimens of tetragonal boron as grown ($) T ?;. Godfrey and I 3 E . TT.arren, J . Chciu. Phys., 18, 1121 but comprehensive diffraction studies3 show that, a t least in the work cited,I a single structural type (19501 ( 5 ) E. R . Howells, D. C. Phillips and D. Rogers, Actn CYSSL.,3, ?io ( m o ) . ( I ) A . 1%’. Laubengayer, D. T . Hurd, A. E. NeFkirk and J. 1,.
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Hoard, THIS JOURNAL, 6 6 , 1924 (1943). (2) J . L. Hoard, S. Geller and R. E. Hughes, i b i d . , 73, 1892 (1951). (3) J. L. Hoard, R. E. Hughes and D. E. Sands, to be submitted to
THISJOURNAL.
(0) H . Lipson and 11,IT,Woolfson, i b i d . , 6, 080 (lQ.52). (7) H . K. Clark and J. L, Hoard, THIS J O U R X A L , 6 6 , 2115 (1943); G. S Zhdanov and N. G Sevast’yanov, Compt. rend. acad. sci. CT.R. S .SI 32, 432 (1941).
