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breaking is irreversibles or, in the case of a single transition-state process, bond breaking has not occurred to an extent which may be detected by this means. I n contrast t o the extremely rapid reactions of BloHlo-*, B12H19-* does not react with aryldiazonium ions in acetonitrile. Acknowledgments.-The authors wish to thank the Advanced Research Projects Agency through the Army Research Office (Durham) and the Alfred P. Sloan Foundation for generous support of this work. The authors also wish to thank Mr. Eugene Pier of Varian Associates for the 32 Rlc./'sec. llB n.m.r. spectra. (8) I, Melander, "Isotope Effects o n Reaction R a t e s , " T h e Ronald Press C o , New Y o r k , N . Y . , 1960, p 103. (9) Alfred P . Sloan Research Fellow.
DEPARTMENT OF CHEMISTRY M . FREDERICK HAWTHORNEg THEUSIVERSITVOF CALIFORSIA FREDRIC P. OLSES RIVERSIDE, CALIFORNIA RECEIVED AVCUST 14, 1964
Exchangeable Hydrogen in Chlorophyll and the Path of Hydrogen in Photosynthesis'
Sir : Whether labile hydrogen atoms in chlorophyll are involved in photosynthesis is an important question that has attracted much attention but which has not received an unambiguous answer.2 The recent demonstration by the method of proton magnetic resonance that chlorophylls a and 6 in solution have readily exchangeable hydrogen atoms a t the C-10 and 6methine positions3 has aroused interest in the possibility t h a t an exchange of hydrogen between water and chlorophyll, particularly a t the 6 position, occurs during photosynthesis. The availability of fully deuterated photosynthetic organisms4 now permits a new approach to these questions. Hydrogen incorporated into the chlorophyll of fully deuterated organisms either by chemical exchange or by metabolism can be readily detected by proton magnetic resonance and, because the proton resonances in chlorophyll have been assigned,%the location of the incorporated hydrogen can be determined. In a typical experiment, fully deuterated S c e n e d e s m u s ob1iqiiii.s grown in 139.7% D 2 0 6 with C 0 2 as the sole carbon source was harvested by centrifugation and immediately resuspended in fresh HzO medium. The resuspended cells were then allowed to continue photosynthesis, receiving 57c COz in nitrogen and a light intensity of 1100 ft.-candles. After 16 hr., the (11 Based on work performed under t h e auspices o f t h e I' S Atomic Enei-gy Commissinn ( 2 ) B Coleman a n d W Vishniac, .Vali A c a d S c i -.Val1 Res. Council, Publ , No. 1146, 213 (196:3), W. Yishniac a n d I . A Rose, . V a l u v ~ 182, , 1089 (10381, W. \-ishniac, "Chemical Participation o f Chlorophyll in Photosynthesis." in "Cornpal-ative Biochemistry of Photoreactive Systems." Si B. .411en, E d , Academic PI-ess. New Yiil-k, N Y , 1960, p p 377-386, J W. Weigl a n d I
