400 



VITAMIN Bi2 



B. CHEMICAL AND PHYSICAL PROPERTIES 



Vitamin B12 is a dark-red crystalline compound which darkens to black 

 at about 212° and does not melt up to 320°.^' ^ The refractive indices of 

 the crystals are L619 (a), L649 (/3), and L659 (7).^ It is optically active; 

 the rotations reported by two laboratories are [a] eses = — 59 ± 9°/^ and 

 [«] 6438 = — 1 10° ± 10 %J The absorption spectrum of vitamin B12 in aqueous 

 solution shows the characteristic bands: 278 m^i (■E'l^m. =" US); 361 m^ 

 (EVcm. = 204); and 550 mju {E\'^^rn. = 63) ^^ (Fig. 1). The absorption spec- 

 tmm is not markedly affected by small changes in pH. Magnetic suscepti- 

 biUty measurements of vitamin Bi2 indicated that it is diamagnetic and that 

 it is a trivalent cobalt complex with octahedral d-sp^ bonding.^o The infra- 

 red absorption spectrum has been published.^ In neutral solution, vitamin 



ABSORPTION SPECTRA 

 IN WATER of: 



VITAMIN B|j 



VITAMIN Biio 



VITAMIN B,jo 



(FROM S GRISEUS) 



380 420 460 



WAVELENGTH IN M/u. 



Fig. 1. Absorption spectra of vitamin B12 compounds. Reproduced with permis- 

 sion from /. Ain. Chem. Soc. 73, 355 (1951). 



B12 shows a well-defined polarographic step with a half-wave potential at 

 — 1.53 V. (against the saturated calomel electrode) surmounted by a char- 

 acteristic maximum. '^ 



Early investigations on vitamin B12 revealed the presence of cobalt and 

 phosphorus,^- 2^- -^ which were later shown to be present in the ratio of 1:1 

 in the vitamin B12 molecule. ^^ Ebullioscopic determination of molecular 

 weight indicated a value of 1490 ± 150.^^ This value is in good agreement 

 with the minimum molecular weight of about 1300 which is the value de- 

 termined by calculation based on a cobalt content of about 4.5%. Esti- 



" N. G. Brink, D. E. Wolf, E. Kaczka, E. L. Rickes. F. R. Koniuszy, T. R. Wood 



and K. Folkers, /. Am. Chem. Soc. 71, 1854 (1949). 

 20 J. C. Wallmann, B. B. Cunningham, and M. Calvin, Science 113, 55 (1951); H. 



Diehl, R. W. Vander Haar, and R. R. Sealock, /. Am. Chem. Soc. 72, 5312 (1950); 



F. Grun and R. Menasse, Experientia 6, 263 (1950). 

 " E. L. Smith, Nature 162, 144 (1948). 

 " E. L. Rickes, N. G. Brink, F. R. Koniuszy, T. R. Wood, and K. Folkers, Science 



108, 134 (1948). 



