A hexacarboxylic acid, obtained by the degradation of vitamin B
12
by Cannon, Johnson & Todd in 1953 has been examined by X-ray analytical methods. These lead to a solution of both the crystal and chemical structure of the acid. The crystals are orthorhombic,
a
= 24·58,
b
= 15·52,
c
= 13·32 Å, space group,
P
2
1
2
1
2
1
,
n
= 4. The asymmetric unit is found to consist essentially of one molecule of hexacarboxylic acid, C
46
H
58
O
13
N
6
. CoCl, two molecules of water and one of acetone. The hexacarboxylic acid molecule has a central cobalt atom in approximately octahedral co-ordination attached to one cyanide group, one chlorine atom and four nitrogen atoms of the corrin nucleus. The nucleus itself is substituted by acetic and propionic acid groups, a lactam ring and a number of methyl groups. The position of the cobalt atom in the crystal structure was first found from Patterson projections and the remaining atomic positions then derived from a series of calculated approximations to the three-dimensional electron density distribution. For these calculations, phases were derived from structure factors calculated on gradually increasing numbers of selected atomic positions from the stage of
ρ
1, where only the cobalt atom sites were known, to
ρ
10 where 73 atoms, not counting hydrogen, had been placed. The process was not quite straightforward; particular difficulty was experienced in finding the positions of the atoms of one side-chain which may be affected by disorder. The parameters of the atoms have been refined by two cycles of least-squares calculations. A number of observations were made in the course of the analysis which bear on the further use of non-centrosymmetric Fourier syntheses in the study of complex structures. An appendix by A. Vos deals with intensity anomalies observed on the X-ray photographs of the hexacarboxylic acid which provide evidence of its absolute configuration. An appendix by K. N. Trueblood summarizes various aspects of the analysis of the hexacarboxylic acid, seen as a whole.
Threonine 57 is required for the post-translational activation ofEscherichia coliaspartate α-decarboxylase