Hill H A, Pratt J M, Thorp R G, Ward B, Williams R J
Biochem J. 1970 Nov;120(2):263-9. doi: 10.1042/bj1200263.
The following equilibrium constants (given as logK in units of m(-1)) were determined for the substitution of co-ordinated H(2)O in aquocobalamin by glycine (bound through N) 5.8, cysteine (bound through S) 6.0 or 8.3, depending on the value chosen for the pK of the thiol group, and phenolate 2.9. The spectrum of the phenolate cobalamin shows an additional intense absorption band at 468nm with a molar extinction coefficient of 1.1x10(4), which is assigned to a charge transfer from the phenolate to the cobalt ion. Equilibrium constants have also been determined for the equilibria between adenylcobamide cyanide and CN(-), HO(-) and H(+), which show that the adenine is more easily displaced by CN(-) and HO(-) than is 5,6-dimethylbenziminazole in vitamin B(12), but can be protonated by acid while still remaining co-ordinated to the cobalt. It is shown that in the binding of corrinoids to proteins and polypeptides the formation of hydrogen bonds is far more important than co-ordination by the metal.
测定了以下平衡常数（以m(-1)为单位的logK），用于甘氨酸（通过N键合）、半胱氨酸（通过S键合，取决于为硫醇基团的pK选择的值，分别为6.0或8.3）和酚盐（2.9）取代水合钴胺素中配位的H(2)O。酚盐钴胺素的光谱在468nm处显示出一个额外的强吸收带，摩尔消光系数为1.1x10(4)，这归因于从酚盐到钴离子的电荷转移。还测定了腺苷钴胺素氰化物与CN(-)、HO(-)和H(+)之间平衡的平衡常数，结果表明，与维生素B(12)中的5,6-二甲基苯并咪唑相比，腺嘌呤更容易被CN(-)和HO(-)取代，但在仍与钴配位时可被酸质子化。结果表明，在类咕啉与蛋白质和多肽的结合中，氢键的形成比金属配位重要得多。
