Further heterolytic cleavage of the C(4)-O produces a terminal carbonyl group (i.e., an aldehyde), from which the CO is eliminated to form the product ion K1 (m/z 171).
The heterolytic cleavage of the C(4)-O bond of the resulting product ion B3 (m/z 263) produces the intermediate ion B4 (pathway a, Scheme 4).
The heterolytic cleavage of the O-H bond results in forming the corresponding anions.
The natural charge distributions of all anions, formed by heterolytic cleavage of O-H bonds of the DHBAs, are presented in Figure 4.
This scheme conforms to the heterolytic cleavage of the enzyme-bound [H.sub.2], i.e., one of the enzyme-bound [H.sub.2] has the rate constant ([k.sub.a]) for the isotope exchange reaction 10 times as that ([k.sub.b]) of the other, but the mode of heterolysis is not as simple as what was initially suggested as in reaction 3.
These data may be considered proof for the heterolytic cleavage of dihydrogen, but strictly speaking, [k.sub.a]/[k.sub.b] ratio can be calculated only when [v.sub.DD]/[v.sub.HD] and [v.sub.p-o]/[v.sub.ex] ratios (from p[H.sub.2]/ [D.sub.2]O) were measured simultaneously.
Heterolytic cleavage of the bound [H.sub.2] produces Ni-R form, in which the hydride is believed to bind at the Ni-Fe bridging site, (77,98-100) and [H.sup.+] on the S atom of [Cys.sup.*] ([Cys.sup.*] is Cys546 for DvM enzyme).