Monodeuterated water

The participation of a cysteine thiol in the abstraction of hydrogen from ribose C-3 coenzyme B12, which is not a feature of any other of the reactions of Table 1, leads to exchange of H between this thiol group, the 5 -methylene group of coenzyme Bi2 and water (22). It was also shown that ribonucleotide reductase catalyzes the conversion of adenosylcobalamin labeled with one deuterium atom at C-5 (initial R/S ratio = 3 1) to monodeuterated coenzyme with R/S ratio = 1. This result shows that the cobalt-carbon 0-bond is reversibly cleaved to a 5 -deoxyadenosyl radical, which permits rotation about the C-47C-5 0-bond. 

A change to a less basic solvent can influence the stereochemistry of unimolecular elimination. In nitrobenzene or acetic acid, erythro-3-deutcro-2-butyl tosylate yields deuterium-free m-2-butene and monodeuterated /ra 5-2-butene, suggestive of a jy/i-elimination (104). In more basic solvents, such as water, acetamide and 80% aqueous ethanol, the labelling of olefins is reversed and a/i/i-elimination is followed. The change in stereochemistry is attributed to the relative basicities of the tosylate anion and solvent and hence the tendencies of these species to accept a beta proton from the carbonium ion (105). 

The best clue to why this is so comes from the strange coincidence of the chemical shifts of the OH, NH, and SH protons in these molecules. They are all the same within experimental error 4.90 ppm for glycine, 4.80 ppm for the aminothiol, and 4.84 ppm for EDTA. In fact all correspond to the same species HOD, or monodeuterated water. Exchange between XH (where X=0, N, or S) protons is extremely fast, and the solvent, D2O, supplies a vast excess of exchangeable deuteriums. These immediately replace all the OH, NH, and SH protons in the molecules with D, forming HOD in the process. Recall that we do not see signals for deuterium atoms (that s why deuterated solvents are used). They have their own spectra at a different frequency. 

If double-deuterated pyridoxamine is used for the same reaction in water, half of the deuterium is lost and the other monodeuterated pyridoxamine enantiomer is obtained. Furthermore, if an a-deuterated L-amino acid is used, the deuterium is transferred stereospecifically to give only one of the two possible stereoisomers of monodeuterated pyridoxamine. 

Monodeuterated water HDO has an abundance in natural water of approximately 600 ppm. One of the methods used in early attempts to separate the two isotopes was fractional distillation, for which the separation factor at 1 atm was found to be 1.026. Although this procedure was ultimately superseded by the more efficient chemical-exchange processes for larger-scale production of heavy water, distillation remains the separation method of choice for upgrading small amounts of heavy water that have been contaminated by atmospheric water vapor. Distillation is in this case carried out at reduced pressure to take advantage of the higher separation factor. Suppose we wish to carry out the distillation of H2O-HDO at ambient temperatures, i.e., at subatmospheric... 

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