The reaction of nitric oxide (NO) with metallothionein (MT) has been investigated at neutral pH under strictly anaerobic conditions. It is observed that NO mediates zinc release from MT by destroying zinc-sulphur clusters, but that it does not by itself S-nitrosylate MT in contrast to common belief. Zinc release and loss of thiolate groups under anaerobic conditions is found to be much slower than under aerobic conditions. The observed percentage loss of Zn(2+) and thiolate groups after 3 h of NO treatment are 62 and 39%, respectively. The reaction of NO with cysteine is reinvestigated and it is found that cysteine is quantitatively converted to cystine after 5 min of NO treatment at pH 7.3. At lower pH, a much lower rate of conversion is observed confirming the base-catalysed nature of the reaction of NO with thiols. On the basis of these results, a reaction mechanism involving electrophilic attack of NO on thiolate groups and subsequent formation of a nitrogen-centred radical, MTSN(. )OH, as intermediate is proposed for the reaction of NO with MT that leads to zinc release.

Heptyl radicals formed by y-irradiation of n-C7Hl6, both in neat form and with various concentrations of n-CsDls added before irradiation, are investigated by ESR spectroscopy. It is observed that the presence of deuterated octane in protiated heptane results in a substantial increase in the relative contribution of protiated 1 -heptyl radicals to the radical spectrum. Their formation is attributed to the proton-transfer reaction (n-C7H16*+ + n-CsDl8 -1-C7H15* + n-CsDlgH+) which, in accordance with an earlier and experimentally verified postulate stating that a strict relation exists between the radical site in alkyl radicals formed by proton transfer from alkane radical cations to alkane molecules and the structure of the semioccupied molecular orbital of the parent cation, is expected to yield l-heptyl radicals selectively. The increase in the relative importance of l-heptyl vs other isomeric heptyl radicals as a result of the presence of octane in heptane is confirmed by analysis of the tetradecanes formed by irradiation of heptane/ l-chlorohexane( 1 mol %) and heptane/lchlorohexane( 1 mol%)/octane(9 mol 7%) systems. As to the acceptor site, a study is made by gas chromatography of the yield and isomeric composition of chlorooctanes formed by y-irradiation of n-C.1Hl6, containing 1 mol 5 % l-C6H1&1 and various concentrations of n-CsHls. Formation of chlorooctanes in such systems is largely due to neutralization of protonated octane molecules by chloride ions (CsH19+ + C1--CsH17Cl+ H2). The isomeric composition yields direct information on the proton-acceptor site in the octane molecule. It is observed that mainly 2-chlorooctane is formed, with much smaller yields of 1-and 3-chlorooctane; the formation of 4-chlorooctane is still less pronounced. The selective formation of 2-chlorooctane, which can be rationalized on the basis of structural considerations and of the fact that the proton affinity for primary C-H protonation is smaller than for secondary C-H protonation, confirms that proton transfer to octane molecules takes place from the chain-end position in extended heptane radical cations and indicates that octane solute molecules are packed quite regularly and with conservation of the layer structure in the heptane crystal. Various alternative mechanisms for chlorooctane formation are discussed and are are discarded with respect to the selective formation of 2-chlorooctane on various grounds. The results obtained further support the postulate stating that a strict relation exists between the proton-donor site in the proton transfer from alkane radical cations to alkane molecules and the structure of the semi-occupied molecular orbital of the parent cation.

A study is made of intrinsic acceptor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The research is conducted by studying the relative degree of protonation at the various C-H bonds in undecane in irradiated CCl 3 F/undecane systems at cryogenic temperatures. It is observed that protonation occurs preferentially at the penultimate position, the difference between the other secondary C-H bonds being marginal; very little primary C-H protonation is observed under the experimental conditions.

The reaction of nitric oxide (NO) with metallothionein (MT) has been investigated at neutral pH under strictly anaerobic conditions. It is observed that NO mediates zinc release from MT by destroying zinc-sulphur clusters, but that it does not by itself Snitrosylate MT in contrast to common belief. Zinc release and loss of thiolate groups under anaerobic conditions is found to be much slower than under aerobic conditions. The observed percentage loss of Zn# + and thiolate groups after 3 h of NO treatment are 62 and 39 %, respectively. The reaction of NO with cysteine is reinvestigated and it is found that cysteine is quan-

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