Although a-benzyloxycarbonylamino-acids normally do not racemise when they are condensed with amino-esters to form peptides, benzyloxycarbonyldipeptides may do so when they are coupled by some of the procedures in current use, which therefore have limited applicability in peptide synthesis. The racemisation occurring when acetyl-L-leucine is condensed with glycine ethyl ester by various methods has now been studied. Coupling through the acid azide gave a product of high optical activity, from which no racemate could be isolated ; some racemisation was observed when dicyclohexylcarbodi-imide was the condensing agent (with dichloromethane or tetrahydrofuran as solvent). The racemisation found with the use of diethyl phosphorochloridite or tetraethyl pyrophosphite varied considerably with the conditions. The carbonic mixed anhydride and the " phosphorazo " method caused extensive racemisation. Methods which have been postulated to involve activation of the amino-component may therefore result in racemisation in the carboxylic component. The use of amino-ester hydrochloride with an equivalent of tertiary amine, in place of the free aminoester, led to increased racemisation in several cases.

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Mercury residues were found in eggs of hens fed on a ration of wheat treated with methylmercury dicyandiamide.* The mercury content rose steadily from zero to 8 p.p.m. over a period of 4 weeks and limited evidence indicates a continued rise for another 4 weeks.Over 75% of the mercury is found in the albumen of the egg and was first detected 3 days after starting to feed treated grain. The residues of mercury in the thigh muscle of the hens were of the order of 4 p.p.m. with slightly more in the breast muscle ; the livers contained 8-24 p,p.m. ; the kidneys approximately 8 p.p.m. after 8 weeks. Tissue from control birds contained little or no mercury.Laying and general health of the birds remained normal and autopsy did not reveal any characteristics attributable to mercury poisoning.The analytical method gave recoveries of 8 0 -1 o o~~.

A rapid method for determining mercury residues in potatoes, grain and animal tissues is presented. Digestion with nitric, sulphuric and perchloric acids is used in place of the oxidation with nitric and sulphuric acids and hydrogen peroxide, which is the method recommended by the Joint Mercury Residues Panel. Recovery of added organomercurial is about 90 per cent. Six analyses can be carried out in 1 day by one worker.

Analyses of samples of rice from United Kingdom importers and from retail sources show that levels of mercury are often negligible, although they may rise to 0.01 ppm and occasionally to 0.015 ppm.

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