Urea and plasma protein differ in natural 15N abundance up to 10%. The origin of this difference is the branched nitrogen metabolism in the liver. One main branch is the protein synthesis pathway, the other the urea synthesis pathway. By this branching 15N of precursor amino acids is depleted in urea while it is enriched in protein. With the 15N abundance of precursor amino acids, which may be taken from jejunum tissue, utilization of amino acids in liver metabolism can be calculated from isotope discrimination in either pathway. This was investigated by feeding different proteins to rats. When feeding high quality protein (whey protein) utilization of amino acids in liver metabolism at requirement intake was better than at zero protein intake (> 85% vs. 70%). From this we conclude that the pattern of amino acids available from the metabolic pool at zero protein intake is characterized by an imbalance. This endogenous imbalance can be complemented by exogenous dietary amino acids so that nitrogen excretion may even be smaller than the so-called "obligatory" losses of intakes not exceeding requirement. Thus, the quality of dietary protein is reflected not only by N balance. It also may be quantified by analysis of isotope discrimination in nitrogen metabolism of the liver. In addition, the quality of amino acid pattern available from the metabolic pool is indicated by this method.

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The cyclization of dipeptide esters of α-, β-, γ-, and δ-amino acids can be achieved by using Ni , Pd , or Cu templates. The structure of one of the complexes (1) obtained, which was determined by X-ray crystallography, reveals that the anions form layers and are linked to water molecules by hydrogen bonds.

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trans‐Bis(dipeptide ester)palladium 1–6 complexes with a deprotonated amide group were obtained by dehydrochlorination of trans‐Cl2Pd(dipeptide ester)2 or by direct reaction of Na2PdCl4 with dipeptide ester in the presence of sodium methoxide. The structure of Pd(NH2CH2CONCH2CH2CO2CH3)2 (3) was determined by X‐ray diffraction. Addition of NaOCH3 to Pd[NH2C(H)(R)NCH2CH2CO2CH3]2 [R = CH(CH3)24, CH2Ph 5] results in the formation of the corresponding complexes 7, 8 with cyclic tetrapeptides which also can be directly synthesized from the dipeptide ester and Na2PdCl4.