Homodimers Subject

The 1 1 mixture of 24 and 20 (or 25 and 20) in aqueous media (0.5 mM each) was subjected to the same conditions used for forming 19-20. MALDl results showed that neither 24 nor 25 selectively cross-linked with 20. In sharp contrast to the exclusive formation of 19-20 or 21-22, multiple products were detected from the 1 1 mixture of 24 and 20, among which 24, formed from the self-cyclization of 24, and 20-20 from the homodimerization of 20, represented the major products (Fig. 9.16b). Product 24-20, from the cross-linking of 24 and 20, only appeared as a minor peak. Similarly, when mixed together, 25 and 20 could not be cross-linked exclusively into 25-20 (Fig. 9.16c). The observed distribution of products, that is, the self-cyclized 25, heterodimer 25-20, and homodimer 20-20, can be regarded as being a statistical one. 

The results obtained by Warwel and Winkelmiiller provided the groundwork for the development of product-selective CM. In these reactions, the observed product selectivity was attributed to the diminished CM reactivity of stilbene relative to styrene. Indeed, subjecting an isolated homodimer to CM conditions constitutes a general strategy... 

In homozygous forms of the dysfibrinogenemias, the mutations occur in all fibrinogen molecules, so only abnormal homodimers will be present. However, homozygous mutations are very rare, so most dysfibrinogenemias are heterozygous. As a consequence, fibrinogen molecules in most subjects consist of various proportions of mutant homodimers, normal homodimers, and heterodimers. 

Control experiments show that neither the carbonylcobalt cation nor the anion undergoes prior ligand substitution by the added PBu3 under the reaction conditions. Moreover, the homodimer Co2(CO)6(PPh3)2 and the cross-dimer Co2(CO)6(PPh3)(PBu3) are not subject to subsequent ligand substitution by tributylphosphine to yield the homodimer in Eq. (33). 

The architecture of the homodimers is crucial to understanding how these fascinating synthases operate. This subject is bound to be a high priority for future work both in the PKS and the animal FAS fields. 

A number of isoenzymes that are related to GST p have since been identified. A hepatic enzyme, called GST ip, has been purified in several laboratories (H27, S26), and it has been shown that this homodimeric protein has an N-terminal amino acid sequence identical to that of GST p (All, HI 6). The expression of this additional mu-class form in human liver is also subject to variation (H63). GST p and GST op are homodimers and it is now generally accepted that these two enzymes represent allelic variants encoded at the GST 1 locus identified by Board (B30) and Strange et al. (S41). Less information is available on the heterodimeric enzyme formed by the combination of p-type and ip-type subunits (H16, V4). The existence of this isoenzyme has been demonstrated by means of starch-gel electrophoresis and chromatofocusing (F2, S41) however, so far, it has not been fully characterized. A third hepatic mu-class enzyme, named GST < ), was identified by Stockman and Hayes (S36). This isoenzyme, which was present in only 1 of 20 livers examined, was shown to be immunologically related to GST p and GST ip but could be distinguished from these forms by its lower isoelectric point and the fact that, unlike GST p and GST ip, it has a blocked N terminus (P. 

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