Imino acids

In acid the nitnle is protonated on nitrogen Nucleophilic addition of water yields an imino acid... 

A series of proton transfers converts the imino acid to an amide... 

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

The part of non-imino acid-containing tripeptides amounts to nearly 50%. It is astonishing that at some places in the collagen chain, containing 1044 amino acid units, such sequences are accumulated, forming imino add-free ranges of remarkable length. 

It has been proved by many experiments that in the case of an increasing percentage of imino acids, Tm increases in the case of native collagen as well as in the case of model peptides (Fig. 37 and 39). This can be attributed to the following causes (seee Eq. (5)) ... 

Fig. 37. Dependence of the thermodynamic parameters AH and AS of triple-helix formation on the imino acid content of the peptides (obtained by cleavage of calf skin-type I collagene with cyanogene and subequent isolation by column chromatrography)3) and of the native neutral salt-soluble skin collagene of various animals. The entropy values are denoted by dotted lines...

The interpretation of these results is, however, problematic since no data on the absolute enthalpy and entropy of the respective triple helix and coiled state are available. Though it may be taken as an established fact that the entropy of conformation of a (Pro-Pro-Gly) coil is lower than in the case of a (Pro-Ala-Gly)n coil, we are not sure whether the entropy of the triple helix depends on the imino acid content. 

All results, except those of Sutoh and Noda, seem to confirm that the higher thermostability with increasing imino acid content is caused by an enthalpy overcompensation of the entropy losses. AH° and also AS° are obviously caused by different effects. This is shown by Eq. (10) and Fig. 38... 

While their physiologic role is uncertain, L-amino acid oxidases of liver and kidney convert amino acids to an a-imino acid that decomposes to an a-keto acid with release of ammonium ion (Figure 29-6). The reduced flavin is reoxidized by molecular oxygen, forming hy-... 

Figure 29-6. Oxidative deamination catalyzed by L-amino acid oxidase (i-a-amino acidiOj oxidoreduc-tase). The a-imino acid, shown in brackets, is not a stable intermediate.

Addition of excess /-butyl isocyanide, CO2, or CyNCO resulted in the formation of five-membered ring chelate compounds 107, the metal-bound imino acid 108, and the metal-bound imino amide 109, respectively, as shown in Scheme 47.170... 

The reported use in the Middle East of the seeds of Cucurbita species as tenifuge (198) was traced back to the imino acid cucurbitine (47 (81). 

Proteins are composed of a-amino acids and a-imino acids. 

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