Aromatic nitrogenous derivatives peptides

Table II summarizes those amino acids that contain more than an un-reactive aliphatic chain, namely a reactive site which may be a functional group in the traditional sense such as sulfhydryl, thiomethyl, hydroxyl, carboxyl, carbamide, amino, or guanido, or may be an activated aromatic ring or heterocycle such as the phenolic part of tyrosine, the pyrrole unit in troptophan, and the imidazole part in histidine. Phenylalanine would only be considered in this connection as the reactive di- or tetrahydro derivative ring. iV-Peptides derived from proline and hydroxyproline are in a separate class because they are tertiary amides carrying no proton at the nitrogen atom. It may be possible to utilize this special feature for a preferential cleavage under proper conditions.

This structure does not account for the nitrogen and sulfur content of humic substances. It has been suggested that these elements are derived from parts of other types of molecule, for example, proteins, which are associated with the humic substances. Indeed it has been proposed that humic substances consist of an aromatic core to which peptides, carbohydrates, metals, and phenolic acids are chemically or physically attached. It can be seen that the structure in Fig. 5-9 is an "open" network. In fact, there have been suggestions that organic and inorganic materials associated with humic substances are trapped inside these "holes" in the humic substance structure. " ... 

Much interest and research over the past 15 or so years has been directed towards ascidian metabolites because of the high incidence of pharmacological activity that they display. Few comprehensive reviews exist on the topic the only general ones being those produced by Davidson in 1993 [1,2] and the Marine Natural Product literature reviews by Faulkner which always contain a section on tunicate metabolites [3-13]. Although a number of non-nitrogenous metabolites have been isolated from the tunicates, the majority of compoimds isolated have been derived from amino acids. In particular, the two most commonly encountered classes are cyclic peptides and polycyclic aromatic alkaloids. 

Although the division between the peptides and other nitrogen-containing metabolites derived from ascidians is not a clear cut one, the literature published on the non-peptidic, tunicate-derived alkaloids which contain aromatic (often in combination with heteroaromatic) rings has been summarised. The few linear peptides (which contain aromatic amino acid residues) have also been included because of their apparent structural... 

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