Cysteine derivatives experimentally formed

New experimental evidence [54] indicates that the C-termini of Nephila clavipes major ampullate spidroins, which are present in the high molecular weight fractions of both the proteins derived from the secretions of the glands and the spun thread, are involved in the formation of disulfide bridges. However, it is rather unlikely that such covalent cross-linking has a strong impact on the material properties as the C-termini of the minor ampullate spidroins that form a very stiff fibre do not contain cysteines [50]. 

The biosynthesis of these derivatives is largely known they are formed from DOPA via phenylalanine and tyrosine. The condensation of two molecules of DOPA produced a diketopiperazine intermediate, whose formation was verified by incorporating synthetic analogs (Jeedigunta, Krenisky, and Kerr, 2000). The sulfur atom present in ecteinascidins derives from cysteine, whose incorporation has been experimentally verified (Kerr and Miranda, 1995). A simplified outline of the biosynthesis of ecteinascidins (types E-743 and E-736) is presented in Eigiue 28.37. 

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