Studies on cytochrome c. Part I. Synthesis of the protected hexadecapeptide (sequence 1-16) of Baker's Yeast iso-1-cytochrome c

Revisiting NO2 as Protecting Group of Arginine in Solid-Phase Peptide Synthesis

The protection of side-chain arginine in solid-phase peptide synthesis requires attention since current protecting groups have several drawbacks. Herein, the NO₂ group, which is scarcely used, has been revisited. This work shows that it prevents the formation of δ-lactam, the most severe side-reaction during the incorporation of Arg. Moreover, it is stable in solution for long periods and can be removed in an easy-to-understand manner. Thus, this protecting group can be removed while the protected peptide is still anchored to the resin, with SnCl₂ as reducing agent in mild acid conditions using 2-MeTHF as solvent at 55 °C. Furthermore, we demonstrate that sonochemistry can facilitate the removal of NO₂ from multiple Arg-containing peptides.

The Bakerian Lecture Towards synthesis of proteins

Proteins, as their name indicates, occupy the prime place in Nature’s architecture. It is worth remarking that Nature uses the structural type for diverse purposes, in addition to those familiar in enzymology, immunology, etc. A striking example is provided by the recent publication (Bradbury et al . 1976), showing that the C fragment of lipotropin (residues 61-91) is firmly bound by opiate receptors and is an analgesic 200 times as potent as morphine in molar terms. The pentapeptide comprising residues 61-65 of this sequence is already well known as enkephalin (Hughes et al . 1975). Reference should also be made to the host of hormones from the pituitary gland, hypothalamus, and intestinal tract. An example of the last is gastrin which has been studied at Liverpool, principally by R. A. Gregory in the Department of Physiology. Recently the structure of the big gastrins, isolated by Gregory & Tracy (1972), has been elucidated by Harris & Runswick (1974) following preliminary work in the Robert Robinson Laboratories (Barton, Choud- hury, Dancsi & Kenner 1973) (figure 1). This structure typifies the way in which small fragments are carved from a protein chain in order to provide an active hormone (Smyth 1975). Thus the gastrins originally described (Gregory & Tracy 1964; Kenner & Sheppard 1968) correspond to the sequence 18-34 of the big gastrins. The lysyl glutaminyl bond is enzymically cleaved, as can be achieved in the laboratory by trypsin, and then the N-terminal glutaminyl residue cyclizes to the pyroglutamyl structure (pyrrolidone carbonyl). The pyroglutamyl structure at the N-terminus of the big gastrins must have arisen in the same way. It should also be noted that the C-terminal amide, which is essential for biological activity (Tracy & Gregory 1964), very likely arises from enzymatic aminolysis of a protein chain extending further to the right; the extensive studies by Morley (1968) of structure- activity relations in the gastrin tetrapeptide indicate that the gastrin tetrapeptide embedded in a protein chain would not have hormonal activity until aminolysis had occurred, and therefore the gastrin sequence can be stored in a prohormone. The sequence of human big gastrin 1 (figure 1) has been synthesized by a colla­borative effort between our laboratory at Liverpool and that of Professor E. Wünsch in the Max Planck Institute at Munich (Choudhury et al . 1976).

Semisynthesis of the native sequence of horse heart cytochrome c-(66-104)-nonatriacontapeptide

The syntheses of some derivatives of horse cytochrome c-(66-79)-tetradecapeptide are presented. The syntheses are so designed that analogues of this phylogenetically well preserved sequence can be obtained also. The compounds were intended as synthons for the semisynthesis of 65-homoserine-cytochrome c, which we described earlier. A requisite for this project was the C-terminal tetracosapeptide fragment of the protein, accessible through degradation of cytochrome c with cyanogen bromide. The five epsilon amino groups in this compound are reversibly protected with the 2-(methylsulfonyl)ethyloxycarbonyl function, which is resistant to acid and causes little impairment of solubility. The condensation of the fragments leading to the native sequence of horse cytochrome c-(66-104)-nonatriacontapeptide is presented also. The syntheses were performed using the solution strategy. Some unexpected ring closing reactions involving tyrosine and tert.-butyl prolylasparaginylcarbazate, are described.