A novel cyclic GRF analog, cyclo(Asp8‐Lys12)‐[Asp8,Ala15]‐GRF(1‐29)‐NH2, i.e. cyclo8.12[Asp8,Ala15]‐GRF(1‐29)‐NH2, was synthesized by the solid phase procedure and found to retain significant biological activity. Solid phase cyclization of Asp8 to Lys12 proceeded rapidly (∼2h) using the BOP reagent. Substitution of Ala12 with d‐Ala2 and/or NH2‐terminal replacement (desNH2‐Tyr1 or N‐MeTyr1) in the cyclo8.12[Asp8,Ala15]‐GRF(1‐29)‐NH2 system resulted in highly potent analogs that were also active in vivo. Conformational analysis (circular dichroism and molecular dynamics calculations based on NOE‐derived distance constraints) demonstrated that cyclo8.12[Asp8,Ala15]‐GRF(1‐29)‐NH2 contains a long α‐helical segment even in aqueous solution. A series of cyclo8.12 stereoisomers containing d‐Asp8 and/or d‐Lys12 were prepared and also found to be highly potent and to retain significant α‐helical conformation. The high biological activity of cyclo8.12[N‐MeTyr1,d‐Ala2,Asp8,Ala15]‐GRF(1‐29)‐NH2 may be explained on the basis of retention of a preferred bioactive conformation.

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Thermal polymers of arginine, glutamic acid, glycine, histidine, phenylalanine, and tryptophan have melanocyte-stimulating activity. The fact that similar polymers lack such activity indicates that the effect is related to the specific amino acid residues. The active polymers are discussed as a model of an evolutionary precursor of contemporary melanocyte-stimulating hormone.

Using a variety of activating agents, a kinetic study was carried out to evaluate the rate of solid phase side‐chain to side‐chain cyclization of Asp3 to Lys12 in the model peptide‐resin, [Ala15]‐GRF(1–29)‐BHA‐resin. Asp3 and Lys12 were introduced in the peptide chain by using Nα‐Boc amino acids in conjunction with the OFm/Fmoc side‐chain protection scheme. The OFm and Fmoc side‐chain protecting groups were shown to be stable to diisopropylethylamine and selectively deprotected on treatment with 20% piperidine in DMF. Solid phase side‐chain to side‐chain cyclization (lactamization) was shown to proceed to completion within 2 h using benzotriazol‐1‐yl‐oxy‐tris(dimethylamino)‐phosphonium hexafluorophosphate (BOP reagent) while the reaction was only 55% completed in 24h using DCC/HOBt. Solid phase side‐chain to side‐chain cyclization by the BOP procedure not only proceeded more rapidly but also gave a purer cyclic product.

A series of analogs of thyroliberin (TRH) ([L‐3Pro3]‐TRH, [D‐3Pro3]‐TRH, [L‐3‐MeHis2, L‐3Pro3]‐TRH) in which proline was replaced by L‐ or D‐3, 4‐dehydroproline was synthesized. The analogs exhibited approximately the same biological activity as the corresponding proline‐containing peptides. These analogs and others in which 3, 4‐dehydroproline is present at the NH2‐terminal, COOH‐terminal or internal positions in the peptide were successfully reduced with deuterium or tritium to provide the 3, 42‐H2‐proline or 3, 43‐H2‐proline analogs, respectively, with near theoretical values of substitution. A novel procedure for the chemical resolution of DL‐3, 4‐dehydroproline is also described.