Leptin, a multifunctional hormone, controls various processes in both the central nervous system and in peripheral tissues. Because of the presence of multiple leptin/receptor (ObR) interaction sites and diverse leptin activities, the literature lacks truly monofunctional leptin protein derivatives or fragments. To date, selective ObR antagonists have not been reported. We developed short, pharmacologically advantageous peptide analogs of ObR-binding site III of leptin that acted as selective ObR inhibitors without any partial agonistic activity. These reduced leptin-dependent growth and signaling in cancer cell lines at picomolar and low nanomolar concentrations. In immunocompromised mice the peptides suppressed the growth of rapidly proliferating orthotopic human breast cancer xenografts by 50% when administered either intraperitoneally (i.p.) or subcutaneously (s.c.) for 38 days at a 0.1 mg/kg/day dose. The peptides were distributed to the brain, and when added to growing C57BL/6 normal mice i.p., s.c., or orally, the lead antagonist accelerated normal weight increase without producing any toxic effects. Weight gain increases could not be observed after 10-12 days of treatment indicating that the mice became resistant to the central nervous system activity of leptin antagonists. However, in normal growing rats the intranasal administration at 0.1 mg/kg/day for 20 days resulted in a 2% net total body weight gain without signs of resistance induction. In addition to the potential of these peptides in drug development against primary and metastatic tumors and cachexia, our data confirm that resistance to leptin resides at the blood-brain barrier.

Molecular Dynamics simulations of a β-hairpin miniprotein, CLN025, were performed to examine the conformational stability of the peptide in H 2 O at 278 K, 300 K, 333 K and 363 K as well as in TFE, MeOH and DMSO at 300 K. CLN025 is a variant of the Chignolin miniprotein, in which the terminal Gly residues of Chignolin are replaced with Tyr residues, which leads to a 29.7 K increase in melting temperature. The energy of the intramolecular interactions were calculated using DFT quantum chemical calculations at the BH and HLYP/cc-pVTZ level of theory. CLN025 maintained a β-hairpin conformation in all environments. The β-hairpin is stabilized by hydrogen bonds, an electrostatic interaction between the charged termini of the peptide and weakly polar interactions. The interaction between the backbones of the N and C-terminal strands accounts for −97.32 to −120.87 kcal mol −1 of the stabilization energy. The energy of the CH-π interactions between Tyr2 and Pro4 were between −1.80 and −8.9 kcal mol −1 and the energy of the Tyr2-Trp9 Ar-Ar interaction was between −0.43 and −8.11 kcal mol −1 . Increasing temperature caused the Tyr2-Pro4 CH-π and the Tyr2-Trp9 and Tyr2-Tyr10 Ar-Ar interactions to become less favorable but the Tyr1-Trp9 interaction became more favorable and played an important role in stabilizing the β-hairpin of CLN025 that resulted in the increased melting temperature. The weakly polar interactions play an important role in the structure and stability of CLN025 and other proteins.

Electronic and vibrational circular dichroism are often used to determine the secondary structure of proteins, because each secondary structure has a unique spectrum. In order to determine these spectral features, polypeptides that are known to adopt a particular conformation along their entire length are studied ideally. Little is known about the vibrational circular dichroic spectroscopic features of the β-hairpin. In this study, the VCD spectral features of a decapeptide, YYDPETGTWY (CLN025), which forms a stable β-hairpin that is stabilized by intramolecular weakly polar interactions and hydrogen bonds were determined. Molecular Dynamics simulations and ECD spectropolarimetry were used to confirm that CLN025 adopts a β-hairpin in water, TFE, MeOH and DMSO and to examine differences in the secondary structure, hydrogen bonds and weakly polar interactions. CLN025 was synthesized by microwave-assisted solid phase peptide synthesis with Nα-Fmoc protected amino acids. The VCD spectra displayed a (−,+,−) pattern with bands at 1640 to 1656 cm−1, 1667 to 1687 cm−1 and 1679 to 1686 cm−1 formed by the overlap of a lower frequency negative couplet and a higher frequency positive couplet. A maximum IR absorbance was observed at 1647 to 1663 cm−1 with component bands at 1630 cm−1, 1646 cm−1, 1658 cm−1 and 1675 to 1680 cm−1 that are indicative of the β-sheet, random meander, either random meander or loop and turn, respectively. These results are similar to the results of others, who examined the VCD spectra of β-hairpins formed by DPro-Xxx turns and indicate that observed pattern is typical of β-hairpins.

The tertiary structure of the TC5b miniprotein is stabilized by inter-residue interactions of the Trp-cage, which is composed of a Tyr and several Pro residues surrounding a central Trp residue. The interactions include Ar-Ar (aromatic side-chain-aromatic side-chain), Ar-NH (aromatic side-chain-backbone amide), and CH-pi (aromatic side-chain-aliphatic hydrogen) interactions. In the present work, the strength of the weakly polar interactions found in the TC5b miniprotein was quantified using all of the available 38 NMR structures (1L2Y) from the Protein Data Bank with DFT quantum chemical calculations at the BHandHLYP/cc-pVTZ level of theory and molecular fragmentation with capping of the partial structures. The energies of interaction between the individual residues of the Trp-cage range between -5.85+/-1.41 and -21.30+/-0.88 kcal mol(-1), leading to a significant total structural stabilization energy of -52.13+/-2.56 kcal mol(-1) of which about 50% is from the weakly polar interactions. Furthermore, the strengths of the individual weakly polar interactions are between -2.32+/-0.17 and -2.93+/-0.12 kcal mol(-1) for the CH-pi interactions, between -2.48+/-0.97 and -3.09+/-1.02 kcal mol(-1) for the Ar-NH interaction and -2.74+/-1.06 kcal mol(-1) for the Ar-Ar interaction.