Replacement of Phe⁶, Phe⁷, and Phe¹¹ofd-Trp⁸-Somatostatin-14 withl-Pyrazinylalanine. Predicted and Observed Effects on Binding Affinities at hSST2 and hSST4. An Unexpected Effect of the Chirality of Trp⁸on NMR Spectra in Methanol

Abstract

An alanine scan performed in the 1970s suggested that Phe(6) and Phe(11) are required for the binding of somatostatin (SRIF-14). Molecular modeling studies and replacement of Phe(6) and Phe(11) with a cystine bridge affording ligands with the retention of high biological activity, however, led to the alternate conclusion that Phe(6) and Phe(11) stabilize the bioactive conformation of SRIF-14. Subsequent studies revealed that Phe(11) shields Phe(6) in a "herringbone" arrangement. More recently, a report from this laboratory demonstrated that Spartan 3-21G MO calculations can be invaluable in explaining SARs in medicinal chemistry. For example, the ability of benzene and indole rings to bind the Trp(8) binding pocket for SRIF-14 and the inability of pyrazine to do so was explained through differences in electrostatic potentials. To investigate the role of Phe(6) and Phe(11) more fully, we report here the synthesis of two analogues of D-Trp(8)-SRIF in which Phe(6) and Phe(11) were replaced by the pryazinylalanine congeners, respectively. The NMR spectra in D(2)O and the K(i)s fully support the proposition that Phe(11) stabilizes the bioactive conformation through pi-bonding or aromatic edge-to-face interaction and that pyrazinylalanine(6) can be shielded by Phe(11). The data also show unexpectedly that Phe(6), via the pi-bond, interacts with the receptor, consistent with the original interpretation of the alanine scan. Heretofore it had only been known that Lys(9) interacts with an aspartate anion of the receptor. These conclusions are supported by recent studies of Lewis et al. on the effects on K(i)s of Ala(6)-SRIF-14-amide at the five receptor subtargets. We also found that pyrazinylalanine(7)-D-Trp(8)-SRIF-14 does not bind, suggesting a repulsive interaction with the receptor. Taken together, our results not only validate predictions based on Spartan 3-21G MO analysis but also provide valuable information about the nature of the receptor interaction at the molecular level. Finally, the chirality of Trp(8) was unexpectedly found to have a striking effect on NMR spectra in methanol, especially at lower temperatures.

Fine‐tuning the π–π Aromatic Interactions in Peptides: Somatostatin Analogues Containing Mesityl Alanine

“…As expected, the half‐life of peptide 5 with L ‐Trp8 is only twice as high as that of somatostatin. The surprisingly the long half‐life of analogue 3 (Msa in position 11) relative to peptides 2 and 4 , may corroborate the fact that the unnatural aromatic residue in position 11 shields the residue at position 6, as previously suggested 10b…”

“…This interaction could enable in the stabilization of some of the biologically active conformations of the hormone. A few years later, the same authors detected the Phe6–Phe11 interaction by NMR spectroscopy,10a and hypothesized that it should be perpendicular (edge‐to‐face) rather than parallel (face‐to‐face) 10a,b. However, van Binst and co‐workers subsequently reported that they were unable to observe the Phe6–Phe11 interaction by NMR spectroscopy (either in aqueous solution or in methanol) and that the only interaction that they could observe was that between Phe6 and Phe7 11a,b.…”

The TLQP-21 Peptide Activates the G-Protein-Coupled Receptor C3aR1 via a Folding-upon-Binding Mechanism

“…Furthermore, the C-terminus of peptides active at class B GPCR act as a docking site to initiate the helix formation, and the N-terminal residues are responsible for the activation of the receptors (Parthier et al, 2009; Karnik et al, 2003). Conversely peptides active at class A GPCR (including the C3aR1) (Beck-Sickinger et al, 1994; Neelamkavil, et al, 2005) as well as other peptides of the granin family (Bartolomucci et al, 2011), do not show a common binding mechanism. The hot spots for TLQP-21 activity reside at the C-terminus.…”

“…The latter will have implications in the design of synthetic ligands that mimic α-helical conformations to target C3aR1 and activate lipolysis to treat obesity and obesity-related disorders. Synthetic molecules designed to target GPCRs have already been developed into potent agonists and antagonists by engineering lactam-bridges promoting α-helical structures (Neelamkavil et al, 2005; Ammoun et al, 2003; Grace et al 2007; Murage et al, 2008; Pellecchia et al, 2008). The mouse TLQP-21 sequence represents a molecular template for designing new analogues of this peptide that can serve as agonists or antagonists for C3aR1 having important implication for obesity (Possenti et al, 2012) diabetes (Stephens et al, 2012), hypertension (Fargali et al, 2014), pain (Fairbanks et al, 2014) and potentially immune functions.…”

Peptide aromatic interactions modulated by fluorinated residues: Synthesis, structure and biological activity of Somatostatin analogs containing 3-(3′,5′difluorophenyl)-alanine

“…However, to the best of our knowledge, the effect of introducing fluorinated Phe residues in the context of Somatostatin has not been addressed to date 27 . One of the few reports of Somatostatin analogs containing electron-poor aromatic residues as Phe isosteres was done by Hirschmann and co-workers 28 , who synthesized analogs containing L-pyrazinylalanine (Pyz).…”