No abstract

Received : 30 October 2013; in revised form: 12 November 2013 / Accepted: 13 November 2013 / Published: 25 November 2013 Abstract: The non-natural amino acid mesitylalanine (2,4,6-trimethyl-L-phenylalanine; Msa) has an electron-richer and a more conformationally restricted side-chain than that of its natural phenylalanine counterpart. Taking these properties into account, we have synthesized ten somatostatin analogs containing Msa residues in different key positions to modify the intrinsic conformational flexibility of the natural hormone. We have measured the binding affinity of these analogs and correlated it with the main conformations they OPEN ACCESSMolecules 2013, 18 14565 populate in solution. NMR and computational analysis revealed that analogs containing one Msa residue were conformationally more restricted than somatostatin under similar experimental conditions. Furthermore, we were able to characterize the presence of a hairpin at the pharmacophore region and a non-covalent interaction between aromatic residues 6 and 11. In all cases, the inclusion of a D-Trp in the eighth position further stabilized the main conformation. Some of these peptides bound selectively to one or two somatostatin receptors with similar or even higher affinity than the natural hormone. However, we also found that multiple incorporations of Msa residues increased the life span of the peptides in serum but with a loss of conformational rigidity and binding affinity.

We prepared the two enantiomers of 3-(3'-quinolyl)-alanine (Qla, 1) in multigram scale by asymmetric hydrogenation. These amino acids, protected as Fmoc derivatives, were then used in the solid-phase synthesis of two new somatostatin 14 (SRIF-14) analogues 8 a and 8 b, tetradecapeptides in which the tryptophan residue (Trp8) is replaced by one of the two enantiomers of 3-(3'-quinolyl)-alanine (Qla8) and therefore lack the N--H bond in residue 8. The selectivity of these new analogues for the somatostatin receptors, SSTR1-5, was measured. Substitution with L-Qla8 yielded peptide 8 a, which was highly selective for SSTR1 and SSTR3, with an affinity similar to that of SRIF-14. Substitution by D-Qla gave the relatively selective analogue 8 b, which showed high affinity for SSTR3 and significant affinity for SSTR1, SSTR2 and SSTR5. The biological results demonstrate that bulky and electronically poor aromatic amino acids at position 8 are compatible with strong activity with SSTR1 and SSTR3. Remarkably, these high affinity levels were achieved with peptides in which the conformational mobility was increased with respect to that of SRIF-14. This observation suggests that conformational rigidity is not required, and might be detrimental to the interaction with receptors SSTR1 and SSTR3. The absence of an indole N proton in Qla8 might also contribute to the increased flexibility observed in these analogues.

Besser etwas unbeweglich: Somatostatin‐Analoga mit starreren Konformationen als die Stammverbindung wurden ausgehend von der nativen Sequenz durch Austausch der Phe‐ gegen Mesitylalanin(Msa)‐Reste erhalten (siehe Struktur). Die hohe Affinität dieser Analoga für SSTR‐Rezeptoren belegt, dass die genaue Einstellung nichtkovalenter Wechselwirkungen zwischen Aminosäure‐Seitenketten die Affinität und Selektivität von Peptiden modulieren kann.

Die aromatischen π‐π‐Wechselwirkungen zwischen den Aminosäuren 6, 7 und 11 im natürlichen Hormon Somatostatin sind für die konformative Stabilität entscheidend. In der Zuschrift auf beschreiben M. J. Macias, A. Riera et al., dass peptidische Analoga, die durch Phenylalanin‐Mesitylalanin‐Austausch erhalten wurden, konformativ starrer sind als das Stammhormon. Auf diesem Weg wurden die ersten 3D‐Strukturen von Somatostatin‐Analoga aus 14 Aminosäureresten erhalten.