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Replacement of Phe 6 , Phe 7 , and Phe 11 of d -Trp 8 -Somatostatin-14 with l -Pyrazinylalanine. Predicted and Observed Effects on Binding Affinities at hSST2 and hSST4. An Unexpected Effect of the Chirality of Trp 8 on NMR Spectra in Methanol

2005; American Chemical Society; Volume: 48; Issue: 12 Linguagem: Inglês

10.1021/jm058184l

1520-4804

Santhosh Neelamkavil, Byron H. Arison, Elizabeth T. Birzin, Jin-Jye Feng, Kuo-Hsin Chen, Atsui Lin, Fong‐Chi Cheng, Laurie L. Taylor, Edward R. Thornton, Amos B. Smith, Ralph Hirschmann,

Monoclonal and Polyclonal Antibodies Research

An alanine scan performed in the 1970s suggested that Phe6 and Phe11 are required for the binding of somatostatin (SRIF-14). Molecular modeling studies and replacement of Phe6 and Phe11 with a cystine bridge affording ligands with the retention of high biological activity, however, led to the alternate conclusion that Phe6 and Phe11 stabilize the bioactive conformation of SRIF-14. Subsequent studies revealed that Phe11 shields Phe6 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 Trp8 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 Phe6 and Phe11 more fully, we report here the synthesis of two analogues of d-Trp8-SRIF in which Phe6 and Phe11 were replaced by the pryazinylalanine congeners, respectively. The NMR spectra in D2O and the Kis fully support the proposition that Phe11 stabilizes the bioactive conformation through π-bonding or aromatic edge-to-face interaction and that pyrazinylalanine6 can be shielded by Phe11. The data also show unexpectedly that Phe6, via the π-bond, interacts with the receptor, consistent with the original interpretation of the alanine scan. Heretofore it had only been known that Lys9 interacts with an aspartate anion of the receptor. These conclusions are supported by recent studies of Lewis et al. on the effects on Kis of Ala6-SRIF-14-amide at the five receptor subtargets. We also found that pyrazinylalanine7-d-Trp8-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 Trp8 was unexpectedly found to have a striking effect on NMR spectra in methanol, especially at lower temperatures.