THE SOLUTION STRUCTURE OF A CYCLIC ENDOTHELIN ANTAGONIST, BQ-123, BASED ON H-1-H-1 AND C-13-H-1 3 BOND COUPLING-CONSTANTS

A cyclic pentapeptide endothelin antagonist, cyclo(dTrp-dAsp-Pro-dVal-Leu), recently reported (K. Ishikawa et al., 13th Am. Pept. Symp., Cambridge MA, 1991) has been studied by NMR spectroscopy and molecular modeling. A stable structure has been determined without the use of nuclear Overhauser effects and is based primarily on homonuclear and heteronuclear three bond coupling constants. The C-13-edited TOCSY experiment is demonstrated at natural abundance and approximately 30 mM peptide concentrations. Three bond C-13-H-1 coupling constants obtained by this method are shown to reduce the ambiguity in phi-angle determination which exists when only interproton coupling constants are used. Three out of four phi-angles were determined uniquely by this method and the fourth was reduced to two possible values. The proline phi-angle was determined to be -78-degrees based on the 3J(H-alpha,H-beta) and 3J(H-alpha,H-beta) coupling constants. Comparison of amide proton temperature dependence, chemical shifts and vicinal proton coupling constants in a 20% acetonitrile/80% water solvent mixture and in (CD3)2SO indicates that the structure is similar in both solvents.