CIDNP study of the aromatic side chain interactions in myotoxin alpha.

CIDNP and COSY measurements were applied to study aromatic side chain interactions and conformations in myotoxin alpha, a Crotalus venom toxin which acts as blocker of the Ca2+ influx in the sarcoplasmic reticulum calcium pump. New evidence for the existence of a hydrophobic aromatic cluster at the amino terminus was obtained. This cluster consists of Tyr1, His5, His10, and (possibly) F12. The CIDNP data clearly establish that the usual order of the tyrosine 2, 6 and 3, 5 proton signals of Tyr1 is inverted, because of the large diamagnetic shielding effects of one ring on the other. The lines of the 2, 6 ring protons of Tyr1 and proton 4 in each of His5 and His10 are significantly broadened, an outcome of the side-chain hydrophobic interaction. The aromatic cluster could possibly present a hydrophobic sticky patch for binding of toxin by Ca2+ ATPase.