The Use of Paraquat as an NMR.-and charge-transfer-probe for solvent-exposed aromatic amino-acid side-chains

✍ Scribed by Jan W. Verhoeven; Anne-Marie A. Verhoeven-Schoff; André Masson; Robert Schwyzer

• Publisher: John Wiley and Sons
• Year: 1974
• Tongue: German
• Weight: 629 KB
• Volume: 57
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19740570823

✦ Synopsis

Abstract

The purpose of this investigation was to find new and more potent charge‐transfer probes for the study of certain aspects of polypeptide and protein conformation, especially of solvent‐exposure of aromatic amino‐acid side chains. N,N′‐dimethyl‐4,4′‐dipyridylium ion (paraquat) was shown by NMR. to complex specifically with tryptophan, tyrosine, and phenylalanine. The long‐wavelength absorption (electronic transitions) typical of charge‐transfer complexes was detectable with tryptophan and tyrosine, not with phenylalanine. Paraquat forms slightly stronger complexes than N(1)‐methyl‐nicotinamidium ion, and appears to have different steric requirements. Im human calcitonin and human calcitonin‐(11–32)‐dokosipeptide all the aromatic amino‐acid side‐chains (Phe, Tyr) are accessible to paraquat in solution. This is true also for at least one of the two tyrosines and the tryptopha of ACTH‐(1–24)‐tetrakosipeptide, and for Trp (62) of chicken egg‐white lysozyme. Paraquat is of special interest, because it generates a CT.‐absorption band not only with tryptophan, but also with tyrosine, and because of its strong diamagnetic shielding effect.