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The Use of Paraquat as an NMR.‐and charge‐transfer‐probe for solvent‐exposed aromatic amino‐acid side‐chains
Author(s) -
Verhoeven Jan W.,
VerhoevenSchoff AnneMarie A.,
Masson André,
Schwyzer Robert
Publication year - 1974
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19740570823
Subject(s) - chemistry , tryptophan , side chain , aromatic amino acids , tyrosine , phenylalanine , steric effects , amino acid , stereochemistry , organic chemistry , biochemistry , polymer
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.