Dynamics of the aromatic amino acid residues in the globular conformation of the basic pancreatic trypsin inhibitor (BPTI)

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Abstract
The basic pancreatic trypsin inhibitor (BPTI) was investigated by high resolution1H NMR techniques at 360 MHz. Observation of the amide proton resonances of the polypeptide backbone showed that the globular conformation of BPTI determined by X-ray studies in single crystals is maintained in aqueous solution over the temperature range from 4‡ to 87‡. NMR studies over this temperature range of the aromatic amino acid residues of BPTI. i.e. 4 tyrosines and 4 phenylalanines, led to complete assignments of all the aromatic spin systems in the protein. From this, information was obtained on the rotational motions about the Cβ-Cγ bond axis of the aromatic rings in the globular form of BPTI. At 25‡, two tyrosine rings and one phenylalanine ring are rotating rapidly on the NMR time scale. For the other rings the transitions from slow to rapid rotational motions were investigated at variable temperatures and energy barriers for these intramolecular rate processes determined. The studies of the tyrosine resonances had been described in detail in a previous publication. The present paper describes the identification of the phenylalanine resonances and comments on some technical aspects which might be of quite general interest for the analysis of highly resolved1H NMR spectra of proteins. Data for the tyrosines and the phenylalanines are compiled in three tables, i.e. the pK a -values for the tyrosines, the NMR parameters for all eight aromatics, and the parameters δG♯, and, where available, δH♯ and δS♯ for the rotational motions of the rings.

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