V. Viswanatha

The tyrosine ring of oxytocin undergoes hindered rotation when the hormone is bound to neurophysin.

Abstract Tyrosine specifically enriched with 13C in the meta positions has been chemically synthesized and incorporated into oxytocin via solid phase peptide synthesis. The 13C nmr spectrum of a 1:1 mixture of the enriched hormone complexed to neurophysin was obtained. The spectrum consisted of three peaks. The two outer peaks, representing 85% of the total intensity, were of equal area, had shifts of −0.9 and +2.4 parts per million relative to the free peak, and each had a linewidth of 100 hz at 20°C, with increasing linewidths at higher temperatures. These two peaks arise from a binding mode in which tyrosine ring rotation is hindered by interaction with neurophysin. The rotation rate at 20°C is 130s−1, and at 42°C is 900s−1. The central peak occurred at the position of the resonance due to free hormone, had a temperature independent linewidth of 30–40 hz, and represented about 15% of the total intensity. We believe this peak is due to a binding mode in which tyrosine ring rotation is rapid, 104–108s−1.