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

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.