Abstract
Thyrotropin-releasing factor (TRF) is a peptide which causes the release of thyroid-stimulating hormone (TSH) from the pituitary in mammals [l] . The sequence of this hormone has recently been determined [ 1,2] to be L-pyroglutamyl-l-histidylL-prolinamide. Carbon-13 nuclear magnetic resonance (13C NMR) has proven useful in detecting cis and trans isomers of proline in peptides [3-61. The purpose of this study was to investigate cis-trans isomerism about the histidyl-prolinamide bond in TRF and to measure the spin-lattice relaxation times, Tr , of all the carbons in the hormone. TRF in water is found to have 14% of the proline present as the cis isomer the amount varies with the solvent. The proline resonances are sensitive to the nature of the group attached to the proline nitrogen. 13C NMR is shown to be sensitive to the presence of D and L forms of histidine. The presence of D or L histidine in TRF has a profound influence on the 13C chemical shifts of proline, whereas those of pyroglutamate are unaffected.
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