NMR analysis of the ultraviolet photolytic behavior of several tryptophan-rich growth hormone releasing peptides
by McKim, Steve; Hinton, James F.; Deghenghi, Romano
Aqueous solutions of three tryptophan-rich growth hormone releasing hexapeptides, GHRP-6 ( His-D-Trp-Ala-Trp-D-Phe-Lys-NH2), hexarelin (His-D-2-Me-Trp-Ala-Trp-D-Phe-Lys-NH2 ), and EP7458 ( His-D-Trp-Ala-2-Me-Trp-D-Phe-Lys-NH2), were exposed to varying durations of ultraviolet (UV) light. Using NMR spectroscopy, first-order rate constants for the UV photolytic degradation of the tryptophan(s)/2-methyl tryptophan residues within each peptide were obtained by plotting the decrease in the area of the indole N-H resonances with respect to UV photolysis time. A significant differential photolytic effect was observed between the two tryptophan residues of GHRP-6 and the tryptophan/2-methyl tryptophan residues of EP7458. A somewhat smaller differential photolytic effect was observed between the tryptophan/2-methyl tryptophan residues of hexarelin. In addition, the three peptides were degraded at different rates, suggesting that the effect of UV light on each peptide is dependent on whether a tryptophan or 2-methyl tryptophan is the second or fourth residue in the primary sequence. (C) 1997 John Wiley & Sons, Inc.
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