NMR analysis of the ultraviolet photolytic behavior of several tryptophan-rich growth hormone releasing peptides

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. © 1997 John Wiley & Sons, Inc. Biospectroscopy 3: 317–323, 1997