Thermal perturbation spectrophotometry of luliberin and model His-Trp peptides

Thermal perturbation (TP) spectra of luliberin were measured at pH 5-8, and compared with the model chromophores N-Ac-Tyr-NH2, N-Ac-Trp-NH2, t-Boc-His-Trp-NH2, H-His-Trp-OH, Ac-His-Trp-OH, tryptophan and cyclo-[His-Trp] (all L-isomers). Between pH 5 and neutrality, the major TP extremum of the Trp3 residue of luliberin increases by about 50%. A similar effect is seen for luliberin acetylated on Tyr5. The effect with luliberin is attributed to the protonation of the His2 residue. One proposed explanation is that the protonated imidazole orients water around the nearby indole in a different way than does unprotonated imidazole. The Tyr5 residue of luliberin behaves like N-Ac-Tyr-NH2, and is considered to be well exposed to solvent. The TP spectra of N-Ac-Trp-NH2, t-Boc-His-Trp-NH2, Ac-His-Trp-OH, and cyclo-[His-Trp] are pH-independent from pH 5 to 8. The TP spectrum of H-His-Trp-NH2 has a bell-shaped pH dependence, rising from normal at pH 3.5 to above normal at pH 6, and returning to normal at pH 8. Luliberin and model peptides show that fluorescence and TP spectra of His-Trp sequences can respond differently to pH.