Receptor inactivation by dye-neuropeptide conjugates: 2. Characterization of the quantum yield of singlet oxygen generated by irradiation of dye-neuropeptide conjugates

Different neuropeptide analogues of the neurotransmitter FMRFamide were covalently attached to a tethered dye, forming dye-neuropeptide conjugates capable of stably binding to the FMRFamide receptors. Singlet oxygen (1 delta O2) generated by laser irradiation of the conjugates bound to this receptor should inactivate it if (a) the distance 1 delta O2 must diffuse to reach the photo-sensitized receptor is less than 1000 A, (b) the conjugate binds the receptor with the same affinity as the indigenous neurotransmitter, and (c) the quantum yield (phi) of 1 delta O2 is sufficient. Previous studies determined that the first two constraints are satisfied. The results of the present study confirm that the third constraint is also satisfied, as the phi of 1 delta O2 resulting from the laser irradiation of the conjugates were uniformly large, exceeding those for the dye itself, ranging from 0.25 at pD 6.0 to 0.93 at pD 9.0.