Receptor Inactivation by Dye-Neuropeptide Conjugates: 2. Characterization of the Quantum Yield of Singlet Oxygen Generated by Irradiation of Dye-Neuropeptide Conjugates
Section snippets
METHOD
Deuterium oxide (99.9%) was obtained from Aldrich Chemical CO. (Milwaukee, WI). The pD of buffers made from deuterium oxide solvent was obtained by adding 0.4 to the apparent pH measured with a glass electrode [12]. Relative 1ΔO2 yields were obtained by measuring the intensity of the singlet emission at 1270 nm following pulsed laser excitation by a phase-R model DL-1200 organic dye laser. The laser was used to generate 633-nm pulses of 300 ns duration and 10 mJ of energy to excite each of
RESULTS
As shown in Table 1, the φ 1ΔO2s for the conjugates were quite large. Indeed, the φ 1ΔO2s for all three conjugates Az-CFMRFa, Az-CFMRF, and Az-CLRFa were always greater than those for either azure B or methylene blue alone from pD 6.0 to 9.0. As methylene blue is widely regarded as an excellent source of 1ΔO2 and the quantum yields of 1ΔO2 for the conjugates exceeded that for methylene blue at every pD measured, their φ 1ΔO2s should be more than sufficient to inactivate the FMRFa receptor. The 1
DISCUSSION
The φ 1ΔO2s for the three conjugates were unexpectedly the same, despite the absence of a Met residue in the tetrapeptide, which was expected to enhance the φ 1ΔO2 of this conjugate through a consequent diminution of 1ΔO2 quenching. Apparently, the close proximity of amino acids capable of quenching singlet oxygen (e.g., methionine, k = 1.5 × 107M−1 s−1) [8] does not trap a significant portion of the singlet oxygen generated. Alternatively, quenching by electron transfer from the sulfur present
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