Synthesis and Photochemical Properties of a Novel Iron−Sulfur−Nitrosyl Cluster Derivatized with the Pendant Chromophore Protoporphyrin IX¹

Abstract

The novel Roussin red-salt ester (PPIX-RSE) with a pendant porphyrin chromophore was prepared and investigated as a precursor for the photochemical generation of nitric oxide. PPIX-RSE has the general formula Fe(2)(NO)(4)[(mu-S,mu-S')P] (where (S,S')P is the bis(2-thiolatoethyl) diester of protoporphyrin IX. The photoexcitation of PPIX-RSE with 436- or 546-nm light in an aerated chloroform solution led to the photodecomposition of the cluster with the respective quantum yields (5.2 +/- 0.7) x 10(-4) and (2.5 +/- 0.5 x 10(-4)) and the concomitant release of NO. PPIX-RSE is a significantly more effective NO generator at longer wavelength excitation than are other Fe(2)(mu-SR)(2)(NO)(4) esters for which R is a simple alkyl group such as CH(3)CH(2)- because of the much higher absorptivity of the pendant PPIX chromophore at these wavelengths and a modestly higher quantum yield. Fluorescence intensity and lifetime data indicate that the photoexcited porphyrin of PPIX-RSE is largely quenched by the energy transfer to the Fe(2)S(2)(NO)(4) cluster's core. However, a small fraction of this emission is not quenched, and it is proposed that PPIX-RSE may exist in solution as two conformers.

Multi-photon excitation in uncaging the small molecule bioregulator nitric oxide

“…Both SPE at 365 nm and TPE at 775 nm led to NO release which was detected by electron paramagnetic resonance (EPR) spectroscopy using Fe(MGD) 2 (MGB = N-methyl-D-glucamine dithiocarbamate) as a trapping agent to give the EPR active Fe(MGD) 2 (NO) [64]. Since the fluorescent ligand was also released from the metal cluster as the result of the photolabilization of NO followed by oxidative secondary reactions, as previously seen with PPIX-RSE and Fluor-RSE [18,60], the net fluorescence from the solution increased as the photoreaction progressed. These workers also demonstrated using both HeLa and Cos-7 cancer cell cultures that the NO release generated by either SPE or TPE of Fe 2 (NO) 4 (μ-SR ) 2 led to light-dependent cytotoxicity, although the magnitude of the effect was greater in the SPE experiments owing to the greater flux of NO generated with direct UV excitation.…”

“…For example, we and others have worked to design molecular species and larger constructs that are photoactive for NO release at longer excitation wavelengths in the visible [10,[18][19][20] and even in the NIR ranges [21]. However, doing so requires a compound that has appropriate absorption properties and reactivity when excited by longer wavelength light.…”

“…Thus, by varying the R-groups, it should be possible to incorporate desired optical or biological properties into the RSEs. In this context, Conrado et al [18] in this laboratory prepared an RSE with a pendant protoporphyrin IX derivative PPIX-RSE which was shown to have significantly enhanced rates of NO production at longer visible-range λ irr (e.g. 546 nm), relative to a simpler RSE such as Fe 2 (NO) 4 (μ-SEt) 2 .…”

Photo-Controlled Release of NO and CO with Inorganic and Organometallic Complexes

“…Ester derivatives of RRS have been used as photoNORMs, and these Roussin's red esters (RSE) are also illustrated in Figure 5 [45,[76][77][78]. The RSE have analogous photochemistry to the RRS, where the largest quantum yields for NO release are seen in aerated solutions [77].…”

“…The RSE have also been used to create red-light activatable NO-releasing compounds, where the ester derivative has a red-light absorbing dye as antenna [45,76,78] in analogy to the systems described by Figure 2. For example, PPIX-RSE [76], demonstrates enhanced rates of NO production upon red light photolysis owing to the much greater absorbances of the porphyrin Q bands at those wavelengths.…”