Raghoottama S. Pandurangi

Chemistry of bifunctional photoprobes. 3 -- Correlation between the efficiency of CH insertion by photolabile chelating agents and lifetimes of singlet nitrenes by flash photolysis: First example of photochemical attachment of {sup 99m}Tc-complex with human serum albumin

Systematic functionalization of perfluoroaryl azides with chelating agents capable of complexing transition metals produces a new class of bifunctional photolabile chelating agents (BFPCAs). The strategy to shield the azide functionality from the electronic and steric influence of the electron-rich metal Pd through ester and amide bridges raised CH insertion efficiency to unprecedented levels (>92%) in a model solvent (cyclohexane). In contrast, perfluoroaryl azides attached to chelating agents via hydrazones show no significant CH insertion in cyclohexane upon photolysis. Measurements of the lifetimes of the singlet nitrenes derived from these agents by flash photolysis techniques correlate well with the efficiency of CH insertion by demonstrating longer lifetimes (10−50 times) for singlet nitrenes derived from azidotetrafluorinated esters and amides compared with the related hydrazones, which failed to yield significant CH insertion. A representative BFPCA 12 is chelated to diagnostic radionuclide 99mTc...

High yields of nitrene insertion into unactivated C–H bonds. First example of X-ray crystallographic and 19F NMR analysis of the photochemically produced C–H inserted adduct

Photolysis of 4-azido-tetrafluorobenzonitrile results in the highest yield reported to date, (75–80% as estimated from 19F NMR spectroscopy) for nitrene insertion into the unactivated C–H bond of cyclohexane; the photochemical adduct is characterized by 1H and 13C NMR spectroscopy and the structure is confirmed by X-ray crystallography for the first time.

Photolabeling of human serum albumin by 4-azido-2-([14C]-methylamino) trifluorobenzonitrile. A high-efficiency, long wavelength photolabel.

N-alkyl derivatives of perfluoroarylazides are excellent candidates for photolabeling of proteins since they have absorption spectra in the 340-350 nm range permitting photolabel absorption without direct protein photolysis. The [14C]-N-methylamino derivative of 4-azido-tetrafluorobenzonitrile has been used to demonstrate that 80% of the photo-induced nitrene transient becomes covalently attached to HSA during photolysis. Multiwavelength detection of the photoprobe-protein separation by size exclusion chromatography is shown to be an effective tool for assessing the conjugation of the photoprobe to the protein.

Recent Trends in the Evaluation of Photochemical Insertion Characteristics of Heterobifunctional Perfluoroaryl Azide Chelating Agents: Biochemical Implications in Nuclear Medicine

Achieving high efficiency covalent attachment of molecular probes to biomolecules by photolabeling techniques remains an elusive goal of chemists and biologists. Developments in the understanding of the fundamental photochemistry of perfluoroaryl azides suggest a new means of attaching diagnostic and therapeutic radionuclei to proteins and antibodies through heterobifunctional chelating agents containing a photoactive terminus. Such an approach requires establishment of the coordination chemistry of ligating systems, appropriate molecular design of the photolabel, analytical method development for critical analysis of photolabeled products and retention of immunoreactivity of antibodies in the postlabeling stage. In this review, we summarize recent developments in photolabeling technology and discuss new challenges for application of the technology in the field of nuclear medicine.

Novel type 1 photosensitizers: viability of leukemia cells exposed to reactive intermediates generated in situ by in vitro photofragmentation

Photodynamic therapy of tumors involving Type 2 photosenstizers has been conspicuously successful, but the Type 1 process, in contrast, has not received much attention despite its considerable potential. Accordingly, several classes of molecules containing fragile bonds such as azido (-N=N=N), azo (-N=N-), sulfenato (-S-O-) and oxaza (-N-O-) functional groups that produce reactive intermediates such as radicals and nitrenes upon photoexcitation were prepared and tested for cell viability using U397 leukemia cell line. The azido photosensitizer was conjugated to leukemia cell binding peptide, SFFWRLS, for targeted cell viability study. The cells were incubated with the photosensitizer at various concentrations, and were illuminated for 5, 10, and 20 minutes. The results show that all the photosensitizers caused cell death compared to the controls when exposed to both the photosensitizers and light. Most importantly, selective cell death was observed with the azido peptide conjugate 6, which clearly demonstrates that these Type 1 sensitizers are useful for phototherapeutic applications.

Phosphorus hydrazides as building blocks for potential photoaffinity labels. Synthesis and co-ordination chemistry of perfluoroaryl azide conjugates of phenylphosphonothioic dihydrazide

The phosphorus hydrazide PhP(S)(NMeNH2)21 reacted with the azido functionalized aldehyde 4-N3C6F4CHO to give a mixture of Schiff-base adducts with one or two perfluoroaryl azido substituents on 1. However, the reaction with R′CHO (R = 4-O2NC6H4, 3-O2NC6H4 or 2,4-Me2C6H3) resulted in a considerable selectivity of incorporating one aromatic substituent in the terminal hydrazide unit of 1via Schiff-base coupling to produce PhP(S)(NMeNH2)(NMeNCHR′)(R′= 4-O2NC6H43, 3-O2NC6H44 or 2,4-Me2C6H35). These mono-Schiff-base adducts 3–5 undergo further Schiff-base coupling with azidotetrafluorobenzaldehyde to produce the azido-functionalized phosphorus hydrazides PhP(S)-(NMeNCHC6F4N3)(NMeNCHR′)(R′= 4-O2NC6H47, 3-O2NC6H48 or 2,4-Me2C6H39). The ligating properties of the representative phosphorus hydrazides 2 and 9 with palladium (II) were investigated. The crystal structure of the complex [PdCl2{PhP(S)(NMeNH2)(NMeNCHC6F4N3-4)}] reveals that the palladium(II) is bound in a chelating cis arrangement to the phosphorus chalcogenide and the hydrazine nitrogen via a five-membered metallacyclic framework: monoclinic, space group P21/n, a= 7.826(10), b= 18.051 (20), c= 18.330(3)A, β= 98.196(7)°, Z= 4, R= 0.038 and R′= 0.053.

Novel visible light activated type 1 photosensitizers

Photodynamic therapy of tumors involving Type 2 photosenstizers has been conspicuously successful, but the Type 1 process, in contrast, has not received much attention despite its considerable potential. Accordingly, several classes of molecules containing fragile bonds such as azido (-N=N=N), azo (-N=N-), and oxaza (-N-O-) functional groups that produce reactive intermediates such as radicals and nitrenes upon photoexcitation with visible light were prepared and tested for cell viability using U397 leukemia cell line. The cells were incubated with the photosensitizer at various concentrations, and were illuminated for 5, 10, and 20 minutes. The results show that all the photosensitizers caused cell death compared to the controls when exposed to both the photosensitizers and light.