Electron Transfer From Photoexcited Singlet and Triplet Bacteriopheophytin

Photodynamic inactivation is a rapidly developing antimicrobial treatment that employs a nontoxic photoactivatable dye or photosensitizer in combination with harmless visible light to generate reactive oxygen species that are toxic to cells. Tetrapyrroles (e.g., porphyrins, chlorins, bacteriochlorins) are a class of photosensitizers that exhibit promising characteristics to serve as broad-spectrum antimicrobials. In order to bind to and efficiently penetrate into all classes of microbial cells, tetrapyrroles should have structures that contain (i) one or more cationic charge(s) or (ii) a basic group. In this report, we investigate the use of new stable synthetic bacteriochlorins that have a strong absorption band in the range 720 to 740 nm, which is in the near-infrared spectral region. Four bacteriochlorins with 2, 4, or 6 quaternized ammonium groups or 2 basic amine groups were compared for light-mediated killing against a Gram-positive bacterium (Staphylococcus aureus), a Gramnegative bacterium (Escherichia coli), and a dimorphic fungal yeast (Candida albicans). Selectivity was assessed by determining phototoxicity against human HeLa cancer cells under the same conditions. All four compounds were highly active (6 logs of killing at 1 M or less) against S. aureus and showed selectivity for bacteria over human cells. Increasing the cationic charge increased activity against E. coli. Only the compound with basic groups was highly active against C. albicans. Supporting photochemical and theoretical characterization studies indicate that (i) the four bacteriochlorins have comparable photophysical features in homogeneous solution and (ii) the anticipated redox characteristics do not correlate with cell-killing ability. These results support the interpretation that the disparate biological activities observed stem from cellular binding and localization effects rather than intrinsic electronic properties. These findings further establish cationic bacteriochlorins as extremely active and selective near-infrared activated antimicrobial photosensitizers, and the results provide fundamental information on structure-activity relationships for antimicrobial photosensitizers.

show abstract

“…The yields of the excited triplet state are in the range 0.48 to 0.53. These values are comparable to that of 0.54 for bacteriopheophytin a (17).…”

Section: Vol 54 2010 Bacteriochlorins As Antimicrobial Photosensitisupporting

confidence: 75%

Stable Synthetic Cationic Bacteriochlorins as Selective Antimicrobial Photosensitizers

Huang

Mróz

et al. 2010

Antimicrob Agents Chemother

Self Cite

117

112

View full text Add to dashboard Cite

show abstract

“…79 For organic sensitizers, electron transfer from singlet excited states generally yields low cage escape yields while triplet states yield redox products in quantitative yield. 80,81 The heavy Ru metal center effectively mixes the spin state through spin orbit coupling and cage escape yields that are intermediate between these extremes are generally reported. 79 Outer-sphere adducts formed within the solvent cage may also influence the cage escape yield.…”

Section: A Imentioning

confidence: 99%

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I⁻ Oxidation and I₃⁻ Photodissociation

et al. 2009

View full text Add to dashboard Cite

Direct 355 or 532 nm light excitation of TBAI 3 , where TBA is tetrabutyl ammonium, in CH 3 CN at room temperature yields an iodine atom, I • , and an iodine radical anion, I 2 -• . In the presence of excess iodide, the iodine atom reacts quantitatively to yield a second equivalent of I 2 -• with a rate constant of k ) 2.5 ( 0.4 × 10 10 M -1 s -1 . The I 2 -• intermediates are unstable with respect to disproportionation and yield initial reactants, k ) 3.3 ( 0.1 × 10 9 M -1 s -1 . The coordination compound Ru(bpz) 2 (deeb)(PF 6 ) 2 , where bpz is 2,2′-bipyrazine and deeb is 4,4′-(C 2 H 5 CO 2 ) 2 -2,2′-bipyridine, was prepared and characterized for mechanistic studies of iodide photo-oxidation in acetonitrile at room temperature. Ru(bpz) 2 (deeb) 2+ displayed a broad metal-to-ligand charge transfer (MLCT) absorption band at 450 nm with ε ) 1.7 × 10 4 M -1 cm -1 . Visible light excitation resulted in photoluminescence with a corrected maximum at 620 nm, a quantum yield φ ) 0.14, and an excited state lifetime τ ) 1.75 µs from which k r ) 8.36 × 10 4 s -1 and k nr ) 5.01 × 10 5 s -1 were abstracted. Arrhenius analysis of the temperature dependent excited state lifetime revealed an activation energy of ∼2500 cm -1 and a pre-exponential factor of 10 10 s -1 , assigned to activated surface crossing to a ligand field or MLCT excited state. Steady state light excitation of Ru(bpz) 2 (deeb) 2+ in a 20 mM TBAI acetonitrile solution resulted in ligand loss photochemistry with a quantum yield of 5 × 10 -5 . The MLCT excited state was dynamically quenched by iodide with K sv ) 1.1 × 10 5 M -1 and k q ) 6.6 ( 0.3 × 10 10 M -1 s -1 , a value consistent with diffusion-limited electron transfer. Excited state hole transfer to iodide was quantitative but the product yield was low due to poor cage escape yields, φ CE ) 0.042 ( 0.001. Nanosecond transient absorption was used to quantify the appearance of two photoproducts [Ru(bpz -)(bpz)(deeb)] + and I 2 -• . The coincidence of the rate constants for [Ru(bpz -)(bpz)(deeb)] + formation and for excited state decay indicated reductive quenching by iodide. The rate constant for the appearance of I 2 -• was about a factor of 3 slower than excited state decay, k ) 2.4 ( 0.2 × 10 10 M -1 s -1 , indicating that I 2 -• was not a primary photoproduct of excited state electron transfer. A mechanism was proposed where an iodine atom was the primary photoproduct that subsequently reacted with iodide, I • + I -f I 2 -• . Charge recombination Ru(bpz -)(bpz)(deeb) + + I 2 -• f Ru(bpz) 2 (deeb) 2+ + 2I -was highly favored, ∆G o ) -1.64 eV, and well described by a second-order equal concentration kinetic model, k cr ) 2.1 ( 0.3 × 10 10 M -1 s -1 .

show abstract

“…In the fi rst of a series of articles, Holten et al (50) studied the photochemistry of BPh, with p-benzoquinone (Q) as acceptor, under conditions where electron transfer from the fi rst excited singlet state (BPh*) and the fi rst excited triplet state (BPhT) to Q could be distinguished. Furthermore, they monitored the fo rma tion of the 7r-cation radical (BPht) from its excited state precursors and measured the kinetics of its formation and decay.…”

Section: Picosecond Measurementsmentioning

confidence: 99%

“…Since that time (see 25,50,121) measurements with fl uorescence, microsecond and nanosecond flash photolysis, electron spin resonance, and nuclear magnetic resonance techniques have demon strated that excited triplet states of photosynthetic pigments can donate electrons to acceptors, which results in the production of the radical ions (oxidized donor and reduced acceptor). Seely (104) pyrochlorophyll can be quenched by electron transfer to nitrobenzenes, and that the efficiency of the quenching correlates with the ability of the nitrobenzene to act as an electron acceptor.…”

Section: Introductionmentioning

confidence: 98%

Picosecond Flash Photolysis in Biology and Biophysics

Holten¹,

Windsor²

1978

Annu. Rev. Biophys. Bioeng.

Self Cite

View full text Add to dashboard Cite

Three years ago one of us was asked by a funding agency to dust off his crystal ball and assess the fu ture importance of picosecond studies to chemistry. After a careful "objective" review of what had been accomplished up to then, and trying hard not to appear self-serving, he boldly concluded that a modest increase in the level of fu nding for picosecond instrumental developments appeared to be justified. In the light of what has occurred since then, this recommendation can only be considered cowardly. Almost all application of picosecond techniques in biology and biophysics has taken place in the past 2 or 3 years. Valuable new insights have been gained and in some areas, notably photosynthesis, striking ad vances in understanding have been made. Why? What special magic do picosecond techniques hold for biology? To answer this question we must trace its roots back to chemistry. The vast majority of chemical and biochemical reactions are composed of many elementary steps. The primary steps, which involve energy migration, bond breakage, molecular rearrangements, and charge transfer, all occur in the time range fr om about 10-14 to 10-8 sec. The "slowness" of an overall reaction is usually determined by other considerations, such as steric fa ctors and diffusion of reactants to the site of reaction. Picosecond spectroscopy enables us for the fi rst time to take a look at these primary steps.In this review we fo cus on the new insights that picosecond studies have provided in several important areas of biology. The area that has received the greatest emphasis to date is photosynthesis, which undoubtedly is appropriate, since we are all dependent upon photosynthesis both for our origin and for our continued survival. Picosecond fluorescence studies in both plants and bacteria show that excitation energy migrates fr om the antenna or light-harvesting system to the . reaction center in times that range fr om 10 psec to about I nsec, depending upon 189

show abstract

Electron Transfer From Photoexcited Singlet and Triplet Bacteriopheophytin

Cited by 103 publications

References 26 publications

Stable Synthetic Cationic Bacteriochlorins as Selective Antimicrobial Photosensitizers

Stable Synthetic Cationic Bacteriochlorins as Selective Antimicrobial Photosensitizers

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I⁻ Oxidation and I₃⁻ Photodissociation

Picosecond Flash Photolysis in Biology and Biophysics

Contact Info

Product

Resources

About

Electron Transfer From Photoexcited Singlet and Triplet Bacteriopheophytin

Cited by 103 publications

References 26 publications

Stable Synthetic Cationic Bacteriochlorins as Selective Antimicrobial Photosensitizers

Stable Synthetic Cationic Bacteriochlorins as Selective Antimicrobial Photosensitizers

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I− Oxidation and I3− Photodissociation

Picosecond Flash Photolysis in Biology and Biophysics

Contact Info

Product

Resources

About

Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I⁻ Oxidation and I₃⁻ Photodissociation