Metals in Medicine Flashcards
(136 cards)
1
Q
What are the problems with photofrin?
A
- Drug purity - exist as a mixture of compounds
- Drug targeting and cytotoxicity - localises in both cancerous and healthy cells, slowing leaves body.
- Light interaction - absorbs strongly in blue region, for PDT blue light has little tissue penetration so red light is more efficient.
there is a limitation to shifting the wavelength of light absorption further into the red.The photosensitiser triplet state drops, and eventually is not sufficient to generate singlet oxygen, range = >600 nm - <900 nm
2
Q
what is purpurin and why is it being studied in PDT?
A
Purpurin is a porphyrin derivative found in chlorophyll
It has a strong absorption at 600 nm and long lived triplet excited state so is a good singlet oxygen generator.
When Sn bounds abs shifted by 30 nm so has excellent optical properties.
3
Q
What are metallotexaphryin and advantages and disadvantages?
A
extended porphryins that contain longer bridges between heterocyclic rings.
When M is Zn or Cd, these show excellent PS (photosensitiser) properties.
intense abs at 730-770
high triplet state yields
low fluorescence yields.
4
Q
List the most common elements in the body and elements present in trace amounts.
A
O,C,N and H are the most common. other elements include Fe, Ca, K, P and Na. Zn and Cu exist in trace amounts for enzymes function.
5
Q
Describe the does response for any essential element
A
- survival
- deficiency
- optimal
- toxicity
- lethaility
6
Q
How can unnatural elements be used in treatment?
A
As Antagonism. They function biologically as they are recognised by the body as natural metal elements.
7
Q
what are the attractive properties of metals?
A
- flexible bond orders and geometries - unique geometries an a number of connectivities
- charged (cationic and anionic) so are water soluble.useful for water soluble metallo - intercalations
- paramagentic/ radioactive - used in imaging and trageted radiotherapy
- redox active
- Lewis acids/ electrophillic so can bind to many e rich sites eg DNA base sites
8
Q
what is the therapeutically active form of arsenic?
A
Arsenic oxide
9
Q
How do arsenoxides help HAT?
A
HAT is sleeping sickness. Parasite invades CNS and causes exhaustion and torpor and sleep like unconsciousness. Used a masked form of an arsenic that crosses the blood brain barrier. (draw this compound)
10
Q
how does glutathione protect the cell from oxidative stress?
A
reacts with reactive oxygen species that cause oxidative stress. GSH + ROOR - GSSG + ROH
11
Q
how do trypnosomes work?
A
They carry out the function og glutathione but use tyrponothione antimonal - drugs will interfere with the enzyme that produces antimonal and therefore target the parasite an d not the drug
12
Q
how do trypnosomes work?
A
They carry out the function og glutathione but use tyrponothione antimonal - drugs will interfere with the enzyme that produces antimonal and therefore target the parasite and not the drug
13
Q
how does APL cause cancer?
A
Genetic mutation that leads to the production of a protein that blocked the expression of the genes involved in the differentiation of white blood cells
14
Q
how does ATRA/As203 work against APL?
A
disrupts APL proteins so get differentiation
Arsenic increases ROS conc which leads to apoptosis
15
Q
how does soft tissue imaging work?
A
uses radioactive isotope
16
Q
what is the role of radio pharmaceuticals?
A
Uses gamma photons in single photon emission computerized tomography. Couples 2D images to make 3D images. Radionucletide provide output and pharmaceutical provides selectivity.
17
Q
what are the requirements for diagnostic imaging?
A
Penetrate body - alpha is too ionising and beta does damage.
short half life but live long enough to be detected and synthesised.
Detectable outside the body - needs a camera and have a high emission energy
Should be relatively cheap
18
Q
what is wrong with Carbon (11)?
A
It is too expensive as it has to be made in a cyclotron.
19
Q
what are the benefits of Tc?
A
has 20 known isotopes but 99mTc is most common as it is:
Readily available through 99Mo/99mTc generator
has a 6 hr half life and displays gamma emission at 140 KeV so an expensive instrument is not required
wide range of oxidation states
20
Q
How does the Tc generator work?
A
Mo decays into Tc (Mo from uranium in nuclear fission)
Tc increase charge of nucleus and is less held to the column. Elute column with Nacl and CL ions displace TC ions
21
Q
what are the problems with Tc?
A
Concentration is too dilute, so structure can not be inferred by NMR. the other isotope is used for that. HPLC used to infere the structure of Tc(99).
Chemistry is complex due to variable oxidation states and coordination numbers. Each ligand has to be tailored to each ion
The imaging complex needs to be produced in one step.
22
Q
what are the problems with Tc?
A
Concentration is too dilute, so structure can not be inferred by NMR. the other isotope is used for that. HPLC used to infere the structure of Tc(99).
Chemistry is complex due to variable oxidation states and coordination numbers. Each ligand has to be tailored to each ion
The imaging complex needs to be produced in one step.
23
Q
what are the two classifcation of radiopharmaceuticals?
A
Biological distribution is determined by perfusion or ultimately determined by specific interactions.
24
Q
what is perfusion?
A
Passage of fluid through the circulatory or lymphatic system.
25
there are two way RFC can be if they are biologically distrubuted by specific interaction. what are they?
so RPC can be metal essential - distribution depends on properties of coordination complex
or Metal tagged - properties of carrier molecule determine distribution
26
what are first generation Tc imaging agents?
They are biologically distributed by blood perfusion and this distribution is due to properties of the coordination complex. METAL ESSENTIAL. they target heart, liver, brain, bone and kidneys.
27
what are the two criteria for cerebral imaging agents?
must cross the BBB so must be neutral and fairly lipophillic?
28
how does TcOPnAO form a macrocycle?
It can deprotonate at neutral pH. coordination to metal centre makes it more acidic. Hydrogen bond forms an you get macrocylcle?
29
what is the benfits andproblem with TcOPnAO?
known to corss BBB and be taken up by the brain but diffuses out the brain too quickly
30
why does ceretec and neurolite cross the BBB?
They are lipophillic
31
how important is the stereochem of neurolite in imaging the brain?
V important. If the ester group is pointing down it wont work. wont image brain or even cross BBB
32
How does neurolite accumulate in the brain?
Esterase present in the brain whcih breaks down ester group in neurolite to an CA, this makes it more hydropillic so it accumulates in the brain
33
What do Tc cebral images show?
Using single photon emission computerised tomography.
High Tc conc will appear red indicating good blood flow (2D images) and where there is poor blood flow or non at all will appear as holes (3D image)
34
what are the uses for Tc based spec?
Diagnosing reduced blood flow areas form conditions such as a stroke.
Imagining the locus of epileptic seizures for more precise treatment
facilitating the diagnosis of psychiatric disorders such as schizophrenia.
35
How is cardolite an improvement to t- butylisonitrile?
the latter was too readily absorbed by liver and clearance. the methoxy groups in Cardolite and hydrolysed which increases hydrophilicity and faster liver clearance.
36
How does cardiolite and myoview end uo in the heart
At first monocationic Tc complexes used as K+ ions analogues to be used to ATP driven K+ active transport but now thought to passively diffuse and electrostatically bind to mitochondria.
37
how does Tc-MAG3 image kidney function
the very polar and anionic complex is rapidly transported to the kidnet and cleared from the body through urine. used to test kidney function as it will accumulate in kidney if kidney is not healthy.
38
What are second generation Tc agents?
All biologically distributed by highly specific interactions. They are designed to bind to ther target with high affinities. Distribution may or may not be dependent on the coordination complex itself.
They are classified according to their receptor site or the biological function they target.
39
what are the requirements for targeting neuroreceptors ?
these are associated with a number of disorders. to target sites you need a Tc complex that is the right size, charge and lipophilicity and one that has a binding site.
40
describe the dopamine system?
Dopamine receptor sites called Dopamine transporters (DAT) are found in nuerons involved in dopamine signalling. Dopamine is associated with parkinsons and schizophrenia.
41
how is the dopamine system imaged?
using neutral 99mTc - TRODAT- 1 which crosse BBB and images dopamine receptors.
this consists of diaminodithiol ligand coordinated to a TcO3+ core
42
what is hypoxic tissue and what is needed to image it?
Tissue with low oxygent content, hypoxia is associated with heart disease and solid cancer tumours. Cause it has a low ox content it is more reducing that normal tissue so a redox active marker is needed to image hypoxia such as imidazole.
43
what us wrong with 18F-labelled imidazole imagine agents for hypoxic tissue?
it is too costly
44
why is Bru - 52 better at imaging hypoxic tissue?
More hydrophillic and shows better targeting
45
draw the structure of HL 92 and what is the significance of this compound?
it shows better imaging results that imaidazole derivatives.
46
how are tumors imaged using folic acid?
humans need folic acid for cell division and since cancerous cells are rapidly developing they need more and so overexpress the folate receptor compared to human cells. Attach a folic acid molecule to Tc imaging agent.
47
how are tumors imaged using folic acid?
humans need folic acid for cell division and since cancerous cells are rapidly developing they need more and so overexpress the folate receptor compared to human cells. Attach a folic acid molecule to Tc imaging agent.
48
What are MDR
multidrug resistance. MDR tumors can pump out cytotoxic material They overexpress glycoprotein Pgp which is a trans membrane pump. they are active against lipophilic moncationic complexes
49
what are MDR modulators?
Agents which block the action of PgP
50
how are MDR cells imaged
monocationic Tc complexes used to image the heart can be used as they are subsrtates for pgp recognition. The peripheral methoxy groups enhance pgp recogniton. this has eld to the develpment of complexes that target MDR cells. They are taken up by MDR cells but will only image in the presencs of a MDR modulator
51
Can antibodies bind to natural and non natural targets?
Yes only if they have been engineered to.
52
How are Tc agents bound to antibodies?
By attaching a Tc centre to reduced disulfide linkages. The anitbody is a protein so there are lots of disulphide bridges and this can be reduced so Tc can coordinate.
53
what are the benefits of using antibodies?
They are v specific and bind to their target with high affinity.
54
what are blood clots and what feature means that they can be detected?
aggregation of fibrin, platelets and red blood cells. a D- dimer protein is found on the surface of a active clot (express this once clotting starts)
55
how can Tc labelled antibodies be used to image clots?
An antibody recognizes the D- Dimer and so a lablled version of this anitbody can be used
56
What inspired resenberg to grow bacterial cells in the presence of a fluctuating electromagnetic field
The similarly between the structures is mitosis and the field lines when poles of opposite electromagnetic polarity are brought together
57
What was the experimental conditions of resenberg?
Medium enriched with glucose and MgCl2
Current oscillation 50- 100,000 per sec by a Pt electrode
58
What were the results from Rosenberg studies?
Cell division stopped under application of a current (cells formed long filaments)
But only in presence of Oxygen, so thought that a new oxidising species made
Could it be from Pt???
59
How could the pt complex be formed in the Rosenberg experiment
Pt(IV) salts from the electrolytic process
In presence of light, her diamine complexes (4cl). Only active form when amines are trans to each other
60
what inspired rosenberg to grow bacterial cells under a fluctuating electromagnetic current
the similarity between the structures in mitosis and the field lines when poles of opposite electromagnetic polarity are brought together
61
what was the experimental conditions of Rosenberg study
grown in a medium enriched with glucose and MgCl2
| Application of current 50 -100,000 times per sec by Pt electrodes
62
what can be deduced from his study?
Cell division stopped but only in presence of oxygen so therefore a new oxidising chemical species made. possibly Pt?
63
What was the Pt complex formed in Rosenberg study?
Pt(IV) salts formed from the electrolytic process. In presence of light get diamine complexes (4Cl and 2 Amine) active form when amines are cis to each other
64
what is the active form of cisplatin?
When Cl2 and ammonia groups are cis to each other - Pt(II) or Pt(IV)
65
How is cisplatin used as an anticancer drug?
Stops cell division so can halt growth of cancerous cells
66
what is the evidence that DNA is the cellular target for cisplatin?
Cells treated with one dose of cisplatin showed preferentially inhibition of DNA synthesis compared to RNA and protein synthesis. The effect lasted the duration of the experiment
67
what is the evidence that cisplatin directly interacts with DNA
DNA polymerase not affected by addition of the drug and cisplatin non competitively inhibits the binding of the intercalator ethidium bromide, therefore, Pt is covalently bound to DNA
68
describe the aq chemistry of cisplatin?
The extent of dissociation depends on the ambient chloride concentration.
In plasma: [Cl-] is high so neutral lipophillic cisplatin dominates
In cell: [Cl-] is low so hydrolysis of cisplatin occurs
low charge and lipophilicity means it can cross BBB and pass across membranes
69
How does cisplatin bind to DNA
There are 4 coordination sites for binding Pt adducts. these are N1 on adenine, N3 on cystosine and N7 of guanine. N1 and N3 less available die to H bonding. Since the N7 of G is a stronger Lewis base than N7 in A, Therefore N7 of G is the preferred binding site for Pt
adducts.
70
what are the majority of Pt adducts?
Binding to two sites in a single guanine and disrupting H bonding has not been observed. Intrastrand and DNA protein cross-linking only account for ~6% of all adducts.
Majority of adducts formed are intrastrand d(GpG) lesions.
71
what are the structural effects of intrastrand adducts on a single strand of DNA
1. Guanosine base planes are destacked
2. dihedral angle of bases changed
3. sugar conformation changed
4. Restriction of conformation flexibility of DNA
72
what are the structural effects of intrastrand adducts on double stranded DNA
1. large bend in duplex structure (GG dihedral angle is 59)
2. Structure is part compressed and part open
3. these distortions flatten and open the mino groove - more available for binding.
73
Cisplatin prevent cell division but this does not give rise to its anticancer properties. What does?
Cisplatin prevents transcription (inhibts)
74
how does cisplatin prevent transcription?
High mobility group domain proteins bind to cisplatin.
HMG is a DNA binding motif and is present in a large number of proteins including transcription factors. they bind to minor groove and bend DNA o binding. This means that HMG only binds 1,2 cis -GG and 1,2 cis - GA and not 1,3 intrastrand or transplatin. This is the anticancer property of Cisplatin.
75
what are the problems with cisplatin?
1. Emetic and toxic
2. Delivered IV, requires hospital stay
3. it has low aq solubility, reaching limit of cytotoxic drugs delivered into blood stream
4. Many tumours become resistant to it or develop resistance during treatment
76
which oxidation states of Pt are active in 2nd generation Pt drugs?
Pt(III) and Pt(II)
77
what is the active form of 2nd gen Pt drugs?
cis-[PtX2(Am)2] or cis-[PtX2Y2 (Am)2].
amine is inert and has at least one NH moiety.
X is an anion and has an intermediate binding strength to Pt with weak trans effect. If ligands are too labile then drug is toxic but if they are inert then drug is inactive
78
draw carboplatin and give its strengths and weaknesses over cisplatin
the chelating dicarboxylate groups mean slower aquation, less toxicity and better water solubility. All mean it can be administered in higher doses.
But tumours have shown cross resistance to cisplatin and carboplatin
79
Draw oxaliplatin and give its strengths and weakneses
all strengths as carboplatin as it also has a dicarboxylate group. The cyclohexane means lack of cross resistance with some cisplatin resistance cancer.s
can be given in higher doses - 2 x
80
criteria of satraplatin?
must adhere to structure function relationship. show good water sol and relatively lipophillic
81
what is the mechanism for drug resistance
Glutathion (GSH) coordinating to drug. Coordination of ligand in sq planar Pt complexes via an associative mechanism.
82
how can Pt drugs be engineered to combat drug resistance?
Sterically hinder the Sq plane
83
draw the structure of picoplatin and gives its benefits and cons?
slower substution than cisplatin and reacts slowly with DNA, but does fully platinate complex. Displays enhanced toxicity to cisplatin resistant cancers
84
Can trans platinum drugs be therapeutic?
They do bind to DNA but are too labile so are deactivated.
85
Draw a trans Pt drug that shows good in vivo activity even with cisplatin resistant cancers.
BBR3464
86
Why do we expect multinuclear pT complexes to form different DNA lesions?
They are oligometallic and charged, also trans
87
Even though multiuclear pt complexes are highly charged how do they cross cell membranes?
They are relatively lipophillic
88
Even though multiuclear pt complexes are highly charged how do they cross cell membranes?
They are relatively lipophillic
89
what do DNA binding show with BBR3464?
1. rapidly binds to DNA, the half life is 40 mins
2. shows bifunctional DNA binding
3. Forms more intrastrand lesion than cisplatin
4. also forms long range intrastrand lesions
90
what is the problem with BBR3464?
Too active and has been liked to renal toxicity
91
what are the therapeutic properties of cis-(DMSO)4RuCl2?
Shown modest antitumour activity and cos for shows modest activity against solid tumours. slight inhibition on metastasis. Shows no cross resistance with cisplatin
92
How is Ru(II) binding switched on cancer cells?
Ru (III) preferentially transported by Fe carrier proteins. Hypoxic tissue is realtovely reducing due to low oxygen content. Ru(III) - Ru (II). Ru(II) with pi donor ligands are stabilised by back bonding.
93
what are NAMI and KP - 1019?
prevent metasis
94
What is the oxidation state of the metal in Nami and KP 1019
Ru(II) it is stabilised by reducing conditions
95
what is the evidence that KP 1019?
As oxygen conc decreases binding to DNA increases
96
Briefly describe how NAMI works?
Antimetastatic agent. Increases the thickness of connective tissues around tumor and its blood vessels
97
What is the known about the physiological of NAMI
Only a small fraction of NAMI reaches a tumour
• Its activity is largely independent of intracell concentration
• Mechanism DOES NOT involve its DNA binding properties
• It stops cell dividing, prevents blood supply to tumour developing
98
What is the known about the physiological of NAMI
Only a small fraction of NAMI reaches a tumour
• Its activity is largely independent of intracell concentration
• Mechanism DOES NOT involve its DNA binding properties
• It stops cell dividing, prevents blood supply to tumour developing
99
What Ru(II) compounds are used in chemotherapy?
Ones that are facially coordinated found to be as cytotoxic as carboplatin. they bind to G bases in DNA and form monoadducts
100
what are the features of DNA that makes them targetable?
Anionically charged phosphodiester backbone
Major and minor grooves
base pair stacks
101
what are the three ways in which things bind to DNA
Electrostatic - DNA naturallt exists as a polyanion so cationic metals eg mg can form loose complimentary electrostatically interactions with polyamines in the phosphodiester backbone. little scope for binding sequence specifity
Groove Binding - molecules approach within the VDW contact and reside in DNA groove. Hydrophobic and/Hydrogen bonds play key role in binding and provide stabilisation. Major and minor groove binders are known they show high affinity and can display good sequence selectivity, without major structure change in DNA.
Intercalation - Polycyclic aromatic rings - get stacking between base pairs. This lengthens and untwists DNA
102
Why use metals complexes in DNA binding?
1. charged so are water sol
2. Cationic so show enhanced DNA binding
3. Metal can acts as a scaffold and arrange ligands in optimal positions for DNA binding
4. Can introduce redox and photchemically functinalities to DNA binding system
103
what are Possible medical/biotechnological applications of molecules that reversibly bind to DNA
New treatment of cancer and genetic disease
• Medical probes for specific sequence/structures of DNA.
•New diagnostic agents testing for specific sequences
• Perhaps even gene modulation
104
what did early work with Ru(phen)3]2+ show about octahedral complexes and DNA binding
Cation binds DNA by hydrophobic contacts in major grrove and Phen may intercalate base stacks but the latter disproved
prefer to bind the right handed isomer
105
what is the significance of [Ru(bpy)2(dppz)]2+
Showed high binding affinity to DNA but not specificit
| These are DNA light switches they are not luminescent in solution until DNA added
106
why is [Ru(bpy)2(dppz)]2+ not specific?
NH acceptor groups
107
what forms does [Ru(phen)2(dppz)]2+ resolve into
Λ-form and Δ form both of which show different luminescent lifetimes
108
how binding modes does the Λ-form of [Ru(phen)2(dppz)]2+ show
2 - major (binds perpendicular) and minor (binds parallel)
109
what is the evidence that changing ancillary ligands can control DNA binding?
Adding methyl's groups to [Ru(phen)2(dppz)]2+, the complex can not bind DNA. cause on unfavourable steric interactions between the ancillary groups and DNA
110
why does the achiral derivative show the same affinity for binding DNA as [Ru(bpy)2(dppz)]2+ but is more specific?
Binds to GC rich DNA sequences due to favourable anciliary and DNA interactions
111
describe the binding of [Rh(phen)2(phi)]3+
intercalate with the long axis of the phi ligand parallel to the long axis of the base pair.
Photoexcitation leads to redox cleavage of DNA at site of intercalation
binds with some sequence specificity (CCAG - 3`)
Slots into base pairs unlike dppz which binds at right angles
112
what is the effect of adding hydrogen bond donors onto ancillary ligands in phi complexes
Get higher binding specifity with a preference for 5- CG -3 compared to analogues containing only S showed no preference
113
The phen based ligand in this complex
| contains a what group
guanadinium group - a moeity known to bind G N7 and O6 atoms. so these would target a more extended sequence
114
what base pair sequences does the two different forms of the molecule able to recognise extended sequence bind to
Δ-form showed a significant preference for a 6
base pair sequence; 5’-CATCTG-3’, while Λ-
form recognised 5’-CATATG-3’
115
what have Consequent studies Λ-form shown (extended sequences)
binds with affinities that are comparable to
transcription factors and can even selectively inhibit transcription factor binding when the target contains the 5’-CATATG-3’ sequence
116
```
what is the complex containing the 5,6-
chrysinequinone diimine (chrysi) ligand
designed to be
```
too large to easily intercalate with normal base pairs so is used to target mismatched sites. The extra ring prevents binding to base pairs
117
what are the three types of mismatch in DNA
wrong base so no base pair formed
XS base so there is a bulge in the backbone
Missing base - no bulge, DNA is still a duplex and backbone is right length.
118
how are mismatched cells detected?
Mismatch recognized by insertion of the
intercalator.
= The rigid inserting ligand displaces the
flexible mismatch base-pair
119
How can mismatch base pairs be used to treat cancer?
Complexes shown below are cytotoxic to cells
| that are deficient in mismatch repair (~20% of solid cancers show this deficiency)
120
why is it important to Understand the dynamic organisation and structure of biomolecules like DNA
within living cell.
will provide an insight into important cellular process and many genetically based diseases - including cancer
121
What is required to image cells and organelles?
A stain
122
What are the problems with current organic dyes?
1. Low water solubility
2. photbleaches easily - can kill cells and reduces time to look at cells
3. small stoke shifts
4. often toxic
5. Emission lifetime is slow, cant tell from indogenous flourphores
123
why are metal better stains?
1. large stoke shift
2. more photstable
3. longer emission lifetimes as from triplet state
124
how are complexes used to image quadruplex DNA
guanine can form tetrad structures, which may function in forming four stranded DNA. Using groove binding light switch complexes which are desogned to stain duplex and quadruplex DNA. They give different emission signals.
125
What is the emission wavelength of quadruplex DNA?
620 - 640
126
how can imaging probes be used in cytoxicity?
Intercalting light switch complexes, imaging propeties allow the mechanism of cytotoxicity to be easily probed.
127
what did injections of hematoporphyrin lead to?
Led to florescence in tumours
128
how does PDT work
Excitation of molecules can lead to many processes
| Draw graph
129
what is PDT used to treat?
To treat cancers and skin conditions
130
what was found about cationic metalloporphyrins?
Cytotoxic to cancer in vitro. But DNA scission only observed when oxygen present so toxicity thought to be PD
131
how are texaphyrins used in PD
Based on lanthanide Lu. previous Cd complexes have poor water solubility and are toxic.
texaphyrin are water soluble and non toxic, they have a absorption max at 730nm
132
why are Pc being used as photosensitisers as PDT?
They have a strong absorption at 670nm
133
how are Metalloporphyrins conjugates cytotoxic
Cisplatin/PDt conjugates. In the dark they are more cytotoxic than carboplatin and cisplatin. But when light is switched on they are even more cytotoxic (more so than Hp and cisplatin mixtures)
134
what is the function of NO in the body
NO is a signalling molecule but high levels can become harmful.
Macrophages produce high local NO concentrations (>10 µM) which are cytotoxic to the pathogen/cancer
cells.
NO reacts with O to for N2O3 which initrosation agent attacking ammines and thiols.
NO also reacts with peroxide (macrophages produce this also). They form Peroxynitrile (O=NOO–) which is an oxidising agent capable of destroying most biomolecules
135
How can NO be used in PDT
complexes will release NO in toxic levels when when photoexcited with long wavelength visible light.
136
By describing the photo-induced processes involved, with the aid of diagrams outline why PDT requires these three components (presence of light, oxygen and a photosensitizer)
In the PDT process light photo-excites the sensitizer into a singlet state that then undergoes ISC into a triplet state. This activated excited state can react directly with biomolecules but better still is the sensitization of ground state triplet state dioxygen either into a reactive radical species (type I mechanism) or, even better again, into the reactive singlet state (type II mechanism). The final mechanism is preferential as the sensitizer acts a catalyst to generate singlet oxygen:
