Senge's Photochem Flashcards
(24 cards)
What does hv do to a double bond?
Cis/Trans isomerisation - rotation requires breakage of double bond as alkene is not freely rotatable
Colour vs Pigmentation
Colour (think of dyes) - chemical structure is that colour, absorption of light and emission of light as photons
Pigmentation - reflection/diffraction of light in certain way so that it appears a colour
What type of structures are generally able to absorb and emit light?
Rigid Structures - cannot disperse energy through vibrational or rotational means, have to disperse as photons.
May contain carbonyl groups
Normally large structures
Conjugated systems
Solvatochromism
Instead of mixing two colours to get colour you want from dyes, can sometimes absorb in different polar solvents where the dipole energy stabilises ground state thus increasing energy of HOMO LUMO gap and therefore the colour of the dye!
What state does oxygen exist at in room temp?
Triplet state as the triplet state is lower in energy than the ground singlet state. To undergo a chemical reaction, triplet oxygen has to undergo the spin forbidden transition to a singlet state.
Fluorescence vs phosphorescence
Fluorescence - emission of electromagnetic radiation within about 10^-8 seconds after absorption
phosphorescence - slow emission of electromagnetic radiation as electrons drop to triplet state and then to ground singlet. It is slower because these are spin forbidden transitions.
How does photorepair work in our bodies?
As light hits DNA, photomutation may occur and “wrong” base pairs added creating this ‘bulge’ in the dimer. Occurs millions times a day. CPD photolyase binds to damaged dimer, and repairs via photolyase -absorption of light >300 nm (violet/blue end of spectrum) and refills with complementary base pairs and unbinds.
How do humans protect their skin from sun damage?
Absorbers of UV radiation,
Reflectors of UV radiation (sunscreen)
Anti-oxidants (help repair mechanism)
Stilbene Photoisomerisation
E vs Z stilbene - Opsin (protein) contains retinal in eye, in dark cis double bond, in light is trans, Change in shape changes shape of protein which changes shape of next protein etc., until it hits tunnel protein.
Tunnel protein
Na+/K+ pump in neurons, something that pumps protons of cations to create an electrochemical gradient which causes neurons to fire.
How can energy absorbed by light be dissipated?
- Light to light - fluorescence, phosphorescence
- Light to chemistry - Chemical change e.g. singlet or triplet state.
- Light to heat - Physical - Internal conversion or inter system crossing.
Photosensitisation
Doing photo chemistry with a compound that doesn’t absorb light, e.g. use a compound that is able to absorb light and let it affect the compound we want to change - e.g. with triplet oxygen, absorb light turn to singlet oxygen, substrate is then oxidised by higher energy oxygen and get an oxidised substrate.
Dyes structure vs fluorescent dyes
dyes - have extended conjugated pi system, molecular structure determines colour. Absorption of light energy and conversion to vibrational energy (heat) - i think absorb energy opposite on colour wheel to what we see
Fluorescent dyes - more rigid structures and therefore conversion to vibrational energy not possible therefore release energy in form of fluorescence.
Hypsochromic shift
Blue shift - higher energy, lower wavelength shift
Bathochromic shift
Red shift - lower energy, higher wavelength
Hyperchromic vs Hypochromic shift
Hyperchromic shift - positive, increased absorption, hypochromic = decreased
Types of photosensitisation
Type 1 - Activate sensitizer with light, make triplet sensitiser. Sensitiser reacts with substrate to form radical substrate and radical sensitizer, then reaction with triplet oxygen to form oxidised products.
Type 2 - Activate sensitizer with light, make triplet sensitiser, react with triplet oxygen first to make singlet oxygen and that oxidises substrate.
Which functional groups go through pi to pi* transitions (singlet or triplet) upon exposure to hv
Aromatics and polyenes
Which functional groups go through n to pi* transitions (singlet or triplet) upon exposure to hv
Mainly carbonyls
What is LLLT
Low Level Light Therapy
Describe Neonatal jaundice blue light treatment and why is it necessary
When bruised - blood vessels burst releasing red blood cells into tissue, cells slowly degrade and haemoglobin released, proteins on their own aren’t very stable and start to degrade.
Porphryin and light - form singlet oxygen which is TOXIC, therefore the ring breaks open to form bilverdin and then further reduction gives bilirubin. Bilirubin is initially formed as Z alkene but under blue light (460-90 nm) it goes to E bilirubin which is passed safely from body.
UNSURE: i think that that isomerisation allows more solubility in water to be passed from body. Diseases associated with this is indication of liver failure. Difference between biliverdin and bilirubin is like one double bond in the difference
PUVA
Psoralen and UV-A Treatment - organic molecule, light and oxygen - uv activates chromophore which produces oxygen which takes part in photochem reaction which kills bacteria causing disease
What is Porphyrias?
In body, we synthesize uroporphyrin 3 which turns into iron(II) protoporphryin heme (which is not a photosensitizer). Feedback inhibition from uroporphyrin 3 tells enzyme ALA to stop making uroporphyrin, however sometimes there is wrong ‘AP’ configuration (uroporphyrinogen 1) which because of its different configuration, doesn’t tell ALA to stop making molecule. This uroporphyrin is sensitive to light and insoluble, reacts iwht light to form singlet oxygen which is TOXIC (but only on exposure to light). Causes red urine among other side effects.
Photodynamic cancer therapy
Inject with photosensitiser, may make it targetted to accumulate in cancer cells. Hit with light which forms singlet oxygen and cell death. Want intense absorption in red or nIR region (greater tissue penetration) Limitations : low penetration in skin.
