Relaxation methods

A chemical equilibrium is thrown off-balance, and we measure the rate of reaching the new equilibrium.

E.g.: temperature jump,

Measurement of the rate of protein folding:The fluorescence of tryptophane (ns time-scale) is influenced by the polarity of the medium. Protein folding tends to exclude water from the globule. Principle of the measurement: the protein is defolded by cooling, then a temperature jump returns the temperature to physiological conditions. The tryptophane fluorescence is measured on the ms timescale, thus we can follow the change of polarity near the tryptophane.

Revival of the temperature jump method

Time-correlated single photon counting

Instead of measuring the decay of fluorescence in real time, we measure the time between the excitation and emission, and based on the statistics we estimate the lifetime of the fluorescence.

Photochemistry of the >C=C< chromophore: cis – transphotoisomerization

Photostationary state

cis trans

tc

ct

cis

trans

trans

cis

][

][

Stilbene (1,2-diphenylethane), 313 nm:93%cis, 7% trans, since cis 2400 dm3mol-1s-1, trans 16200 dm3mol-1s-1, ct = 0,35, tc = 0,5

Additive colours

Substractive colours

How the human eye works

Electrocyclization

Practical example: synthesis of vitamin D

Stereochemistry of cycloaddition

Cycloaddition within DNA

Relative weight of DNA photodamage

Chemistry Relative weight

Thymine dimer 1

Cytozin hydration 0,5

DNA-protein crosslink 0,0017

DNA crosslink 0,00025

Strand break 0,00025

Photochemistry of proteins

Amino acid Absorbance

at 254 nm

Quantum efficiency

effectiveness

ε·Φ·104

Cystine(-S-S-) 270 0,13 35,1

Tryptophane 2870 0,004 11,5

Phenylalanine 140 0,013 1,8

Thyrozine 320 0,002 0,6

Correlation between enzyme photoinactivation and cystine content

Spectrum of the main photoproduct in the lens

Thephoto-

chemistryof beer:

thiolproductioninduced by light