Chiroptical Spectroscopy

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Chiroptical Spectroscopy

Compiled by Krisztina Pál

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Introduction

• Chiroptical spectroscopic methods enable us to distinguish enantiomeric/diastereomeric forms of chiral compounds

– Polarimetry (optical rotation at a selected wavelength),

– ORD /optical rotation dispersion/ spectroscopy (optical rotation as a function of wavelength)

– CD /circular dichroism spectroscopy/ (difference of the absorption of light polarized circularly left-handed and right-handed)

• Chiroptical spectroscopic methods are based upon the interaction between the optically active sample and the polarized light.

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Optically active or chiral materials

• capable of rotating the plane of vibration of polarized light to the right - Arago, quartz crystal (1811)

– Biot, tartaric acid aqueous solution (1838)

– Le Bel és van't Hoff (1874); asymmetric, tetrahedral C-atom

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Aimé Cotton (1869 - 1951) discovered

Optical rotatory dispersion = ORD Circular dichroism = CD

spectroscopic methods for the

determination of stereo structures of chiral compounds

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Chiral: not superimposable on its mirror image.

Origin of word ‚chiral’ is kheir meaning hand in Greek.

Such mirror structures are related as right and left hands.

Optical activity = the plane of polarization of linearly

polarized light is rotated as it travels through certain materials;

samples of chiral molecules are optically active (except racemic mixtures)

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Enantiomers: two stereoisomers that are mirror images of each other

•Symmetric environment: identical physical and chemical properties

•Asymmetric environments:

different properties

Relationships of chiral molecules

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Properties of enantiomers

• Melting point, boiling point, indece of refraction, solubilities – same values

• Identical UV-VIS, IR, NMR spectra

• Different behavior in interaction with chiral agents:

– Different solubilities in chiral solvents – Different reactivities with chiral reagents

(diastereomer salts, basis of resolvation) – Different interaction with ‚chiral’ light

• (diastereomers: isomers of an optically active compounds with two or more stereocenters. The

configuration of at least one center is identical, at least one is different.)

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Types of molecular chirality

C COOH

CH₃ NH₂ H

helical chirality:

hexahelicene

O

OH OH OH

HO OH

O

OH OH OH

HO OH

CHO HO H

H OH HO H HO H

CH₂OH

chiral center

axis of chirality

plane of chirality

kumulén

biaril

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Polarized light

The electromagnetic

radiation is a propagating wave (light) consisting of an oscillating electric and

magnetic field which are orthogonal to each other. In general, the oscillation of the electric field is not restricted to a fixed plane (xy), i.e. the radiation is unpolarized.

x y

z

Unpolarized light

Plane

polarized light

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Plane polarizer

Unpolarized light

The vertically polarized light passes the polarizer

The horizontally

polarized component is absorbed by the filter

A filter converting the unpolarized light into plane polarized light.

Polaroid filter – simplest plane polarizer made of a stretched polymer of oriented chains, which contains an absorbing

compound.

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Optical rotation

• The index of refraction of a chiral sample is different for the two components of plane polarized light (right- and left-handed circularly polarized light)

• n_left≠n_right

• Thus, the velocities of the two circularly polarized components are different, as the plane polarized light passes through the chiral sample

• n=c/v

• Consequence: the plane of polarization of the plane polarized light rotates (by angel  )

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Polarimetry: Measuring the angle of rotation of the plane of polarization at a defined wavelength

/589nm (Na D line)/

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Optical rotation

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Optical rotatory dispersion (ORD)

• The optical rotation as a function of

wavelength (dispersion = dependence on wavelength)

Light source

(lamp + diffraction grating)

Detector

(photoelectron multiplier)

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ORD spectrum

• Plain (monotonously changing) curve – no chromophore in chiral molecule – no absorption

• Cotton effect ‚anomalous curve’ – absorbing chiral molecule

The wavelength at the intercept between the + and – bands is ~ _max in the

absorption spectrum

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Circular dichroism (CD) spectrum

• Circular dichroism: the

absorptions of the right- and left-handed circularly

polarized lights are different

• CD spectrum:

A() = A_left () – A_right (), or  () = _left () – _right ()

=190-800nm, electronic

A A_right

A_left

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CD spectra of enantiomer pairs

 left,(R)= right,(S),  left,(R)= right,(S)

 left, (R)- right,(R) = _(R)=  right, (S)- left,(S) =  _(S)

 left, (S)- right,(S)= _(S)=  right, (R)- left,(R)= _(R)

Chemical structures

CD spectra mirror images

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CD spectrometer

(spectropolarimeter)

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• abs. sp: band

• CD sp: signal

• ORD sp: Cotton effect

• If no abs  no CD signal  plain ORD curve UV, ORD, CD

connections

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Applications of CD spectroscopy

• Determination of enantiomer purity

• Determination of absolute configuration

• Induced CD signal: binding of an achiral molecule to a chiral molecule – CD bands in the absorption range of the achiral

molecule may appear

• Protein CD spectra: secondary structure.

conformational changes

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Enantiomeric excess (ee%)

100 100

 

 

 

 

S R

R S

S R

c c

c ee c

c c

c ee c

Experimental determination

 

  ¹⁰⁰

)

(  

 pureenantiomer

sample ity

opticalpur

ee 



 

  ^¹⁰⁰



 

omer pureenanti

sample ee 



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Importance of optical purity of chiral drugs

• 40% of currently produced drugs are chiral

compounds, many of wich are distributed in form of racemic mixtures

• The pharmacokinetics of enantiomeric pairs can be different, usually they act on different receptors

• In many cases only one of the enantiomers

provides the desired clinical effect, the other

enantiomer is uneffective or has adverse effects

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Thalidomide is a chiral drug molecule, marketed first in 1957, under brand name

Contergan

• It was an efficient sedative to cure panic disorder, anxiety, sleeplesness, psychic traumas and also nausea of pregnant women

• Many babies were born with limb defiencies

Contergan-scandal (thalidomide)

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• (R)-isomer (S)-isomer

teratogene efficient sedative

(may cause birth defects)

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Determination the absolute

configuration by CD spectroscopy

• (1): Comparison of CD spectra: Compound with unknown configurationsimilar compound with

known configuration: CD spectra are similar are close to mirror images

• (2): Empirical rules: The sign of some CD signals can be realted to the absolute configuration (e.g. octant rule)

• (3): Quantum chemical calculation of CD spectrum and comparison of the experimental and theoretically

calculated spectra

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Comparison of CD spectra

O O

H H

(2R,3S)

(-)-2a, (2R, 3S) absolute configuration

O O H

H O O H

H

CH₃

Identical signs of bands suggest identical absolute configuration

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Octant rule for ketones

• n→π* transitions of C=O groups falls to

~ 300 nm. It

produces a weak band in UV abs.

spectrum and a

stronger band in CD spectrum. The sign of the CD band is related to the abs.

conf. of chiral ketones

O

x

y

z O

The 3D model of the molecule is arranged in a coordinate system, with the center of the C=O

The sign of the octant including the majority of the atoms and the sign of the C=O band in the CD spectrum will be the same.

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Application of octant rule

(-) (+)

(+) (-) The majority of the

atoms fall in a (-) octant, predicting that a molecule of this stereostructure has a (-) carbonyl CD band.

(Contributions of

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Induced CD effect

• Chiral host (e. g. cyclodextrin) or binding site (enzyme, DNS)

• Achiral, chromophore guest (dye probe, drug molecule)

• Achiral chromophore may become distrorted in a chiral structure or its electronic transitions are

chirally perturbed – a CD signal is induced in the

absorption range of the chromophore

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Induced CD

cis-parinaric acid (achiral guest, chromophore)

-lactoglobuline (chiral host)

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CD spectroscopy of proteins

• Environmental changes (pH, temperature, etc.) → conformation changes

• CD spectrum sensitive to conformation

• CD spektroscopy is a key technique in protein

studies (denaturation studies, protein binding studies)

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Secondary structure

• The CD signals of

proteins are in the far UV region (180nm - 260nm) – they arise from the

transitions of amide groups

• CD spectrum is affected by the relative orientations of the amide groups → CD spectrum depends on secondary structure

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CD spectra of ‚pure’ secondary structures

• The CD spectrum of a protein can be considered the linear combination of the spectra of secondary structures

-helix

disordered

structure -sheet

-turn

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Linear combination

-helix

-sheet



^⁽^⁾



 ^



^ ⁽^⁾



 ^



^ ⁽^⁾



 ^r

^

^r ⁽^⁾

^

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Thank you for the attention!