Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated electron systems.

Ultraviolet-Visible (UV-VIS) Ultraviolet-Visible (UV-VIS)

SpectroscopySpectroscopy

Gives information about conjugated electron systems

gaps between electron energy gaps between electron energy levels are greater than thoselevels are greater than thosebetween vibrational levelsbetween vibrational levels

gap corresponds to wavelengthsgap corresponds to wavelengthsbetween 200 and 800 nmbetween 200 and 800 nm

Transitions between electron energy statesTransitions between electron energy states

EE = = hh

X-axis is wavelength in nm (high energy at left, X-axis is wavelength in nm (high energy at left, low energy at right)low energy at right)

maxmax is the wavelength of maximum absorption is the wavelength of maximum absorption and is related to electronic makeup of molecule— and is related to electronic makeup of molecule— especially especially electron system electron system

Y axis is a measure of absorption of electromagnetic Y axis is a measure of absorption of electromagnetic radiation expressed as radiation expressed as Absorbance or molar absorptivity (Absorbance or molar absorptivity ())

Conventions in UV-VISConventions in UV-VIS

UV and Visible light cause only two kinds of electronic transitions

• Only organic compounds with electrons can absorb energy in the UV/Visible region• A visible spectrum is obtained if visible light is absorbed• A UV spectrum is obtained if UV light is absorbed

Most stable Most stable -electron -electron

configurationconfiguration

-Electron -Electron configuration of configuration of

excited stateexcited state

* Transition in cis,trans-1,3-cyclooctadiene* Transition in cis,trans-1,3-cyclooctadiene

HOMOHOMO

LUMOLUMO

EE = = hh

* Transition in Alkenes* Transition in Alkenes

HOMO-LUMO energy gap is affected by HOMO-LUMO energy gap is affected by substituents on double bondsubstituents on double bond

as HOMO-LUMO energy difference decreases as HOMO-LUMO energy difference decreases (smaller (smaller EE), ), maxmax shifts to longer wavelengths shifts to longer wavelengths

Substituent EffectsSubstituent Effects

Methyl groups on double bond cause Methyl groups on double bond cause maxmax

to shift to longer wavelengthsto shift to longer wavelengths

CC CC

HH

HH

HH

HH

CC CC

HH

HH CHCH33

maxmax 170 nm 170 nm

CHCH33


A chromophore is the part of a molecule which absorbsUV or visible light

Effect of Conjugation on max


Extending conjugation has a larger effect Extending conjugation has a larger effect on on maxmax; shift is again to longer ; shift is again to longer

wavelengthswavelengths

CC CC

HH

HH

HH

HH

CC CC

HH

HH

maxmax 170 nm 170 nm maxmax 217 nm 217 nm

HH

CC CC

HH

HHHH

UV Spectrum: (+)-carvone

0

0.5

1

1.5

2

2.5

3

3.5

200 220 240 260 280

Wavelength (nm)

Series1

O

C

H

3

C

H

3

C

H

2

What is carvone’s What is carvone’s maxmax??

236 nm236 nm

The Beer–Lambert Law

The molar absorptivity of a compound is a constant thatis characteristic of the compound at a particular

wavelength

A = log(I / I0)c = concentration of substance in solutionl = length of the cell in cm = molar absorptivity

A = cl

200200 220220 240240 260260 280280

10001000

20002000

Wavelength, nmWavelength, nm


maxmax 2630 2630

MolarMolar

absorptivity (absorptivity ())

UV Spectrum of cis,trans-1,3-cyclooctadieneUV Spectrum of cis,trans-1,3-cyclooctadiene



(conjugated (conjugated dienediene))

HH

CC CC

HH

HH CC CC

HH

HHHH

CC CC

HH CHCH33

HH

HH

CC CC

HH33CC

HH CC CC

HH

HH


conjugated conjugated trienetriene plus plus two methyl groupstwo methyl groups

Both the max and increase as the number of conjugated double bonds increases

An auxochrome is a substituent in a chromphore thatalters the max and the intensity of the absorption

Uses of UV/Vis Spectroscopy

• Measure the rates of a reaction

• Determine the pKa of a compound

• Estimate the nucleotide composition of DNA

The Visible Spectrum and Color

VisionVision

Biological / Physiological response

to light stimuli

The Eye and Vision

The Eye and its Neurological Wiring

Vision: Receptive Field (RF)

• Definition: the area of the retina (or visual field) in which light signals evoke responses

• It’s a property of the cell, not a cell or a part of the cell

• It depends largely on the synaptic inputs to the cell and to some degree the biophysical property of the cell itself

Projection from retina to LGN

fixation point

fovea

• Nasal RGC: axons crossover, project to contralateral LGN

• Temporal RGC: axons stay on the same side (ipsilateral)

• Left visual field: right LGN, right V1

• Right visual field: left LGN, left V1

Visual pathway from retina to V1

Optical imaging of orientation map

Retinotopic map

neighboring cells have neighboring RF

retinotopic map is true in the retina, LGN and V1, but it gets fuzzy as you move on to higher visual areas

abc

ab

c

objectretina

Project orderly to LGN and V1

The Retina & Photoreceptive Cells

Structure of the eye

The Basic Retinal Circuit

1. Receptor Cells(Graded potential)

(input)2. Bipolar Cells(Graded potential)3. Ganglion Cells(action potential)

(Output)

Different cells in the retina

Back of eye

Front of eye

4. Horozontal Cells(Graded potential)5. Amacrine Cells(Graded/action potential)

6. Pigment cells

2

light

Fovea: high spatial resolutionPeriphery: low spatial resolution

Direct pathway: Photo receptor Bipolar RGC

Photoreceptor

On bipolar

Off bipolar

On RGC

Off RGC+

-

+

+

Direct pathway is responsible to the RF center

+: excitatory synapse, preserve response direction

-: inhibitory synapse, flip response direction

Indirect pathway

Photoreceptor PhotoreceptorHorizontal cell+ -

Indirect pathway mediated by horizontal is responsible to the RF surround

There are other indirect pathway mediated by amacrine cells

Photoreceptor

On bipolar

Off bipolar

On RGC

Off RGC+

-

+

+

Direct pathway

Photoreceptor PhotoreceptorHorizontal cell+ -

Indirect pathway

Pathways mediated by amacrine cells

Summary of retinal circuit

The Nobel Prize in Physiology or Medicine 1981:

Roger W.Sperry: for his discoveries concerning the functional specialization of the cerebral hemispheres

David H. Hubel & Torsten N. Wiesel: for their discoveries concerning information processing in the visual system

Terpenes in VisionCan diet affect sight?.....It might depend on what you’re

looking at.

• Color vision vs. B&W: Rods vs. Cones • The Photochemical Process

� Lycopenes� Carotenes� Vitamin A� Retinol / Retinal

Humans only see in B&W at night, but some animals like the nocturnal hawkmoth see color. Nature, 922-25, 2002

LycopeneLycopene


orange-red pigment in tomatoesorange-red pigment in tomatoes

Lycopene

Beta Carotene "Pro vitamin A"

OH

HO

Vitamin A

Cis-Trans Isomerization & Vision

B&W ChemistryStep One: Oxidation

OH

CHO

B&W ChemistryStep Two: Trans -> Cis Isomerization

CHO

CHO

B&W ChemistryStep Three: “Hooked on Opsin”

CHO

NH2

Opsin

C N

B&W ChemistryStep Four: “Flash”

C N

C N

picosec~ 5 photons~ 3

QuickTime™ and aVideo decompressor

are needed to see this picture.

B&W ChemistryStep Five: “Repeat Process”

C N

NH2

Opsin

CHO

Vision exercise

http://ep.llnl.gov/msds/orgchem/Chem227/227assign03.html#Vision:

http://ep.llnl.gov/msds/orgchem/Chem227/227assign03.html

Terpenes

•Terpenes are natural products that form from a pyrophosphate and are structurally related to isoprene.

HH22CC CC

CHCH33

CHCH CHCH22

IsopreneIsoprene(2-methyl-1,3-butadiene)(2-methyl-1,3-butadiene)

OP

OP

O

OO

O

O

Isopentenyl pyrophosphate Isoprene

The Isoprene Unit

• An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds)

Two isoprene units:Two isoprene units:

The Isoprene Unit

• The isoprene units are joined "head-to-tail."

headhead tailtail

tailtail headhead

Isoprene LinksHeads or Tails?

Common Carbon Backbone

Terpenes

•Class Number of carbon atoms

•Monoterpene 10

•Sesquiterpene 15

•Diterpene 20

•Sesterpene 25

•Triterpene 30

•Tetraterpene 40

Classification of TerpenesClassification of Terpenes

Terpenes

Representative MonoterpenesRepresentative Monoterpenes

-Phellandrene-Phellandrene(eucalyptus)(eucalyptus)

MentholMenthol(peppermint)(peppermint)

CitralCitral(lemon grass)(lemon grass)

OO

HH

OHOH

Terpenes

Representative MonoterpenesRepresentative Monoterpenes

-Phellandrene-Phellandrene(eucalyptus)(eucalyptus)

MentholMenthol(peppermint)(peppermint)

CitralCitral(lemon grass)(lemon grass)

Terpenes

Representative SesquiterpenesRepresentative Sesquiterpenes

-Selinene-Selinene(celery)(celery)

HH

Terpenes

Representative SesquiterpenesRepresentative Sesquiterpenes

-Selinene-Selinene(celery)(celery)

Terpenes

Representative DiterpenesRepresentative Diterpenes

Vitamin AVitamin A

OHOH

Terpenes

Representative DiterpenesRepresentative Diterpenes

Vitamin AVitamin A

Common Terpenes

OH

Geraniol C10H18O

OH

Nerol C10H18O

OH

Terpineol C10H18O

OH

Linalool C10H18O

Limonene

CH2

CH2

Limonene

CH2

Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated electron systems.

Documents

Transcript of Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated electron systems.