ultraviolet-visible (uv-vis) spectroscopy
DESCRIPTION
Ultraviolet-Visible (UV-VIS) Spectroscopy. Gives information about conjugated p electron systems. Transitions between electron energy states. gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm. - PowerPoint PPT PresentationTRANSCRIPT
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
maxmax 188 nm 188 nm
A chromophore is the part of a molecule which absorbsUV or visible light
Effect of Conjugation on max
Substituent EffectsSubstituent Effects
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 230 nm 230 nm
maxmax 2630 2630
MolarMolar
absorptivity (absorptivity ())
UV Spectrum of cis,trans-1,3-cyclooctadieneUV Spectrum of cis,trans-1,3-cyclooctadiene
Substituent EffectsSubstituent Effects
maxmax 217 nm 217 nm
(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
maxmax 263 nm 263 nm
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
maxmax 505 nm 505 nm
orange-red pigment in tomatoesorange-red pigment in tomatoes
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
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
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)
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)
HH
Terpenes
Representative SesquiterpenesRepresentative Sesquiterpenes
-Selinene-Selinene(celery)(celery)
Terpenes
Representative DiterpenesRepresentative Diterpenes
Vitamin AVitamin A
OHOH
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
