Ultraviolet-Visible (UV-VIS) Spectroscopy Gives

Transitions between electron energy states
Ultraviolet-Visible (UV-VIS)
Spectroscopy
Gives information about conjugated π electron
systems
Δ E = hν
gaps between electron energy
levels are greater than those
between vibrational levels
gap corresponds to wavelengths
between 200 and 800 nm
UV and Visible light cause only two kinds of electronic
transitions
Conventions in UV-VIS
X-axis is wavelength in nm (high energy at left,
low energy at right)
λmax is the wavelength of maximum absorption
and is related to electronic makeup of molecule—
molecule—
especially π electron system
Y axis is a measure of absorption of electromagnetic
radiation expressed as
Absorbance or molar absorptivity (ε)
π→π* Transition in cis,trans-1,3-cyclooctadiene
cis,trans-1,3-cyclooctadiene
ψ4∗
LUMO
ψ4∗
ψ3∗
HOMO ψ2
ψ1
Most stable
π-electron
configuration
• Only organic compounds with π 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
π→π* Transition in Alkenes
HOMO-LUMO energy gap is affected by
substituents on double bond
ψ3∗
Δ E = hν
ψ2
as HOMO-LUMO energy difference decreases
(smaller ΔE), λmax shifts to longer wavelengths
ψ1
π-Electron
configuration of
excited state
1
A chromophore is the part of a molecule which absorbs
UV or visible light
Substituent Effects
Methyl groups on double bond cause λmax
to shift to longer wavelengths
H
H
H
C
C
C
H
H
CH3
C
CH3
H
λmax 170 nm
λmax 188 nm
Effect of Conjugation on λmax
Substituent Effects
O
CH3
H2C
Extending conjugation has a larger effect
on λmax; shift is again to longer
wavelengths
H
H
CH3
UV
Spectrum:
3.5
(+)-carvone
236 nm
3
2.5
2
H
Series1
H
1.5
1
C
H
C
C
H
C
H
0
H
C
200
C
H
λmax 170 nm
0.5
220
240
Wavelength
H
260
280
(nm)
What is carvone’
carvone’s λmax?
λmax 217 nm
2
The Beer–Lambert Law
UV Spectrum of cis,trans-1,3-cyclooctadiene
cis,trans-1,3-cyclooctadiene
A = clε
A = log(I / I0)
c = concentration of substance
in solution
l = length of the cell in cm
ε = molar absorptivity
The molar absorptivity of a compound is a constant that
is characteristic of the compound at a particular
wavelength
H
Substituent Effects
H
C
C
H
λmax 230 nm
εmax 2630
1000
200
220
240
260
280
Wavelength, nm
Both the λmax and ε increase as the number of
conjugated double bonds increases
H
C
λmax 217 nm
(conjugated diene)
diene)
C
H
H
H 3C
2000
Molar
absorptivity (ε)
H
C
C
H
H
C
λmax 263 nm
H conjugated triene plus
two methyl groups
C
H
C
H
C
CH3
An auxochrome is a substituent in a chromphore that
alters the λmax 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
3
The Visible Spectrum and Color
Vision
Biological / Physiological response
to light stimuli
4
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
Visual pathway from retina to V1
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
Optical imaging of orientation map
5
Retinotopic map
The Retina & Photoreceptive Cells
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
c
b
a
Project orderly to
LGN and V1
a
b
c
object
retina
Different cells in the retina
light
The Basic Retinal Circuit
Back of eye
6. Pigment cells
1. Receptor Cells
(Graded potential)
(input)
2. Bipolar Cells
(Graded potential)
3. Ganglion Cells
(action potential)
(Output)
Stru
cture
of th
e eye
4. Horozontal Cells
(Graded potential)
5. Amacrine Cells
(Graded/action potential)
Fovea: high spatial resolution
Periphery: low spatial resolution
Front of eye
2
Direct pathway: Photo receptor  Bipolar  RGC
Indirect pathway
Photoreceptor
-
On bipolar
+
Photoreceptor
+
Off bipolar
+
+
Horizontal cell
-
Photoreceptor
On RGC
Off RGC
+: excitatory synapse, preserve response
direction
-: inhibitory synapse, flip response
direction
Indirect pathway mediated by
horizontal is responsible to the
RF surround
There are other indirect
pathway mediated by amacrine
cells
Direct pathway is responsible
to the RF center
6
Summary of retinal circuit
Direct pathway
-
On bipolar
+
On RGC
Photoreceptor
+
Off bipolar
+
Off RGC
The Nobel Prize in Physiology or Medicine 1981:
Indirect pathway
Photoreceptor
+
Horizontal cell
-
Photoreceptor
Pathways mediated by amacrine cells
Terpenes in Vision
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
Lycopene
Can diet affect sight?.....It might depend on what you’re
looking at.
• Color vision vs. B&W: Rods vs. Cones
• The Photochemical Process
orange-red pigment in tomatoes
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
Lycopene
Beta Carotene "Pro vitamin A"
λmax 505 nm
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
OH
HO
Vitamin A
7
Lycopene
Lycopene
Beta Carotene "Pro vitamin A"
Beta Carotene "Pro vitamin A"
OH
OH
HO
HO
Vitamin A
Vitamin A
Cis-Trans Isomerization & Vision
Lycopene
Beta Carotene "Pro vitamin A"
OH
HO
Vitamin A
B&W Chemistry
B&W Chemistry
Step One: Oxidation
Step Two: Trans -> Cis Isomerization
OH
CHO
CHO
CHO
8
B&W Chemistry
B&W Chemistry
Step Three: “Hooked on Opsin”
Step Four: “Flash”
CHO
Opsin
NH2
C N
~ 5 photons
~ 3 picosec
C N
C N
B&W Chemistry
Step Five: “Repeat Process”
C N
NH2
CHO
Opsin
Terpenes
•Terpenes are natural products that
form from a pyrophosphate and are
structurally related to isoprene.
CH3
H 2C
C
CH
CH2
Isoprene
(2-methyl-1,3-butadiene)
Vision exercise
O
O
P
O
O
O
P
O
O
Isopentenyl pyrophosphate
Isoprene
9
The Isoprene Unit
• An isoprene unit is the carbon skeleton
of isoprene (ignoring the double bonds)
The Isoprene Unit
• The isoprene units are joined "head-totail."
head
Two isoprene units:
Isoprene Links
Heads or Tails?
tail
tail head
Common Carbon Backbone
10
Terpenes
Terpenes
Classification of Terpenes
Representative Monoterpenes
•Class
Number of carbon atoms
•Monoterpene
10
•Sesquiterpene
15
•Diterpene
20
•Sesterpene
25
•Triterpene
30
•Tetraterpene
40
OH
O
H
α-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
Terpenes
Terpenes
Representative Monoterpenes
Representative Monoterpenes
OH
O
H
α-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
α-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
Terpenes
Terpenes
Representative Sesquiterpenes
Representative Sesquiterpenes
H
α-Selinene
(celery)
H
α-Selinene
(celery)
11
Terpenes
Terpenes
Representative Sesquiterpenes
Representative Diterpenes
OH
α-Selinene
(celery)
Vitamin A
Terpenes
Terpenes
Representative Diterpenes
Representative Diterpenes
OH
Vitamin A
Vitamin A
Common Terpenes
Limonene
OH
OH
CH2
Nerol C10H18O
Linalool C1 0H1 8O
OH
OH
Geraniol C1 0H1 8O
Terpineol
CH2
C10H18O
12
Limonene
CH2
13