Transcript Slide 1

Ultraviolet-Visible (UV-VIS)
Spectroscopy
Gives information about conjugated p electron
systems
Transitions between electron energy states
DE = hn
gaps between electron energy
levels are greater than those
between vibrational levels
gap corresponds to wavelengths
between 200 and 800 nm
Conventions in UV-VIS
X-axis is wavelength in nm (high energy at left,
low energy at right)
lmax is the wavelength of maximum absorption
and is related to electronic makeup of molecule—
especially p electron system
Y axis is a measure of absorption of electromagnetic
radiation expressed as
Absorbance or molar absorptivity (e)
UV and Visible light cause only two kinds of electronic
transitions
• Only organic compounds with p 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
pp* Transition in cis,trans-1,3-cyclooctadiene
LUMO
y4*
y4*
y 3*
y3*
HOMO y2
y1
Most stable
p-electron
configuration
DE = hn
y2
y1
p-Electron
configuration of
excited state
pp* Transition in Alkenes
HOMO-LUMO energy gap is affected by
substituents on double bond
as HOMO-LUMO energy difference decreases
(smaller DE), lmax shifts to longer wavelengths
Substituent Effects
Methyl groups on double bond cause lmax
to shift to longer wavelengths
H
H
C
H
H
CH3
C
C
H
lmax 170 nm
H
C
CH3
lmax 188 nm
A chromophore is the part of a molecule which absorbs
UV or visible light
Effect of Conjugation on lmax
Substituent Effects
Extending conjugation has a larger effect
on lmax; shift is again to longer
wavelengths
H
H
C
H
H
H
C
C
H
C
H
H
C
C
H
lmax 170 nm
lmax 217 nm
H
O
CH3
H2C
CH3
236 nm
What is carvone’s lmax?
The Beer–Lambert Law
A = cle
A = log(I / I0)
c = concentration of substance
in solution
l = length of the cell in cm
e = molar absorptivity
The molar absorptivity of a compound is a constant that
is characteristic of the compound at a particular
wavelength
UV Spectrum of cis,trans-1,3-cyclooctadiene
2000
lmax 230 nm
Molar
absorptivity (e)
emax 2630
1000
200
220
240
260
Wavelength, nm
280
H
Substituent Effects
H
C
C
H
H
C
C
H
H
H3C
lmax 217 nm
(conjugated diene)
H
C
C
H
lmax 263 nm
H conjugated triene plus
two methyl groups
H
C
C
H
C
H
C
CH3
Both the lmax and e increase as the number of
conjugated double bonds increases
An auxochrome is a substituent in a chromphore that
alters the lmax 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
Vision
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
c
b
a
a
b
c
object
retina
Project orderly to
LGN and V1
The Retina & Photoreceptive Cells
Different cells in the retina
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)
4. Horozontal Cells
(Graded potential)
5. Amacrine Cells
(Graded/action potential)
Front of eye
2
light
Fovea: high spatial resolution
Periphery: low spatial resolution
Direct pathway: Photo receptor  Bipolar  RGC
-
On bipolar
+
Off bipolar
+
On RGC
Photoreceptor
+
Off RGC
+: excitatory synapse, preserve response
direction
-: inhibitory synapse, flip response
direction
Direct pathway is responsible
to the RF center
Indirect pathway
Photoreceptor
+
Horizontal cell
-
Photoreceptor
Indirect pathway mediated by
horizontal is responsible to the
RF surround
There are other indirect
pathway mediated by amacrine
cells
Summary of retinal circuit
Direct pathway
-
On bipolar
+
Off bipolar
+
On RGC
Photoreceptor
+
Off RGC
Indirect pathway
Photoreceptor
+
Horizontal cell
-
Pathways mediated by amacrine cells
Photoreceptor
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 Vision
Can diet affect sight?.....It might depend on what you’re
looking at.
• Color vision vs. B&W: Rods vs. Cones
• The Photochemical Process
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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
orange-red pigment in tomatoes
lmax 505 nm
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 Chemistry
Step One: Oxidation
OH
CHO
B&W Chemistry
Step Two: Trans -> Cis Isomerization
CHO
CHO
B&W Chemistry
Step Three: “Hooked on Opsin”
CHO
Opsin
NH2
C N
B&W Chemistry
Step Four: “Flash”
C N
~ 5 photons
~ 3 picosec
C N
QuickTime™ and a
Video decompressor
are needed to see this picture.
B&W Chemistry
Step Five: “Repeat Process”
C N
NH2
CHO
Opsin
Vision exercise
Terpenes
•Terpenes are natural products that
form from a pyrophosphate and are
structurally related to isoprene.
CH3
H2C
C
CH
CH2
Isoprene
(2-methyl-1,3-butadiene)
O
O
O
P
P
O
O
O
O
Isopentenyl pyrophosphate
Isoprene
The Isoprene Unit
• An isoprene unit is the carbon skeleton
of isoprene (ignoring the double bonds)
Two isoprene units:
The Isoprene Unit
• The isoprene units are joined "head-totail."
head
tail
tail head
Isoprene Links
Heads or Tails?
Common Carbon Backbone
Terpenes
Classification of Terpenes
•Class
Number of carbon atoms
•Monoterpene
10
•Sesquiterpene
15
•Diterpene
20
•Sesterpene
25
•Triterpene
30
•Tetraterpene
40
Terpenes
Representative Monoterpenes
OH
O
H
a-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
Terpenes
Representative Monoterpenes
OH
O
H
a-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
Terpenes
Representative Monoterpenes
a-Phellandrene
Menthol
(eucalyptus)
(peppermint)
Citral
(lemon grass)
Terpenes
Representative Sesquiterpenes
H
a-Selinene
(celery)
Terpenes
Representative Sesquiterpenes
H
a-Selinene
(celery)
Terpenes
Representative Sesquiterpenes
a-Selinene
(celery)
Terpenes
Representative Diterpenes
OH
Vitamin A
Terpenes
Representative Diterpenes
OH
Vitamin A
Terpenes
Representative Diterpenes
Vitamin A
Common Terpenes
OH
OH
Nerol C 1 0H1 8O
Linalool C10 H 18 O
OH
OH
Geraniol C10 H 18 O
T erpineol
C1 0H1 8O
Limonene
CH2
CH2
Limonene
CH 2