Ultraviolet-Visible (UV-VIS) Spectroscopy

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

2. Transitions between electron energy states gaps between electron energy levels are greater than thosebetween vibrational levels gap corresponds to wavelengthsbetween 200 and 800 nm DE = hn

3. 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)

4. 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 UV and Visible light cause only two kinds of electronic transitions

5. y4* y3* y2 y1 pp* Transition in cis,trans-1,3-cyclooctadiene y4* y3* LUMO DE = hn y2 HOMO y1 Most stable p-electron configuration p-Electron configuration of excited state

6. 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

7. C C C C Substituent Effects Methyl groups on double bond cause lmax to shift to longer wavelengths H H CH3 H H CH3 H H lmax 170 nm lmax 188 nm

8. A chromophore is the part of a molecule which absorbs UV or visible light

9. Effect of Conjugation onlmax

11. C C C C C C Substituent Effects Extending conjugation has a larger effect on lmax; shift is again to longer wavelengths H H H H H H H H H H lmax 170 nm lmax 217 nm

12. 236 nm What is carvone’s max?

13. 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

14. UV Spectrum of cis,trans-1,3-cyclooctadiene 2000 Molarabsorptivity (e) lmax 230 nm emax 2630 1000 200 220 240 260 280 Wavelength, nm

15. H H C C H H C C H H H3C H C C H H H C C H C C CH3 H Substituent Effects lmax 217 nm(conjugated diene) lmax 263 nmconjugated triene plus two methyl groups

16. Both the lmax and e increase as the number of conjugated double bonds increases

17. An auxochrome is a substituent in a chromphore that alters the lmax and the intensity of the absorption

18. Measure the rates of a reaction • Determine the pKa of a compound • Estimate the nucleotide composition of DNA Uses of UV/Vis Spectroscopy

23. The Visible Spectrum and Color

24. Vision Biological / Physiological response to light stimuli

25. The Eye and Vision

26. The Eye and its Neurological Wiring

27. 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

28. Projection from retina to LGN fixation point • 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 fovea

29. Visual pathway from retina to V1

30. Optical imaging of orientation map

31. 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 Project orderly to LGN and V1 b a a b c object retina

32. The Retina & Photoreceptive Cells

33. 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) Structure of the eye Front of eye 2

34. light Fovea: high spatial resolution Periphery: low spatial resolution

35. Direct pathway: Photo receptor  Bipolar  RGC +: excitatory synapse, preserve response direction -: inhibitory synapse, flip response direction Direct pathway is responsible to the RF center

36. Indirect pathway Indirect pathway mediated by horizontal is responsible to the RF surround There are other indirect pathway mediated by amacrine cells

37. Summary of retinal circuit Direct pathway Indirect pathway Pathways mediated by amacrine cells

38. 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

39. Terpenes in VisionCan diet affect sight?.....It might depend on what you’re looking at. • Color vision vs. B&W: Rodsvs.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

40. Lycopene orange-red pigment in tomatoes lmax 505 nm

46. Cis-Trans Isomerization & Vision

47. B&W ChemistryStep One: Oxidation

48. B&W ChemistryStep Two: Trans -> Cis Isomerization

49. B&W ChemistryStep Three: “Hooked on Opsin”

50. B&W ChemistryStep Four: “Flash”