Chem 204. K ow and the Rule of Five. with an extended interlude of how to measure concentra3ons of molecules in fluid solu3on

Transcription

1 Chem 204 K ow and the Rule of Five with an extended interlude of how to measure concentra3ons of molecules in fluid solu3on

2 octanol (CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH) water

3 Now add molecule octanol (CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH) water

4 octanol (CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH) water

5 K ow octanol- water parbbon coefficient EQUILIBRIUM measure of the relabve amounts of the molecule in octanol / water Also called P Can find in the literature as log P Common measure of how hydrophobic (and lipophilic) a molecule is

6 Interlude: Common ways to measure concentrabon of solutes in liquids Ultraviolet- visible spectroscopy (uv- vis) Fluorescence spectroscopy (if molecule fluoresces) Gas chromatography/mass spectrometry (GC/ MS; will skip for now)

7 Spectroscopy: The InteracBon of Light with Ma\er Image taken from Chemistry: The Central Science, 12 th ed.

8 Light as a wave: the speed of light (3 x 10 8 m/s) equals the wavelength of the light (m) Bmes its frequency (s - 1 or hertz, Hz)

9 Light as a parbcle: the energy of a photon (J) equals Planck s constant (h, x J s) Bmes frequency (s - 1, or Hertz, Hz) The wavelengths of light emi\ed from a sample (atomic emission here, fluorescence in molecules) tell you about the energies between orbitals remember the H atom?

10 Details of vibrabonal sublevels in two potenbal energy wells (MO s) showing absorpbon of light (blue arrow) and fluorescence (green arrow, more later). Note that the potenbal wells have energy minima at different nuclear coordinates. That means the shape, and dipole moment, of the molecule are different in the ground and excited states. Blue up arrow: Absorbance (uv- vis measurement) Green down arrow: fluorescence

11 Absorbance (uv- vis) I 0 I b A = absorbance (at a given wavelength) I 0 = source intensity I = detected intensity b = sample path length T = transmittance intensity for a lamp has units of Joule/sec (aka Wa\) per solid angle

12 scan wavelengths, plot A vs wavelength Image taken from Chemistry: The Central Science, 12 th ed.

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14 Absorbance proporbonal to concentrabon (up to a point) Uv- vis spectra of I 2 at various concentrabons. The bo\om curve shows hardly any signal above background. Image taken from Chemistry: The Central Science, 12 th ed.

15 Beer s Law Typical values: A = ; ext. coeff ,000 M - 1 cm - 1 Beer s Law not reliable for too dilute or too concentrated samples.

16 Fluorescence and Phosphorescence (generic: photoluminescence, or emission)

17 Fluorescent molecules have lots of resonance

18 Monochromater selects one wavelength of light (usually 2, 4, 16 nm slit widths; have monochromators in uv- vis spectrometers too). Sample starts to fluorescence once light hits it; some of the light goes into the emission monochromator. Scan emission m.c. and hold excitabon constant. Spectrofluorometer

19 ExcitaBon and Emission Spectra ExcitaBon (blue): hold emission m/c and scan Emission (pink): hold excitabon m/c constant and scan h\p://

20 Fluorescence Intensity Can Be Used For Molecular QuanBficaBon if you run a calibrabon curve Cary 100 manual

21 Back to K ow : Examples 1,2- diphenylethane: log P = 4.82 Caffeine: log P = Hexylpyridinium bromide: log P = From: A. Leo, C. Hansch, D. Elkins, Chemical Reviews 1971, 71,

22 Back to K ow : Problems Octanol does dissolve in water a bit Self- associabon or ionizabon of solute Strong solvabon in one phase over another makes solute molecules inequivalent ph effects

23 J. Med. Chem. 1980, 23,

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28 dichlorodiphenyltrichloroethane (DDT) insecbcide in US, literature log P values: (!)

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34 SBrred, not shaken!

35 Medicinal Chemistry, Drug Discovery and the Rule of Five

36 A History of Drug Discovery Folklore and medicinal plants (accounts for 40% of current drugs) 1980 s: RaBonal Drug Design 1990 s: High Throughput Screening

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38 For orally taken drug Can t be TOO water- soluble or it will just stay in bloodstream/excreted Can t be TOO insoluble in water or it might precipitate out and not make it to target

39 The Rule of Five: Poor drug absorpbon or permeabon are likely when

40 Example: caffeine How many hydrogen bond donors are in this molecule?

41 Example: caffeine How many hydrogen bond donors are in this molecule? ZERO

42 Example: caffeine How many hydrogen bond acceptors are in this molecule?

43 Example: caffeine How many hydrogen bond acceptors are in this molecule? SIX

44 Example: caffeine Caffeine has less than 5 H- bond donors; its MW is less than 500 g/mol; Its log P is close to zero; it has less than 10 H- bond acceptors SO would predict this might be a good drug lead!

45 What you need to know DefiniBon and uses of Kow How to measure concentrabon in solubon using instrumentabon (esp Beer s Law) What is the Rule of Five and do it for a molecule