Ch1b09Lecture23

Ch1b09Lecture23 - L e c tu r e 2 3 - M a r c h 3 , 2 0 0 9...

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Unformatted text preview: L e c tu r e 2 3 - M a r c h 3 , 2 0 0 9 S u m m a r y o f la s t le c tu r e •drug design and HIV protease •intermolecular forces P r e v ie w o f c o m in g a ttr a c tio n s •intermolecular forces, part II •hydrophobic effect •polymers F in a l r e v ie w - T h u r s d a y , M a r c h 1 2 , 7 :3 0 p m 1 d is s o lv e d io n s a r e s o lv a te d in w a te r M z + + nH 2 O → M (H 2 O )n z + 2 ∆Hhydration is the enthalpy change for this process, and reflects ion-dipole interactions http://en.wikipedia.org/wiki/Image:Na%2BH2O.gif fo r a g iv e n io n ic r a d iu s , t h e g r e a te r z , th e la r g e r ∆ H h y d r a tio n ion Mg+2 Ca+2 Ba+2 Fradius (Å) 0.65 0.99 1.35 1.36 ∆Hhyd kJ/mole -1990 -1660 -1364 -474 ion Li+ Na+ K+ Clradius (Å) 0.60 0.95 1.33 1.81 ∆Hhyd kJ/mole -550 -440 -351 -338 Pauling radii; ∆Hhyd http://www.chemsoc.org/networks/LearnNet/data/ds_hydration_enthalpies.htm 3 d ip o le - d ip o le in te r a c tio n s : t w o p a ra lle l, c o llin e a r d ip o le s o f m o m e n t µ = δ a -! a ! -! a ! r r-a r+a % (#$ ) 2 (+$ ) 2 (#$ )($ ) (#$ )($ ) ( µ 2 1389 U= + + + ' *~ 3 "&r r r#a r+a ) r 1/r3 distance dependence (also orientation effects) d ip o le - d ip o le in te r a c tio n s fo r a = 1 Å , δ = 0 .3 , ε = 1 , U = R T w h e n r ~ 5 Å a n im p o r ta n t e x a m p le o f d ip o le - d ip o le in te r a c tio n s in v o lv e s m o le c u le s th a t h a v e H b o n d e d to O o r N !- !+ O—H 4 a s s o c ia tio n o f th e s e m o le c u le s is a c h ie v e d th r o u g h d ip o le - d ip o le in te r a c tio n s k n o w n a s h y d r o g e n b o n d s !- !+ OH H H !-O H !+ !- H O H O-H------O distance = 2.8 Å O-H distance = 1.0 Å s tr e n g th o f h y d r o g e n b o n d in te r a c tio n s 5 s ta n d a r d ∆ H ˚ v a p f o r s tr a ig h t- c h a in a lk a n e s a n d a lc o h o ls liq u id → v a p o r a t 2 9 8 K ~25 kJ/mole slope ~5 kJ/mole/non-H atom each alcohol shares 2 hydrogen-bonds; ~25x(2/2)=25 kJ/mole each non-H atom forms non-bonded contacts ~5 kJ/mole (~2RT) V. Majer & V. Svoboda, Enthalpies of Vaporization of Organic Molecules o r ie n ta tio n a lly a v e r a g e d d ip o le - d ip o le in te r a c tio n s 6 interaction energy depends not only on 1/r3, but also orientation f (") U (", r) = 3 r use the Boltzmann equation to calculate the orientation average < U >= e -(#,r )/RT U (#, r) = 1$ +… RT % U (#, r) ( < U >= " U (#, r)'1 $ * d# = & RT ) ! " U (#, r)P (#)d# = U (#, r)e$U (#,r )/RT d# " U 2 (#, r) " U (#, r)d(#) $ " RT d# first term = 0 second term < 0 " U (#, r)d(#) = 0 since U (#, r) = -U (-#, r) $ 1 ' 2 f 2 (") U (") 1 -* d" = #& 3 ) * d" ~ # 6 %r ( RT RT r 2 inter!ctions are weak (1/r6), but always attractive a 7 S u m m a r y o f n o n c o v a le n t e le c tr o s ta tic in te r a c tio n s r e le v a n t to p o la r m o le c u le s w ith c h a r g e s a n d d ip o le s interaction charge-charge charge-dipole dipole-dipole angle averaged dipole-dipole 1/rn 1/r 1/r2 1/r3 1/r6 U=RT in vac. 550 Å 13 Å 5Å weak (attractive) 8 N o n - c o v a le n t in te r a c tio n s a n d n o n - p o la r m a tte r n o n - p o la r m a tte r c o n ta in s n e ith e r c h a r g e s n o r ( s ig n ifi c a n t) p e r m a n e n t d ip o le s i n e rt g a s e s h y d ro c a rb o n s a r e th e r e a ttr a c tiv e in te r a c tio n s ? y e s - n o n - p o la r m a te r ia l c a n l iq u ify , c r y s ta lliz e , e tc . T h e r e m u s t b e a ttr a c tiv e in te r a c tio n s to o v e r c o m e e n tr o p ic s ta b iliz a tio n o f th e g a s p h a s e . N o n p o la r m a tte r h a s n o p e r m a n e n t d ip o le s , b u t 9 in d u c e d d ip o le s e x is t d u e to p o la r iz a tio n e ffe c ts - o n e c la s s ic a l h y d r o g e n a to m - + + tw o c la s s ic a l h y d r o g e n a to m s + fi n ite in s ta n ta n e o u s d ip o le m o m e n ts c a n in te r a c t; e q u iv a le n t to a n g le - a v e r a g e d d ip o le - d ip o le in te r a c tio n s net result - attractive interactions varying as 1/r6 k n o w n a s L o n d o n d is p e r s io n in te r a c tio n s 10 L e n n a r d - J o n e s “ 6 - 1 2” p o t e n t i a l T h is n o n - c o v a le n t in te r a c tio n i n c lu d e s b o th d is p e r s io n and repulsion (modeled as 1/r12) terms %% $ (12 % $ ( 6 ( AB U ( R) = 12 " 6 = 4#'' * " ' * * '& r ) & r ) * r r & ) +% R (12 % R ( 6 . = #-' min * " 2' min * 0 & r )/ ,& r ) (Ch1b lecture 9) r Lennard-Jones potential ! σ -ε well depth (< RT) Rmin parameters from Atkins Physical Chemistry m a g n itu d e o f d is p e r s io n in te r a c tio n s a r g o n c r y s ta l - s p h e r ic a l a to m s s p a c e d b y 3 .7 6 Å c lo s e d p a c k e d w ith 1 2 n e a r e s t n e ig h b o r s 11 •the lattice energy of argon = -7.7 kJ/mole •to evaporate solid argon, 12 shared, or 6 full, i n te r a c tio n s m u s t b e d is r u p te d p e r a to m •each interaction ~ -7.7/6 = -1.3 kJ/mole 12 s u m m a r y o f in te r m o le c u la r in te r a c tio n s a ll ty p e s o f a to m s e x p e r ie n c e ( 1 ) a ttr a c tiv e d is p e r s iv e in te r a c tio n s ( 2 ) r e p lu s iv e i n te r a c tio n s th a t p r e v e n t o v e r la p p o la r m o le c u le s w ith c h a r g e s a n d p e r m a n e n t d ip o le s h a v e a w id e r r a n g e o f e le c tr o s ta tic in te r a c tio n s 13 M ix tu r e s a n d s o lu tio n s w h a t h a p p e n s w h e n y o u m ix to g e th e r tw o s u b s ta n c e s , starting with either both polar or both nonpolar? A + B → A+B E n tr o p y o f m ix in g 14 m ix in g ta k e s p la c e d u e to th e in c r e a s e d v o lu m e a v a ila b le to t h e d iffe r e n t m o le c u le s # VA + VB & # VA + VB & "S = nA R ln% ( + nB R ln% (> 0 $ VA ' $ VB ' ( C h 1 b le c tu r e 1 3 ) m ix tu r e o f p o la r a n d n o n p o la r , s p e c ifi c a lly w a te r a n d a lk a n e s CH3 15 H O H O H H O H H H2C + H2C CH2 = ?? CH2 H3C 16 http://www.cleanupoil.com/images/prestigerocks.jpg The hydrophobic effect H H O H O H3C 17 H O H H CH2 CH2 CH3 H H O H H2C O H H2C presence of apolar groups in an aqueous solution is energetically unfavorable due to restrictions on water orientation and/or disruption of water-water hydrogen bonding interactions Near room temperature, water molecules surrounding an apolar molecule can maintain their hydrogen bonding interactions to other water molecules; however, there are rH strictions on the possible orientations that permit this. e H H O H O H3C H O H H 17a H O H CH2 CH2 CH3 O H H H2C H2C O H O H H O H H “orientational restrictions” = “entropically unfavorable”, (ie ∆S˚ < 0) so that the solution is more ordered than the separated components. This leads to limited solubilities of hydrocarbons and apolar molecules in water "#G˚ since "T = $#S˚ , ∆G˚ for this process becomes more positive with temperature, so that the aqueous solubilities of hydrocarbons decreases with temperature. s o lu b iliz a tio n o f h y d r o c a r b o n s in w a te r : d e te r g e n ts S O4 2- 19 SDS =sodium dodecyl sulfate O H2O O H2O O O O H2O O H2O O OO H2O O O H2O O shorthand notation: tail ----------- head detergent micelles: • polar exterior • apolar interior spontaneously assemble above a certain critical concentration d e te r g e n ts a r e a ls o c a lle d a m p h ip h ile s 20 th e p a c k in g o f a m p h ip h ile s ( d e te r g e n ts a n d lip id s ) in to s e lfa s s e m b le d s tr u c tu r e s r e fl e c ts th e ir g e o m e tr ic a l s h a p e s Israelachvili, Intermolecular & Surface Forces Lipids and cell membranes phosphate glycerol HOCH2-CHOH-CH2OH 21 choline p o la r h e a d g ro u p a p o la r t a il fatty acids: palmitic - sat C16 oleic - unsat. C18 polar headgroup T h e m e m b ra n e b ila y e r 22 apolar tail phospholipid S.J. Singer’s fluid mosaic model Science 175, 720 (1972) phospholipids assemble into a bilayer through the “hydrophobic effect”; the apolar interior of the membrane is largely impermeable to water, ions and other polar molecules. Membrane proteins are required for transport of these species into and out of cells Molecular dynamics simulation of phospholipid bilayer 23 40 Å 20 Å = phosphate location Heller et al. J Phys. Chem. 97, 8343 (1993), fbilayer.pdb Polymers ...
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This note was uploaded on 09/25/2010 for the course CH 104 taught by Professor Bopanna during the Fall '09 term at UMBC.

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