the physical-organic chemtstry of surfaces, and its

16
THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS RELEVANCE TO MOLECUI,AR RECOGNITTON G. M. Whitesides and Hans Biebuyck Department of Chemistry Harvard University Cambridge, Massachusetts 02138 Molecular recognition in most biological systems is the result of summing large numbers of small, compensating energies between the parts of the interacting conlonents. rn biological systems, there are, as a rure, -threl najor interacting components a protein (a receptor or enzyme), a ligand (the morecurar being recognized), and water (Figure 1) . other participating morecules -- ions, membranes, small organic morecures, additionar ligands m?y be important in some circumstances, but canr dS a f irst approxirnation, be neglected. How should one think about the interactions between these three rnajor participants? Protein-water and ligand- water interactions should be easier to understand than protein-ligand interactions, since water is a homogeneous medium (or at least more homogeneous than protein 5r Iigand). Even for interactions involving later, however, the intuition of the chemist or biochemilt is less than perfect. rn principre, and in due course, alr wirl be carcurated by the cornputer, and advances in molecular mechanics and molecular dynamics have been rapid in the last years.I rhe fact r"iuins, however, that at the present time most of the success in practicat applications of molecular recognition especiarly to drug aLsign and molecurar pharmacology have been based on tne tifre- honored patterns of classicar medicinar chemistry. we have been interested in a problen whose relevance to molecular recognition apparently lies at the farthest boundaries of the subject that ir, the physicar- organic chemistry of wetting. our interesl is to .understand how f luids , especiat Iy water, interact with

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Page 1: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITSRELEVANCE TO MOLECUI,AR RECOGNITTON

G . M . W h i t e s i d e s a n d H a n s B i e b u y c k

Department of Chemist ryHarvard Un ive rs i t yC a m b r i d g e , M a s s a c h u s e t t s 0 2 1 3 8

Molecu la r recogn i t i on in mos t b io log ica l sys tems i sthe resu l t o f summing la rge numbers o f sma l l , compensa t ingenerg ies be tween the par ts o f the in te rac t ing con lonen ts .r n b i o l o g i c a l s y s t e m s , t h e r e a r e , a s a r u r e ,

- t h r e l n a j o r

in te rac t ing componen ts a p ro te in (a recep to r o re n z y m e ) , a l i g a n d ( t h e m o r e c u r a r b e i n g r e c o g n i z e d ) , a n dwate r (F igu re 1 ) . o the r pa r t i c ipa t ing morecu les - - i ons ,m e m b r a n e s , s m a l l o r g a n i c m o r e c u r e s , a d d i t i o n a r l i g a n d sm?y be impor tan t i n some c i r cumstances , bu t canr dS af i r s t approx i rna t ion , be neg lec ted .

How should one th ink about the in teract ions betweenthese th ree rna jo r pa r t i c ipan ts? Pro te in -wa te r and l i gand-wate r i n te rac t ions shou ld be eas ie r to unders tand thanpro te in - l i gand in te rac t ions , s ince wa te r i s a homogeneousmed ium (o r a t l eas t more homogeneous than p ro te in 5 rI i gand) . Even fo r i n te rac t ions invo lv ing la te r , however ,the in tu i t i on o f the chemis t o r b iochemi l t i s l ess thanp e r f e c t . r n p r i n c i p r e , a n d i n d u e c o u r s e , a l r w i r l b ecarcurated by the cornputer , and advances in molecularmechan ics and mo lecu la r dynamics have been rap id in thel a s t y e a r s . I r h e f a c t r " i u i n s , h o w e v e r , t h a t a t t h epresen t t ime mos t o f the success in p rac t i ca t app l i ca t ionso f mo lecu la r recogn i t i on espec ia r l y to d rug aLs ign andmolecurar pharmacology have been based on tne t i f re-honored pa t te rns o f c lass ica r med ic ina r chemis t ry .

we have been in terested in a problen whose re levanceto mo lecu la r recogn i t i on apparen t l y l i es a t the fa r thes tboundar ies o f the sub jec t tha t i r , t he phys ica r -o rgan ic chemis t ry o f we t t i ng . ou r in te res l i s to.unders tand how f l u ids , espec ia t I y wa te r , i n te rac t w i th

Page 2: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

F i g u r e 1 .

Hzo ---_Lb....-

van der \Vaals

H-Bond

Electrostat ic

Water

Conformational ChangeAG=AH-TAS

A schernat ic i l lus t ra t ion o f thebetween a protein and a l igand

+ HzO

in teract ioni n w a t e r .

Page 3: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

//

/

s o l i d s o r i n n i s c i b l e I i g u i d s . 2 r n b r i e f , w h a t r n o l e c u l a rin te rac t ions de te rmine the energ ies o f i n te r facesl i g u i d - s o r i d , s o r i d - v a p o r , a n d l i q u i d - v a p o r ? T h e s econ f igu ra t ions a re re levan t to rno lecu la r recogn i t i on andrecep to r - l i gand b ind ing , because the same in te rac t ionsde te rmine we t t ing and recogn i t i on . rn we t t i ng , however ,the sys tems a re s t ruc tu ra r l y ve ry s impre in p r inc ip reno more than one mo lecu la r l y homogeneous , smooth l i qu iain te rac t ing w i th an equa l ry ho rnogeneous , smooth su r face( o f a s o l i d o r . r i q u i d ) . r n m o r e c u r a r r e c o g n i t i o n , b ycon t ras t , the in te rac t ing componen ts a re rough (on am o l e c u l a r s c a r e ) a n d h e t e r o g e n e o u s . N o n e t h e r e s s , b yu n d e r s t a n d i n g t h e i n t e r a c t i o n o f a l a r g e , h o m o g e n e o i = ,m o d e l s u r f a c e w i t h w a t e r r w € s h o u r d b e - a b r e t o h e l p t ounders tand the in te rac t ion o f a s rna l r pa tch o f p ro le inh a v i l g s i m i l a r p h y s i c a l c h a r a c t e r i s t i c s w i t h w a l e r . B yexamin ing a number o f su r faces mode l t i ng d i f fe ren t pa r tso f the p ro te in , we may be ab le to con t r ibu te tounders tand ing the g robar in te rac t ions o f a p ro te in (o r al i g a n d ) w i t h w a t e r , a n d , u l t i m a t e r y , o f a p i o t e i n w i t h al i gand . conversery , i f we canno t unders tand thein te rac t ion o f homogeneous su r faces w i th pure wa te r , ou rab i l i t y to unders tand the much more comp l l x i n te ra " t ionsoccur r ing in b io log ica l sys tems w i I I rema in unsa t i s fac to rya t any rever deeper than emp i r i ca l rnode l l i ng .

rn o rder to s tudy so l id - I i gu id in te rac t ions us ing theapproaches o f phys ica l -o rgan ic chemis t ry r w€ mus t be ab leto p repare su r faces hav ing wer l -de f ined s t ruc tu res , tovary the st ructures of these sur faces, and to measure oneor more p roper t ies o f the sys tem tha t g i ve in fo rmat ionabou t the energe t i cs o f the in te r faces o f i n te res t . ou rwork has been addressed p r ina r i l y to two sub jec tsdeve lopment o f rne thods fo r the p iepara t ion o f s t ruc tu ra l l ywer l -de f ined o rgan ic su r faces us ing a g roup o f techn iquescar led co l lec t i ve ly ' tmo lecura r se l f -as iembry , t r and thein fe rence o f l i qu id -so l id in te rac t ions th rough themeasurement o f con tac t ang les .3 ,4

se l f -Assernb led Mono layers . The p r inc ip res o f se l f -assembry a re wer l i r tus t ra ted by the bes t deveroped andmost thoroughly invest igated of the systems that formorgan ic mono layers long-cha in a tky l th io rs HS (cH2 ) nxadsorbed on gord . rn th i s techn ique , one exposes a c reango ld f i l n to a so lu t ion o r vapor o r an o rgan ic th io l( F igure 2 ' ) .5 The sul fur atonl o f the th io l coord inate tothe go ld and a re conver ted (by p resen t ry unde f inedmechan isms) to go ld th io la tes hav ing a geomet ry de te r rn inedby the roca l coord ina t ion chemis t ry o f tne su r iu r and.go ld . The a rky l cha ins a r range themse lves to n in in i ze

Page 4: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

BA

F igure 2

lll)' ') r

/

/C

There are two approaches to forn ing sel f -assemb led mono layers on go ld . rn i cheme A , ago ld f i rm i s immersed in a d i l u te so lu t ion o fa n a l k a n e t h i o l ; f i l m f o r m a t i o n i s t y p i c a r l ycornplete, as j udged by macroscopic p ioper t i .=o f the f i l n , i n one o r two minu tes . rn schemeB ' the gord f i rn i s exposed to an a lkane th io rin the gas phase , e i the r i n a i r o r under vacuumcondi t ions. Both rnethods of preparat ion resur ti n c r y s t a r l i n e f i l m s w i t h a c i n t - a n g r e o f 3 0 " ;c . F i lm fo rmat ion i s d r i ven by the s t rong ,spec i f i c i n te rac t ion o f the th io l w i th th6 gords u b s t r a t e .

Page 5: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

20

t5

Aro

oo

Vapr Phase formatlon of I\f onolayerSolutlon phase formatlon of l\tonolaver

18 20

Number of Carbons in Alkanethiol

The th i ckness o f the f i l n on go ld i s eas i l ycontro l led by changing the length of thea lkane th io l . The average th i ckness o f the f i lmin angstroms, der ived f rom the measurement ofthe in tens i t y o f the pho toe lec t ron s igna l o fgo ld us ing an X- ray pho toe lec t ronspectrophotorneter ( XPS ) , is pI o t ted as afunct ion of to ta l number of carbons in thealkaneth io l . F i lms hrere made e i ther byexposing the substrate to a vapor of thea l k a n e t h i o l i n a i r ( b 1 a c k d o t s ) o r t o aso lu t ion o f the a lkane th io l i n e thano l (wh i ted o t s ) . T h e l i n e r e p r e s e n t s t h e b e s t f i t o f t h etwo se ts o f da ta i t he s lope o f th i s l i ne showsthat the th ickness of the f i lm changes byapproxinate ly an angstrom per carbon in thea lkane th io l . Techn iques used to in fe r theth ickness of a monolayer f rom the photoelect ronin tens i t y measured by XPS a re g iven in C .D,B a i n a n d G . M . W h i t e s i d e s , J . P h y s . C h e m , l - 9 8 9 ,9 3 , L 5 7 0 .

t6l 4t210

F igure 3 .

Page 6: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

t he i r energy . Themonolayers havingc r y s t a l l i n e o r d e r .

sys tems can be shown to beI o c a l l y ) w e I l - d e f i n e d

r e s u l t i n g

Iua leas t

The advan tages o f th i s p repara t i ve techn igue( re la t i ve to Langmui r -B todge t t p rocedures o r to thef u n c t i o n a l i z a t i o n o f p r e - e x i s t i n g s o l i d s ) f r o m t h e v a n t a g eo f o rgan ic su r f ace chemis t ry a re many . i f r . mono layers a reunder thermodynanic contro l , and the ord.er that theyexhib i t represents a rn in imum in energy. They arere ra t i ve ry f ree o f de f ec ts . They w i r l f o r rn on anyappropr ia te l y exposed su r face , even on the ins ides o fob jec ts and on ve ry rough o r po rous su r faces . Assembly i se a s y t h e g o r d s u r f a c e i s s i m p l y e x p o s e d t o t h e t h i o lfor one or two hours at room temperature, renoved, andwashed . Mos t i rnpor tan t l y , morecura r se r f -assenb lyp rov ides g rea t con t ro l over ce r ta in o f the phys ic l rp roper t ies o f the monorayers . rn par t i cu rd r , theth ickness o f the mono layer can be con t ro l red inapprox imate ly one-angs t rom s teps by va ry ing the leng th o ft h e p o l y m e t h y l e n e c h a i n s ( F i g u r e 3 ) , a n d t h e s t r u c t u r e o fthe so r id - l i qu id in te r face can be con t ro l red by va ry ingthe terrn inal group X.

on ly some o f the ru les fo r bu i rd ing wer l -o rderedmono layers a re p resen t ry known. rn par t i cu la r , thegues t ion o f the in f luence o f i ncommensura te s i zes o f theth io la te head g roups and the ta i l g roup x on the s t ruc tu reo f the mono layer i s no t we l l unders tood , and a lmos t a r Is t u d i e s h a v e u s e d t h i o l s o f t h e s t r u c t u r e H S ( c H 2 ) n X .None theress , f o r re la t i ve ly smar r o rgan ic func t lo i i a lg r o u p s ( X = F , C l , B r , I , C N , C O 2 H , C O N H 2 , e t c . ) t h e s emono layers cons t i tu te read i ry ava i lab le -ys tems tha tp resen t shee ts o f o rgan ic func t iona l i t y to I i qu id wa te r .B y w o r k i n g w i t h a n e x t e n d e d r a n g e o f t n i o t s H s n x ( y , 2 . . . ) ,both as pure compounds and as mixtures , vJe bel ieve that i tw i l l even tua t l y be poss ib le to make a ve ry b road range o fsu r faces .

- we t t i ng and con tac t Anqre . we have exp lo red theinteract ion between sol id and f iqu id by nealur ing contactangres (F igu re 4 ) . Th is techn ique has th ree g re i tadvan tages : F i r s t , i t i s exper i rnen ta l l y ve rys t r a i g h t f o r w a r d ( a t l e a s t f o r s t a t i c d r o p s ) . s e c o n d , i tis very sensi t ive to the st ructure and composi t ion o i theou te rmos t pa r t o f the su r face . we w i l l show be low tha tthe wa te r f r senses ' the so l id to a dep th ress than l_o E ,and p robabry less than 5 A a dep th sens i t i v i t y g rea te rthan that of many of the much more complex techniques ofvacuum phys ics . Th i rd , i t i s app l i cab le to the

Page 7: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

F igure 4

'Ilvcos(e)=Ysv-YSL

The f igure shows the egui l ibr ium shape of ad r o p o f a l i q u i d o n a f l a t , s o l i d s u r f a c e . T h econ tac t ang le o f the l i qu id i s the ang lebetween the sur face and the l ine drawn tangentto the sphere o f l i qu id where the l i qu id meetsthe su r face . Young 's equa t ion desc r ibes there lat ionship between the contact angle of theI iqu id and the su r face tens ions be tween each o ft h e t h r e e p h a s e s ( s o l i d , l i q u i d , a n d 9 a s ) ,w h e r e 7 S L , ' / S V , a n d ' y L V a r e t h e s o l i d - I i q u i d ,so l id -vapor , and l i qu id -vapor su r face tens ions .

Page 8: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

expro ra t ion o f so l id - r iqu id in te r faces and i s no tres t r i c ted to so l id - , r l po r ( vacuum) in te r faces . I t su f fe rsf rom two co r respond ing d isadvan tages . I t i s a macroscop icra ther than a morecu la r techn ique tha t i s , i t measures? p roper ty de te r rn ined by the gp l leq l i ve behav io r o f ala rge number o f mo lecures ( 1o r2 -1o l -3 o rgan ic func t iona lg roups occupy the in te r face be tween so l id and l i gu id fo r ad rop o f vo lume 1 pL) . r t a l so measures a ra t io 6 r su r faceenerg ies ra ther than any s ing re energy . rn p rac t i ce , thel igu id -vapor in te r fac ia l f ree

c o s 0 - ( r S V ^ / S L ) / t 6 V

e n e r g y ( t h a t i s , t h e s u r f a c e t e n s i o n ) i s w e r r k n o w n r s o- tha t the p roper ty measured d i rec t ry i s the d i f fe renceb e t w e e n t h e s o r i d - v a p o r a n d s o l i d - r i q u i a f r e e e n e r g i e s .Vary ing the p roper t ies o f the con tac€ ing r iqu id - - - i o ,exampre the s i ze o f i t s cons t i tuen t no recures o r i t shydroph i l i c i t y can p rov ide some ins igh t i n to the na tu reo f t h e f o r c e s d u e t o t h e s o l i d .

en t I ave S e v i to f Wet t ing . We have used nono lJyerscomponent to invest igate a number of

c o m p r i s i n g a s i n g l ephenomena. One o f

the deptha s o l i d d o e s a

re levance to mo lecu la r recogn i t i on i ssens i t i v i t y o f we t t i ng . Ho* fa r i n tol i q u i d r f s e e r r ?

Two exper iments i l lust rate the approach that we haveused in s tudy ing th i s ques t ion . rn on6 , we p reparedm o n o r a y e r s o f a s e r i e s o f t h i o l s H S ( c H 2 ) r r o ( L H z j n c H r i nwhich a potar oxygen funct ionat i ty was- ;6; i t io f f i : i i u i , ac o n t r o r l e d ( b y n ) d i s t a n c e f r o m t h e s o l i a - r i q u i din te r face , and examined the we t tab i r i t y o f thesenono layers as a func t ion o f the ca rbon number D . We foundtha t / the in f luence o f the leng th o f the cha in becameconstant for nwet tab i f i t y o f the e ther -con ta in ing mono iayer su r face wasthe same as the we t tab i l i t y o f a pu re hydr6carbonmono layer (such as tha t f rom HS(cH2) rzcHg) . we conc ludethat the l iqu id water was able to sense the nbur iedr f e thergxygen (by wha tever mechan ism d ipo le -d ipo le in te rac t ionth rough the super f i c ia l a tky l g roups o r pene t ra t ion o fwa te r i n to the mono layer l on ly - th rough r l ss than 5 - to A o fa l k a n e .

fn the second-exper iment r w€ examined the we t tab i f i t y: l l ono l ,Ry : r= o f s inp le a lky r th io l s HS (cH2 ) ncHs on go td .we found f , tha t the we t tab i r i t y o f these-mof ro lay6r= u i=o.began to increase f or n

Page 9: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

120

90

60

30

e

0.0

-1.0 0

-cos e

F igure 5

s ss d\ -$ o$."$ .F

water ,/t ';--o-a

,/

hexadecane

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I Ether I

Fi lns on gord f ormed f rom ar -rr€rcaptoethers ,HS ( CH2 ) fOO ( CHZ ) nCHg , have in te r fac ia lproper t ies that change as the polar ether groupis bur ied underneath the in ter tace. Theadvanc ing con tac t ang les o f wa te r andhexadecane a re p lo t ted as a func t ion o f n , thenumber of rnethyrenes between the ether and thein te r face . A lso shown a re the con tae t angreson po lye thy le le g ryco l (pEG) and on a monorayero f docosane th io r on go ld . The fo rmer su r faceis one in which the ether groups are fu l lyexposed to the con tac t ing l i qu id ; the la t le r i sone in wh ich the re i s no con t r ibu t ion o f ane ther to the in te r fac ia l p roper t ies o f thes u r f a c e .

Page 10: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

0.0

120

ea

300

t2 l4 16 18 20

Number of Carbons in Alkanethiol

The contact angles on monorayers on gold formedf rom a lkane th io l s show p rogress ive ly lessdependence on the underry ing gold substrate asthe leng th o f the a lkane th io r

- i nc reases . The

advancing contact angles of water andhexadecane a re p lo t ted as a func t ion o f thenumber o f ca rbons in the a lkane th io l . The l i nethrough the data is incruded as a guide for thee y e .

4

q

(JI

10

F i g u r e 6 .

water

,,g-O-O-O-Q-6-9'hexadecane

Page 11: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

exper iment does no t d i s t ingu ish be tween d ipo le - inducedd ipore in te rac t ions invorv ing the wa te r ana the h igh rypo la r i zab le go ld subs t ra te f rom in te rac t ions resu l { , i ngf rom penetrat ion of water between the a lky l chains down tothe go ld . In e i the r even t , these exper i rnen ts con f i rm theconc lus ion tha t we t t i ng i s a shor t - range in te rac t ion . fnte rms re levan t to mo lecu la r recogn i t i on , the in te rac t iono f wa te r w i th a p ro te in o r r i gand i s domina ted by thestructure of the outerrnost few angstroms of thesem o l e c u l e s .

ReLa ted exper iments invorv ing func t iona l g roupspos i t i oned a t the su r face o f po rye thy lene p rov ideindependen t con f i rmat ion and i l l us t r l t i on o f the shor t -r a n g e c h a r a c t e r o f w e t t i n g . S

Mixed Monoravers . ou r mos t use fu l exper imenta lsys tems a re those based on mono layers incorpora t ing twod i f fe ren t componen ts , HS ( cH2 ) px ana HS ( cH2 ) ; y . By va ry ingx and Y f o r cha ins o f the same leng th (m = i i 1 , on l caninves t iga te in te rac t ions be tween X and 'y .9 bv mak ing thecha ins o f d i f fe ren t reng ths (m * n ) , one can in t rodu6econtro l led amounts of d isorder in to the outermost par ts oft h e m o n o l a y e r . 1 0 , 1 1

one type of exper iment based on rn ixed monolayers ispar t i cu ra r ry re leva i r t to mo lecu la r recogn i t i on . i { "p repared mono layers con ta in ing a m ix tu re o f n -a lky l th io l shav ing two d i f fe ren t l eng ths , and examined the we l t i ng o fth i s sys tem by l i qu id a lkanes . The ques t ion o f i n te res tconcerned in tercalat ion hras there any tendency forpar t i cu la r s i zes o r shapes o f a l kane mo lecu les to in te rac tpar t i cu la r l y favorab ly w i th these d iso rdered mono layersto f r comp le te f r the monorayer o r to f i l l i t t o a densL ,c rys ta l l i ne s ta te? Th is t ype o f shape-serec t i vein te rac t ion wourd , o f course , cons t i l u te a fo rn o fm o l e c u l a r r e c o g n i t i o n .

fn b r ie f , fo r th i s sys tem (F igure 7 ) , we found noev idence fo f th i s t ype o f shape-se lec t i ve we t t ing . Th isconc lus ion i s no t su rp r i s ing , s ince a I I i nd ica t ions a retha t shape se lec t i v i t y regu i res the p resence o f mo lecu la rcav i t i es hav ing de f ined s i ze and r ig id shape . There i s noreason to expect such r ig id i ty and order in a layer madeup o f l oose ly d i so rdered , f l ex ib le , a l ky l cha ins :

The exper iment does, however , po int to one futured i rec t ion fo r se l f -assemb led mono layers : tha t i s , towardthe deve lopment o f techn iques capab le o f assemb l ing r ig id.componen ts in to mono layers . The ab i l i t y to accomp l i sh

Page 12: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

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\ _ i

\ \ ,

M ixed mono layers on gord fo rmed f rom so lu t ionscon ta in ing m ix tu res o f HS ( CH2 ) Z fCH: andH S ( C H 2 ) f f C H : . T h e a b s c i s s a i s t n e - r a t i o , R , o ft h e c o n c e n t r a t i o n s o f H S ( C H 2 ) Z f C H f t oHS ( CH2 ) f fCHf in so lu t ions o f e thano l . Theupper g raph p lo ts the e l l i psomet r i c th i cknessof the monolayers. The lower graph p lots theadvanc ing con tac t angres o f wa te r , hLxadecane ,?nd b icyc rohexy l . The l i ne th rough the da ta i sinc luded as a gu ide fo r the eye .

40

30

20(o-(a

Figure 7 .

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/t ,/'. /o

Page 13: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

MoIecu Ia r Recoqn i t i on?

I

t h i s t ype o f assemb ly wou ld p rov ide the bas is fo r bu i ld ing

sur faces hav ing shape-se lec t i ve cav i t i es '

unders tand ing we t t ing w i l l ce r ta in l y he lp in

unders tand ing so lva t ion o f p ro te ins and l i gands by wa te r ,

and perhaps i t t unders tand ing p ro te in - l i gand b ind ing 'Whether the f i e ld o f mo lecu la r recogn i t i on pers is ts fo r

some t ime in i t s p resen t mode us ing gua l i ta t i vephys ica l -o rgan ic mode ls fo r impor tan t phenomena o r

rn " tn . t i t rnoves to a new mode based on (more ) success fu lguant i ta t ive methods f or cornputat ion and s imulat ioniema ins to be seen . I f t he io rmer , resu l t s f rom s tud ies

o f we t t i ng w i l l become a par t o f the comp lex p rocess o f

ana log re ison i tg , pa t te rn recogn i t i on , an l i n tu i t i on used

" " * by med ic ina i a r ra b io log ica l chern is ts in the des ign o f

new b ind ing agen ts . I f t he la t te r , we t t i ng w i l l p rov ide

da ta f ro rn we I I -de f ined su r faces rv i th wh ich to tes t

theor ies o f so lva t ion and po ten t ia l func t ions fo r

s imu la t ions . 12 Bo th conneL t ions be tween sur face chen is t ry

and we t t ing and recogn i t i on a re concep tua l l y . i nd i rec t .A re more d i rec t connec t ions poss ib le? I be l ieve the

answer to th i s ques t ion to be t t yes , t t and . p ropose !h "fo l l ow ing top ic j as a reas in wh ich the f i e lds w i I Ip robab ly g row c loser toge ther .

1 ) S t ruc tu ra l l y Complex Sur faces . A p ro te in 91n be

cons iaerea , to one ap f l rox ina t ion , as a la rge =?3 f f : ]9 used

to cons t ruc t a cav i t y o f mo lecu la r d imens ions . r J Th is

v iew o f a p ro te in as a r ig id en t i t y i s p robab ly la rge ly

cor rec t i n some cases ( i .6 . , immunog lob ins ) and incor rec t

in o thers . In any even t , the su r face suppor t ing a se l f -

assernb led mono layer wou ld , i n p r inc ip le , a l so se rve

admi rab ly the func t ion o f sca f fo ld . A t p resen t , we do no t

know how to assemble the cavi ty , but we understand the

pr inc ip les : Use rnu l t i p le r i g id_ componen ts in mak ing up

tne monolayer that wi l l c rysLal l ize on the suppor t in to a

thermoaynai r ica l ly s table, ln ixed, two-d imensional crysta l

w i th su i face roughness appropr ia te to b ind mo lecu les .

A t a more p r im i t i ve leve l , one o f the mos t ve rsa t i l e

approaches to r lcogni t ion employs . par t l t ig l chromatography

t o d e t e c t d i f f e r e n c e s i n r e c o g n i t i o n . I a T h e p r i n c i p l e s o f

se l f -assernb ly a re excep t iona l l y we l l su i ted to the

ra t iona l des ign o f soph is t i ca ted so l id phases to tes t

t h e o r i e s o f a d s o r p t i o n , a n d , h e n c e , o f m o l e c u l a rr e c o g n i t i o n .

Page 14: THE PHYSICAL-ORGANIC CHEMTSTRY OF SURFACES, AND ITS

2 ) u l t r a - T h i n L i s t i d F i l m s . r t i s c r e a r f o r m a n yl iqu ids tha t p rox in i t y o f the r iqu ia to a so r id suppor tmod i f i es the s t ruc tu re and p rope i t i es o f the l i gu ia :15T o t w a t e r , o n e , b u t n o t t h e o n l y , m a n i f e s t a t i o n o f t h i sin te rac t ion i s the hydrophob ic e f tec t . se l f -assembredmonorayers prov ide the oppor tuni ty for s tudying thein te rac t ion be tween o rgan ic su r fa les bear ing comp lexorgan ic func t iona l i t y and wa te r . These s tud ies n .y becar r ied ou t e i the r us ing we t t ing o r th rough vaporadsorpt ion and temperature-progiammed desorpt ion.

3 ) s t u d i e s u s i n g t h e F o r c e B a l a n c e . T h e f o r c eba lance , an ing l rument deve loped by faUor andr s r a e l a c h v i l i , t 0 p r o v i d e s r e m a r k a b i y d e t a i l e d i n f o r m a t i o nabou t the in te rac t ions be tween sur faces a t d i s lances o fnanomete rs . A11 tha t i s requ i red to me ld the t ypes o fs tud ies desc r ibed here us ing se l f -assernb led *on-o iayersw i th more guan t i ta t i ve s tud ies us ing the fo rce ba ] lnce oF ,p e r h a p s , u l t i m a t e l y , t h e a t o m i c f o r c e m i c r o s c o p e , l T i s t h edeveropment o f rou t ine p rocedures fo r p repar ing su i tab ryf l a t s u b s t r a t e s .

. 4 ) M o d e l s o f c e r r s u r f a c e s . M u c h o f p r a c t i c a limpor tance invo lv ing 4q tecu la r recogn i t i on Lakes p lace a tt h e s u r f a c e o f c e r l ; . 1 8 r t i s u n c r e a r h o w i m p o r t a n t t h eprox i rn i t y o f the recep to r to the su r face m igh t be , o r howsur face a t tachment rn igh t be man i fes t . I t i ; p robab le tha tTany in te res t ing aspec ts o f recep to r - l i gand in te rac t ionsin v iYo w i l l depend on the res t r i c t i on

" f the recep to r to

a f l u id , quas i - two-d i rnens ionar p lane . Two con t ra=L ingexampres of e f fects expected to be important are s ter icinh ib i t i on to b ind ing o f l a rge l i gands to recep to rs , dueto prox i rn i ty to the cer l sur f ace ( and to the othercomponents on i t ) and enhancement of b inding of row-a f f i n i t y s y s t e m s ( . . g . v i r a r c e l l - s u r f a c e r e c e p t o r s ) d u eto a fo rm o f nu l t i -po in t , coopera t i ve a t tachmenta t t r i bu tab le to c lus te r ing o f l i gands by d i f fus ion on thece l l and c lus te r ing o f t i gands by s t ruc lu re on the v i rus .

overa l l , we a re conv inced tha t much o f morecura rrecogn i t i on i s due to in te rac t ion be tween mo lecura rs u r f a c e s ( o r m o r e p r o p e r l y , i n t e r f a c e s ) , a n d t h a t t h es tudy o f moder o rgan ic su r faces and in te r faces w i r lu r t i na te ly p rov ide in fo rmat ion o f g rea t va lue inunders tand ing and con t ro r r ing morecu la r recogn i t i on .

Acknowl edgernents

The work desc r ibed here has been ca r r ied ou t by anexcep t iona l ry ab re g roup o f coworkers , among whom a l "

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I

II

IIIV

CoI in Ba in , Randy Ho lmes-Far ley , Pau I La ib in i s , and Kev inPr ime. Cer ta in par ts o f the work a re based on ac o l l a b o r a t i o n w i t h R a l p h N u z z o ( A T & T B e I I L a b o r a t o r i e s ) .Support was provided by the ONR and DARPA through theUn ive rs i t y Research In i t i a t i ve , and by the Na t iona lScience Foundat ion through suppor t o f the HarvardMate r ia l s Research Labora to ry .

Re fe rences

1 ) C . B . P o s t a n d M . K a r p 1 u s , J . A m . C h e m . S o c . , 1 9 8 6 ,1 0 8 , l . 3 1 - 7 . A . T . B r u e n g e r , G . M . C l o r e , A . M .G r o n e n b o r n , a n d M . K a r p l u s , P . N a t I . A c a d . S c i . U S A . ,1 9 8 6 , 8 3 , 3 8 0 1 . C . F . W o n g a n d J . A , M c C a m r n o n , J . A m .C h e m . S o c . , 1 - 9 8 6 t 1 0 8 , 3 8 3 0 . T . P . L y b r a n d , J . A .M c C a m m o n , a n d G . W i p f f , P r o c . N a t l . A c a d . S c i . U S A ,L 9 8 6 , 8 3 , 8 3 3 . J . A . M c C a m m o n , S c i e n c e t L 9 8 7 , 2 3 8 ,4 8 6 . J . D . M a d u r a a n d W . L . J o r g e n s e n , J . A m . C h e m .S o c . , l - 9 8 6 , 1 0 8 , 2 5 L 7 . J . P . B a r e m a n e , G . C a r d i n i ,a n d M . L . K l e i n , P h y s . R e v . L e t t . , 1 9 8 8 , 6 0 , 2 L 5 2 .

2 ) t C o n t a c t A n g l e , W e t t a b i l i t y a n d A d h e s i o n ' , F . M .Kowkes , Ed . , Advan . Chem. Ser . 43 , Amer ican Chemica lS o c i e t y , W d s h i n g t o n D . C . , L 9 6 4 . A . W . A d a m s o n ,I P h y s i c a l C h e m i s t r y o f S u r f a c € s ' , W i l e y - I n t e r s c i e n c e ,New York | 1 -976 | 3 rd ed . F . B rochard , Langmui r , l -986 ,5 , 4 3 2 . F . H e s l o t , N . F r a y s s e , a n d A . l ' 1 . C a z a b a t ,N a t u r e 1 9 8 9 , 3 3 8 , 6 4 0 .

3 ) C . D . B a i n a n d G . M . W h i t e s i d e s , A n g e w . C h e m . f n t e r n .E d . E n q . , i n p r e s s .

4 ) G . M . W h i t e s i d e s a n d G . S . F e r g u s o n , C h e m t r a c t s , 1 9 8 8 ,r , L 7 L .

5 ) S o l u t i o n : C . D . B a i n , E . B . T r o u g h t o n , Y . - T . T a o , J .E v a l l , G . M . W h i t e s i d e s , a n d R . G . N u z z o , J . A m . C h e m .S o c . , L 9 8 9 , 1 1 1 , 3 2 L . V a p o r : G . F . F e r g u s o n , H . A .B i e b u y c k , M . C h a u d h u r y , M . B a k e r , a n d G . M .W h i t e s i d e s , u n p u b l i s h e d .

5 ) M . D . P o r t e r , T . B . B r i g h t , D . L . A l I a r a , a n dC . E . D . C h i d s e y , J . A m . C h e m . S o c . , L 9 g 7 t L 0 9 , 3 5 5 9 .L . S t r o n g a n d G . M . W h i t e s i d e s , L a n g m u i r , l - 9 8 8 , 4 ,5 4 6 .

7 ' ) C . D . B a i n a n d G . M . W h i t e s i d e s , J . A m . C h e m . S o c . ,L 9 B 8 , 1 1 0 , 5 8 9 7 .

I ) 1 " 1 . W i l s o n a n d G . M . W h i t e s i d e s , J . A m . C h e m . S o c . ,L 9 8 8 , 1 1 0 , 8 7 1 8 . S . R . H o l m e s - F a r l e y , C . B a i n a n d G .M . W h i t e s i d e s L a n c r n u i r , l - 9 8 8 , 4 , 9 2 L . S . R . H o l m e s -F a r l e y , R . H . R e a m e y , R , N u z z o , T . J . M c C a r t h y a n d G . M .W h i t e s i d e s , L a n g r n u i r , 1 9 8 7 , 3 t - 1 9 9 .

9 ) C . D . B a i n a n d G . M . W h i t e s i d e s , J . A m . C h e m . S o c . , i np r e s s . C . D . B a i n , J , E v a l l , a n d G . M . W h i t e s i d e s ,

. i b i d . , i n p r e s s .

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1 0 )

1 1 )

]-2)

C . D . B a i n a n d G . M . W h i t e s i d e s ,6 2 .C . D . B a i n a n d G . M . W h i t e s i d e s ,1 9 8 8 , 1 1 0 , 3 6 6 5S . D i e t r i c h , i n

S c i e n c e , L 9 8 8 , 2 4 O ,

J . A m . C h e m . S o c . ,

I Phase Trans i t i ons and Cr i t i ca lP h e n o m e n a ' I C . D o r n b a n d J . L . L e b o w i t z , E d s . , A c a d e m i cP r e s s , N e w Y o r k , l _ 9 8 8 , V o I . L 2 .

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1 4 ) s . A l l e n m a r k , r c h r o m a t o g r a p h i c E n a n t i o s e p a r a t i o n :M e t h o d s a n d A p p l i c a t i o n S , , W i l e y , N e w y o r k , 1 9 8 8 .

1 5 ) B . V . D e r j a g u i n a n d N . V . C h u r a e v , i n r F l u i df n t e r f a c i a l P h e n o m e n a t , C . A . C r o x t o n , E d . , W i l e y , N e wY o r k , 1 9 8 5 , C h a p t e r 1 5 ; C . T a n f o r d , l T h e H y d r o p h o b i cE f f e c t ' , W i l e y , N e w Y o r k I t 9 7 3 .

1 6 ) J . N . I s r a e l a c h v i l i a n d A . T a b o r , P r o c . R o v a l S o c . ,I 9 7 2 b I A 3 3 1 , L 9 . J . N . I s r a e l a c h v i l i , J . C o I I o i dI n t e r f a c e S c i . , l - 9 8 6 , l - 0 0 , 2 6 3 . J . N . f s r a e l a c h v i l ia n d H . K . C h r i s t e n s o n , P h y s i c a A , L 9 I G , 1 4 O A , 2 7 B .

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1 8 ) A . D . L a n d e r , T r e n d s N e u r o s c i e n c e s , 1 9 8 9 , L 2 , 1 8 9 .G . M . E d e l m a n , A n n . R e v . C e l I B i o l , ] - 9 g 6 , 2 , g 1