Water Wettability of Proteins Adsorbed at the Hydrogel-Water Interface

20 Water Wettability of Proteins Adsorbed at the Hydrogel-Water Interface

Downloaded by UNIV OF MISSOURI COLUMBIA on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch020

FRANK J. HOLLY and MIGUEL F. REFOJO Eye Research Institute of Retina Foundation, 20 Staniford Street, Boston, Mass. 02114

The h y d r o g e l s i n g e n e r a l , and c r o s s l i n k e d p o l y ( h y d r o x y e t h y l m e t h a c r y l a t e ) [PHEMA] i n p a r t i c u l a r , a r e p r e s e n t l y b e i n g used as c o n t a c t l e n s m a t e r i a l s and a r e c o n s i d e r e d as b i o m a t e r i a l s o r c o a t i n g s f o r b i o m a t e r i a l s . The w a t e r - s o l u b l e s u r f a c e - a c t i v e s u b s t a n c e s i n the b l o o d plasma and t i s s u e f l u i d s i n c l u d i n g t e a r s a r e proteinaceous i n character. Therefore, the c h a r a c t e r i s t i c s o f t h e i n t e r f a c e between h y d r o g e l s and water and i t s i n t e r a c t i o n with d i s s o l v e d p r o t e i n s are o f importance. P a s t Work L i t t l e i s known o f t h e e n e r g e t i c s o f t h e h y d r o g e l water i n t e r f a c e . Water has an u n e x p e c t e d l y h i g h advanci n g c o n t a c t a n g l e (60-80°) on PHEMA g e l s u r f a c e s (1) and even g e l s o f more h y d r o p h i l i c polymers such as p o l y ( g l y c e r y l m e t h a c r y l a t e ) [PGMA], and p o l y ( h y d r o x y e t h y l a c r y l a t e ) [PHEA] w i t h t w i c e t h e e q u i l i b r i u m water c o n t e n t o f PHEMA e x h i b i t advancing c o n t a c t a n g l e s as h i g h as 40° (2^) . A s o l i d w i t h such a low water w e t t a b i l i t y i s e x p e c t e d t o have an i n t e r f a c i a l t e n s i o n o f cons i d e r a b l e magnitude a g a i n s t water. The low r e c e d i n g c o n t a c t a n g l e v a l u e on h y d r o g e l s , however, i n d i c a t e s t h a t t h i s i s n o t t h e c a s e . Due t o t h e c o n s i d e r a b l e segmental m o b i l i t y o f t h e s u r f a c e polymer c h a i n s o f t h e g e l m a t r i x , o r i e n t a t i o n a l changes a t t h e g e l - w a t e r i n t e r f a c e seems t o l e s s e n t h e i n t e r f a c i a l t e n s i o n s i g n i f i c a n t l y by i n c r e a s i n g t h e d e n s i t y o f t h e h y d r o p h i l i c s i t e s a t the s u r f a c e . Tension a t the gel-water i n t e r f a c e has been e s t i m a t e d t o be f a i r l y low by o t h e r s (_3) · I f such i s t h e c a s e , then t h e w a t e r - s o l u b l e p r o t e i n s are e x p e c t e d t o i n t e r a c t l i t t l e w i t h such an i n t e r f a c e and a d s o r p t i o n would p r o b a b l y e n t a i l l i t t l e i f any i r r e v e r s i b l e c o n f o r m a t i o n a l changes o r d e n a t u r a t i o n o f 267

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

268

HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS

the p r o t e i n m o l e c u l e s . P r o t e i n adsorbed a t h y d r o g e l - w a t e r i n t e r f a c e s has not been c h a r a c t e r i z e d i n d e t a i l . To o u r knowledge, no s y s t e m a t i c attempts have been made t o s t u d y t h e energ e t i c s i n c l u d i n g w e t t a b i l i t y o f p r o t e i n s adsorbed a t the h y d r o g e l - w a t e r i n t e r f a c e .


M a t e r i a l s and Methods We have s t u d i e d t h e e f f e c t o f p r o t e i n a d s o r p t i o n on t h e water c o n t a c t a n g l e on PHEMA g e l s h a v i n g e i t h e r 38.9 o r 40.0% e q u i l i b r i u m water c o n t e n t . Some measurements have a l s o been made on a c r o s s l i n k e d PHEA g e l c o n t a i n i n g 89.9% water a t e q u i l i b r i u m h y d r a t i o n . These g e l s a r e d e s c r i b e d elsewhere i n t h i s book (2) . Occasi o n a l l y p o l y ( m e t h y l m e t h a c r y l a t e ) [PMMA] and p o l y e t h y l ene [PE] were a l s o used as a d s o r b e n t s f o r comparison. The p r o t e i n used i n t h i s s t u d y was b o v i n e serum albumin [BSA] (Cohn F r a c t i o n V, Sigma C h e m i c a l Co., S t . L o u i s , MO) w i t h a m o l e c u l a r weight o f 68,000 g/mole. A h i g h m o l e c u l a r weight, c o m m e r c i a l l y a v a i l a b l e g l y c o p r o t e i n , b o v i n e s u b m a x i l l a r y mucin [BSM] (type I , MW= 4.1x10 g/mole, Sigma C h e m i c a l Company, S t . L o u i s , MO) and egg lysozyme [LYZ] (as c h l o r i d e , MW=14,800 g/mole, M i l e s L a b o r a t o r i e s , Kankakee, IL) were a l s o used i n t h e s t u d y . F o r comparison purposes, s y n t h e t i c , w a t e r - s o l u b l e polymers were a l s o used i n t h e p r e l i m i n a r y measurements . C o n t a c t a n g l e s were measured by u s i n g a Ramè-Hart goniometer w i t h an e n v i r o n m e n t a l chamber so t h a t t h e h y d r o g e l sample c o u l d be k e p t i n an atmosphere s a t u r a t e d w i t h water vapor. S u r f a c e t e n s i o n o f t h e aqueous s o l u t i o n s was measured by t h e Wilhelmy method u s i n g a roughened p l a t i n u m b l a d e and a Cahn E l e c t r o b a l a n c e Mod e l RM-2. Both t h e s u r f ice t e n s i o n and t h e c o n t a c t a n g l e were m o n i t o r e d f o r t h i r t y m i n u t e s . Measurements made a t l o n g e r times i n d i c a t e d t h a t such a time i n t e r v a l was s u f f i c i e n t t o approximate e q u i l i b r i u m a t l e a s t f o r s o l u t i o n s w i t h c o n c e n t r a t i o n s 1 0 " wt% and above. 6

2

E f f e c t o f S y n t h e t i c and B i o p o l y m e r s on A d h e s i o n T e n s i o n The a d v a n c i n g (but s t a t i c ) c o n t a c t a n g l e f o r aqueous polymer s o l u t i o n s was determined on t h e PHEMA g e l , the PHEA g e l , PMMA, and p o l y e t h y l e n e (_2) . I n a d d i t i o n t o t h e s o l u t i o n s o f t h e t h r e e b i o p o l y m e r s a l r e a d y described, the following synthetic or modified natural polymers were employed: P o l y ( v i n y l a l c o h o l ) ( G e l v a t o l 20/90), p o l y ( e t h y l e n e oxide) (WSR-205), h y d r o x y e t h y l c e l l u l o s e , and p o l y ( v i n y l p y r r o l i d o n e ) . The polymers


20.

HOLLY

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Water Wettability

REFOJO

of

Proteins

269

were d i s s o l v e d i n d i s t i l l e d water t o a s o l u t i o n concent r a t i o n o f 0.1% by weight. Aqueous s o l u t i o n s o f sodium s u l f o s u c c i n a t e and methyl c e l l u l o s e (USP grade) a t 0.1% c o n c e n t r a t i o n were a l s o used on PMMA and PE. See T a b l e I f o r the s u r f a c e t e n s i o n o f t h e s e s o l u t i o n s .

Table I


Surface

Tension

o f Aqueous S o l u t i o n s o f S y n t h e t i c and

B i o p o l y m e r s and S u r f a c t a n t *

SOLUTE

SURFACE TENSION

Poly(vinyl

alcohol)

52. ,6

Poly(vinyl

pyrrolidone)

67. ,5

Methyl c e l l u l o s e

55. ,4

Hydroxyethyl c e l l u l o s e

63. ,5

Poly(ethylene

60. ,7

oxide)

BSM

41.,2

B SA

52. ,8

LYZ

58. .0

Sodium s u l f o s u c c i n a t e

30, .8

(dyne/cm)

* a t 0.1% c o n c e n t r a t i o n

and a t30 minutes

F i g u r e 1 shows the r e s u l t s where the a d h e s i o n t e n s i o n o f the s o l u t i o n s t o the g e l o r s o l i d are p l o t t e d as a f u n c t i o n o f t h e s o l u t i o n s u r f a c e t e n s i o n . F o r each s u b s t r a t e , the polymer s o l u t i o n s y i e l d e d d a t a which appear t o e x h i b i t a l i n e a r dependence o f the a d h e s i o n t e n s i o n on the s o l u t i o n s u r f a c e t e n s i o n . The 45° broken l i n e on the graph r e p r e s e n t s zero c o n t a c t a n g l e (adhesion tension = s o l u t i o n surface tension). I t s i n t e r c e p t w i t h the s t r a i g h t l i n e , c a l c u l a t e d by the l e a s t square method f o r each s u b s t r a t e , r e p r e s e n t s a k i n d o f



270


" c r i t i c a l s u r f a c e t e n s i o n " o b t a i n e d w i t h t h e s e aqueous s o l u t i o n s f o r t h e p a r t i c u l a r s o l i d o r g e l . The s l o p e s o f t h e s t r a i g h t l i n e s f o r PE, PMMA, PHEMA, and PHEA a r e e q u a l t o -0.76, -0.18, +0.16, and +0.46, r e s p e c t i v e l y . I t has been r e p o r t e d (4-6,) t h a t aqueous s o l u t i o n s o f s u r f a c t a n t s , when p l a c e d on a n o n p o l a r s o l i d such as p a r a f f i n , form c o n t a c t a n g l e s t h a t e x h i b i t a g e n e r a l r e l a t i o n s h i p with respect t o the surface t e n s i o n o f the s o l u t i o n . The p r o d u c t o f t h e s u r f a c e t e n s i o n o f t h e s o l u t i o n and t h e c o s i n e o f t h e c o n t a c t a n g l e (adhesion tension, W ) v a r i e s l i n e a r l y with s o l u t i o n surface ten s i o n ( Υ χ ) such t h a t T

ν

W_ = γ, .cos0 = Αγ, Τ 'lv lv 1

+ Β

where A < 0

F o r low m o l e c u l a r weight s u r f a c t a n t s , f o r which the Gibbs a d s o r p t i o n e q u a t i o n i s v a l i d , t h e s l o p e , A, has been shown (£,6) t o be e q u a l t o t h e n e g a t i v e r a t i o o f t h e s u r f a c e excess c o n c e n t r a t i o n o f t h e s u r f a c t a n t a t t h e s o l i d - w a t e r i n t e r f a c e (^ ±) and t h a t a t t h e s o l u t i o n - a i r i n t e r f a c e (Γ^ ) . Assuming t h a t t h e s o l u t e s u r f a c e excess c o n c e n t r a t i o n a t the s o l i d - v a p o r i n t e r f a c e i s z e r o , we may w r i t e : s

ν

A

r

r

= - sl/ lv

When A = -1, t h e e x t e n t o f a d s o r p t i o n a t t h e two i n t e r f a c e s i s t h e same and t h e a d h e s i o n energy o f t h e s o l u t i o n t o t h e s o l i d becomes independent o f t h e s o l u tion surface tension. Such a s i t u a t i o n has a c t u a l l y been o b s e r v e d (2). I f A = 0, t h e c o n c e n t r a t i o n o f t h e s o l u t e i s t h e same a t t h e s o l i d - s o l u t i o n i n t e r f a c e as i n t h e b u l k , i . e . t h e i n t e r f a c i a l excess c o n c e n t r a t i o n is zero, r ^ = 0. Such a s u r f a c e t e n s i o n dependence o f a d h e s i o n t e n s i o n was demonstrated by Lucassen-Reynd e r s (6) u s i n g t h e d a t a o f Smolders (8) o b t a i n e d i n a hydrogen gas - aqueous a n i o n i c s u r f a c t a n t s o l u t i o n mercury system a t t h e e l e c t r o c a p i l l a r y maximum (zero i n t e r f a c i a l charge d e n s i t y ) . P o s i t i v e s l o p e s would mean t h a t the s u r f a c e excess c o n c e n t r a t i o n o f the s o l u t e ( s ) i s n e g a t i v e , i . e . t h e s o l u t e c o n c e n t r a t i o n i s lower a t the i n t e r f a c e than i n t h e b u l k . F i g u r e 1 demonstrates t h a t t h e s u r f a c e - a c t i v e polymer s o l u t i o n s behave s i m i l a r l y t o s i m p l e s u r f a c t a n t s o l u t i o n s on n o n p o l a r s o l i d s . W h i l e t h e v a l i d i t y o f t h e Gibbs e q u a t i o n does n o t extend t o macromolecules, i t i s s



20.

HOLLY

Water Wettability of Proteins

A N D REFOJO

271

r e a s o n a b l e t o suppose t h a t t h e magnitude o f t h e s l o p e A r e f l e c t s i n some way t h e magnitude o f t h e i n t e r f a c i a l adsorption o f the polymeric solute o r a t l e a s t i t s e f f e c t on t h e i n t e r f a c i a l t e n s i o n . I f t h i s i s s o , then t h e r e s u l t s i n d i c a t e t h a t con s i d e r a b l e a d s o r p t i o n o f the polymeric s o l u t e takes p l a c e a t the p o l y e t h y l e n e - w a t e r i n t e r f a c e . T h i s i s ex p e c t e d i n view o f t h e l a r g e i n t e r f a c i a l t e n s i o n a t t h e p o l y e t h y l e n e - w a t e r boundary. T h i s e f f e c t i s c o n s i d e r a b l y l e s s f o r t h e more p o l a r PMMA s u r f a c e as shown by the s m a l l e r a b s o l u t e , b u t s t i l l n e g a t i v e , v a l u e o f A f o r PMMA. T h i s i s no doubt due t o t h e g r e a t e r i n t e r a c t i o n o f PMMA s u r f a c e w i t h water which competes w i t h the p o l y m e r i c s o l u t e s f o r t h e a v a i l a b l e a d s o r p t i o n sites. I t i s q u i t e i n t e r e s t i n g t h a t t h e PHEMA g e l i s c h a r a c t e r i z e d by a s t r a i g h t l i n e w i t h a s l i g h t l y p o s i t i v e s l o p e , which may i n d i c a t e no d i f f e r e n c e between i n t e r f a c e and b u l k c o n c e n t r a t i o n o r even lower concen t r a t i o n a t t h e i n t e r f a c e than i n t h e b u l k . The s l o p e o f the l i n e f o r PHEA i s a l s o p o s i t i v e and even g r e a t e r than t h a t f o r PHEMA. The s c a t t e r o f t h e d a t a i s c o n s i d e r a b l e f o r each s o l i d and i t i s e s p e c i a l l y l a r g e f o r t h e g e l s . There a r e i n d i c a t i o n s (9) t h a t t h e type o f t h e s o l u t e s em ployed a l s o a f f e c t the p o s i t i o n o f the s t r a i g h t l i n e i n t h i s Wolfram p l o t (adhesion t e n s i o n v s . s u r f a c e t e n s i o n ) e s p e c i a l l y f o r multicomponent p o l y m e r i c s o l u t i o n s . Thus, c o n s i d e r i n g t h e wide spectrum o f t h e p o l y m e r i c s o l u t e s used i n t h i s s t u d y , t h e r e s u l t s f i t t h e s t r a i g h t l i n e s reasonably w e l l . F i l m Pressure

o f Biopolymers Adsorbed a t the I n t e r f a c e

Attempts have been made t o o b t a i n t h e f i l m p r e s sure o f t h e b i o p o l y m e r s a t the water-PHEMA g e l and water-polyethylene i n t e r f a c e s as a f u n c t i o n o f t h e s o l u t i o n c o n c e n t r a t i o n . The d e c r e a s e o f i n t e r f a c i a l t e n s i o n , due t o t h e a d s o r p t i o n o f t h e b i o p o l y m e r , i s the f i l m p r e s s u r e o f t h e a d s o r b a t e a t t h e i n t e r f a c e . I t i s g i v e n by t h e d i f f e r e n c e between t h e s o l u t i o n s o l i d and t h e s o l v e n t - s o l i d a d h e s i o n t e n s i o n s (10) :

ι

'sol'n

sol'n

f

w

w

The f i l m p r e s s u r e s o f albumin c a l c u l a t e d f o r t h e PHEMA g e l - water and t h e PE - water i n t e r f a c e s , t o gether with the f i l m pressures a t the w a t e r - a i r i n t e r -



272


i n t e r f a c e , a r e shown i n F i g u r e 2. The r e s u l t s show t h a t t h e f i l m p r e s s u r e o f albumin i s s i m i l a r a t t h e w a t e r - a i r and w a t e r - p o l y e t h y l e n e i n t e r f a c e s , e s p e c i a l l y near the p h y s i o l o g i c c o n c e n t r a t i o n range (0.1 - 6%). On t h e o t h e r hand, t h e f i l m p r e s s u r e o f albumin a t t h e PHEMA-water i n t e r f a c e i s n o t s i g n i f i c a n t l y d i f f e r e n t from z e r o w i t h i n t h e r a t h e r l a r g e d e v i a t i o n o f t h e d a t a , a l t h o u g h i t appears t o d e c r e a s e with decreasing s o l u t i o n concentration. The f i l m p r e s s u r e o f mucin and lysozyme show simi l a r v a r i a t i o n w i t h s o l u t i o n c o n c e n t r a t i o n a t t h e PEwater i n t e r f a c e . A t t h e PHEMA g e l - water i n t e r f a c e , however, t h e f i l m p r e s s u r e i s n e g a t i v e a t h i g h s o l u t i o n c o n c e n t r a t i o n s and approximates z e r o as t h e s o l u t i o n c o n c e n t r a t i o n i s lowered. W e t t a b i l i t y o f B i o p o l y m e r s A d s o r b e d onto PHEMA and PE The c o n t a c t a n g l e o f t h e s e s s i l e d r o p l e t o f t h e b i o p o l y m e r s o l u t i o n s on PHEMA and PE s u b s t r a t e s t h a t had been exposed t o a b i o p o l y m e r s o l u t i o n o f t h e same c o m p o s i t i o n and c o n c e n t r a t i o n was d e t e r m i n e d . By u s i n g the same s o l u t i o n f o r t h e a d s o r b i n g medium and f o r t h e w e t t a b i l i t y measurement, we a v o i d e d g r o s s changes i n the i n t e r f a c i a l t e n s i o n a t t h e d r o p l e t - s u b s t r a t e bounda r y due t o d i s s o l u t i o n o r f u r t h e r d e p o s i t i o n o f t h e biopolymers. A f t e r t h e s u b s t r a t e had been i n t h e b i o p o l y m e r s o l u t i o n o f a g i v e n c o n c e n t r a t i o n f o r 1 hour, i t was g e n t l y b l o t t e d w i t h a g r e a s e - f r e e f i l t e r paper t o r e move t h e excess l i q u i d . Then t h e c o n t a c t a n g l e o f a s e s s i l e d r o p l e t was d e t e r m i n e d i n t h e e n v i r o n m e n t a l chamber 30 minutes a f t e r the d r o p l e t o f t h e same s o l u t i o n was d e p o s i t e d on t h e s u b s t r a t e . A f t e r w a r d s , t h e s u b s t r a t e s u r f a c e was r i n s e d w i t h d i s t i l l e d water, b l o t t e d , and t h e c o n t a c t a n g l e was determined u s i n g t h e same b i o p o l y m e r s o l u t i o n . T h i s measurement was r e p e a t e d a t f i v e d i f f e r e n t b i o p o l y m e r c o n c e n t r a t i o n s f o r both substrates. F i g u r e s 3 and 4 show t h e advancing c o n t a c t a n g l e s o b t a i n e d w i t h bovine serum albumin s o l u t i o n on albumin adsorbed on PHEMA g e l and p o l y e t h y l e n e as a f u n c t i o n o f s o l u t i o n c o n c e n t r a t i o n . These graphs a l s o c o n t a i n t h e d a t a o b t a i n e d w i t h albumin s o l u t i o n on t h e c l e a n , a l bumin-free s u r f a c e s . The r e s u l t s o b t a i n e d w i t h mucin and lysozyme a r e n o t shown here b u t a r e q u a l i t a t i v e l y similar (11). A l b u m i n adsorbed from s o l u t i o n s c o n t a i n i n g t h e p r o t e i n a t c o n c e n t r a t i o n s o f t h e same o r d e r o f magnitude as t h a t o f p h y s i o l o g i c ( t e a r and b l o o d plasma)


20.

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Water Wettability

A N D REFOJO

of

273

Proteins

T=25°C ζ 4

ο °CO ζ

Hi 30·


Ο

20·

CO

I ο < 10

20

30

"70

50

60* • >*L

80

90

LIQUID SURFACETNr^ENSION (dyne/cm)

Figure 1. Adhesion tension of aqueous polymer solutions to gels and solids as a function of the solution surface tension. (Large symbols indicate values obtained with pure water also showing standard deviation.)

T=25°C

Θ Θ Θ

Φ

Φ

4

Θ

10

Φ

10

10

SOLUTION CONCENTRATION ( %)

Figure 2. Film pressure of bovine serum albumin at three interfaces: O, water-air; • , water-polyethylene; Δ , water-PHEMA gel. (Vertical lines show standard deviation.)



T-25°C

[3

Water Wettability of Proteins Adsorbed at the Hydrogel-Water Interface

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