Macromolecular Conformation in the Two-Dimensional State

27 Macromolecular Conformation in the

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Two-Dimensional State G. GABRIELLI and M. PUGGELLI Institute of Physical Chemistry, Via G. Capponi 9, Florence, Italy

Experiments

were done on monomolecular

-benzyl-L-aspartate (ΡβΒΑ) tained and

from

a spreading

solvent

poly-γ-benzyl-L-glutamate

spreading

solvent

chloroacetic

acid

solutions.

when a solvent is obtained pyridine.

devoid

from

of polymer

of pyridine

solvents

For monolayers

containing of PγBG

α-helixes on the HCl support dichloroacetic

acid

obtained

on a support

The results

films

of p o l y - β

interface

with and without (PγBG)

of chloroform

-helical monomolecular

films

at the water/air

show

a high

from

a

of HCl or di-

ΡβΒΑ

is spread;

ob

pyridine

that the α are

obtained

β-helical

form

percentage

there is evidence

and the random

of

of the

coil on the

support.

D y

comparing experimental spreading isotherms and their thermodynamic values with theoretical ones (1, 2, 3, 4, 5, 6, 7), we can obtain parameters of the distribution and energies of macromolecules at an interface (8, 9,10,11,12). The macromolecular configuration in the bulk phases (13) and the type of interface (ll, 14, 15, 16, 17, 18, 19, 20) are important i n determining the distribution and polymeric conformation in the two-dimensional state. Thus, by comparing the configurations i n the three-dimensional and two-dimensional phases, we can define the role of the interface i n modifying the configuration found i n the bulk phase. This work was done to obtain various macromolecular forms i n the two-dimensional state for the same polymer. Since monomolecular films at the water-air interface are often treated theoretically (21) as twodimensional solutions, the most convenient comparison appeared to be between two- and three-dimensional solutions. W e studied two polypeptidic polymers which have conformations that can be modified i n solution by various solvents: 347

Goddard; Monolayers Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

348

MONOLAYERS

(a) Two-dimensional films of ΡβΒΑ at the water-air interface, obtained from various spreading solvents on the same liquid substrate. (b) Two-dimensional films of P y B G at the water-air interface, obtained from the same spreading solvent on various liquid substrates. The macromolecular forms at the interface were characterized by determining the spreading isotherms at various temperatures and by the technique of multiple internal reflection ( M I R ). Experimental The polymers P/3BA (mean molecular weight, 4200) and P y B G (mean molecular weight, 100,000) were supplied by Miles Yeda, L t d . , Israel. The solvents, high purity chloroform and dichloroacetic acid, were supplied by Carlo Erba, M i l a n , Italy; the pyridine for chromatography was supplied by Riedel de Haën, Hannover. The support solutions were twice-distilled water, purified by activated charcoal, high purity hydrochloric acid (Riedel de Haën), and dichloroacetic acid (Carlo E r b a ) . Surface pressure, measured by an apparatus already described (8, 9, 10), was ±0.05 dyne/cm; the surface area was 0.02 m / m g . A l l isotherms were constructed by points after each area reached a constant surface pressure to guarantee that surface equilibrium was attained. A l l isotherms were obtained at various surface concentrations to ensure repro2

Figure

1.

Spreading isotherms of ΡβΒΑ at 15°, and 25° C obtained from solvent I

20°,


27.

G A B R I E L L I A N D puGGELLi

0.30

Figure

2.

Macromolecular

0.40

0.50

Spreading isotherms of ΡβΒΑ at and 25°C obtained from solvent II

Conformation

349

0.60

15°, 20°

ducibility and perfect spreading. Special care was taken in forming ΡβΒΑ monolayers, starting from pyridine-rich spreading solutions. To avoid introducing drops into the lower phase, the spreading was done slowly, with the microsyringe held nearly parallel to the surface. The spreading solutions, prepared for solubilizing the polymer in the solvent or mixture of solvents were used 24 hr after preparation. IR spectra were recorded with a Perkin-Elmer 225 spectrophotome ter with accessories for M I R from Wilks Scientific Co., model 50 with a germanium prism, 52.5 X 20, 1 mm thick. To transfer polymer films from the liquid substrate to the germanium plate, the Langmuir-Blodgett technique was used (22, 23); oleic acid was the piston oil, and the withdrawal rate was 1 m m / m i n . The germanium plate was washed with a detergent ( T i d e ) , carefully rinsed with water, acetone, and chloroform, and dried for 3 hr in vacuo. Cleaning was always controlled by the IR spectrum of reflection on the germanium plate only. Results and Discussion ΡβΒΑ Monolayers. A l l ΡβΒΑ monolayers were obtained on twicedistilled water, starting from two different spreading solvents. Spreading solvent I, composed of chloroform containing 0.8 vol % dichloroacetic acid, ensured complete solubilization of the polymer; spreading solvent II


350

MONOLAYERS

was composed of solvent I and 80 vol % pyridine. Figure 1 shows the spreading isotherms at 15°, 20°, and 25°C for ΡβΒΑ monolayers obtained from solvent I. Figure 2 shows the monolayers obtained from solvent II. Since the π-Α plot (Figure 2) was nearly independent of temperature, the line shows the mean values, and the symbols refer to the various temperatures. For spreading solvent I (Figure 1), all the isotherms have a surface pressure "plateau" above 10 dynes/cm; thus, the curves are composed of two parts. F o r pressures lower than the plateau, we derived from Ή—A experimental values the corresponding equations of the two-dimensional state ΤΓΑ—ΤΓ. F r o m their coefficients, using an accepted procedure (8, 9, 10,11), we derived the following parameters: ( 1 ) Bi, the coefficient of the first degree term i n the two-dimensional state equation, deduced theoretically by M . L . Huggins (7). (2) z\ the coordination number of the two-dimensional pseudo-lat tice of Singers theory ( 1 ). (3) f , the partial submersion factor i n the Frish and Simha theory m

(4).

(4) η /ζ, which, in the treatment by Motomura and Matuura (6), represents the interaction energy between macromolecular segments at the interface. These parameters are shown together at the hmiting area A (ob tained by extrapolating the straight portion of the high pressure range of the π-Α isotherms ) i n Table I. Table I shows that ( a ) the values of A agree satisfactorily with those found by others (24, 25, 26), but they do not constitute a valid criterion 2

0

0

Table I.

Parameters of P/3BA in the Two-Dimensional State

Temperature, A °C (m /mg) B Pyridine-free Spreading Solutions 15 0.650 0.997 20 0.680 1.000 25 0.698 1.010 0

2

z'

x

f 1

v /z 2

m

a

2.22 2.22 2.22

0.81 0.81 0.81

-31.5 -46.5 -16,0

Spreading Solutions Containing 80% Pyridine 2.23 15-25 0.535 1.008

0.83

-87.0

1

The η /ζ values are in cal/mole of monomeric unit. 2

Table II.

Thermodynamic Spreading Values of ΡβΒΑ from Pyridine-Free Spreading Solutions

A (m /mg)

A S ergs/cm Κ

Δ Η ergs/cm

0.72 0.70 0.68 0.66 0.64

+0.10 +0.16 +0.38 +0.68 +0.82

+ 28.7 + 45.6 +108.8 +193.7 +231.9

2

2

8

3


2

27.

G A B R E E L L I A N D PUGGELLi

Macromolecular

Conformation

Δ Η» 240 200 160 1208040-

0-I 1.9

1

2.0 3.

Figure

,

,

,

1

1

2.1

2.2

2.3

2.4

2.5

ΔΗ

vs. 1/A polymer

8

2

for

y^ ΡβΒΑ

for distinguishing and characterizing the macromolecular form present at the interface; (b) the values of z' and consequently of ω flexibility (19) [even considering the estimates made i n their deduction (8, 9 , 1 0 ) ] agree with a configuration at the two-dimensional helix which is not very flexible. Of course, even these parameters do not indicate a definite macromolecular conformation at the interface; (c) the values of f show that nearly 20% of the macromolecules are submerged in the aqueous substrate, typical of most of the polypeptidic type polymers; (d) the negative values of η /ζ indicate (8, 9, 10) that the main surface energies are attractive. W e also calculated the thermodynamic spreading functions [enthalpy ( ΔΗ ) and entropy ( AS ) ] to further our knowledge of polymeric distri bution at the interface (9, 10, 11, 27, 28, 29). Their values at 20°C for five surface areas are shown in Table II. Since the spreading functions are substantial, the polymer-interface system ( 27, 28, 29 ) is not athermic. In particular, the positive sign of the enthalpy means that i n the mono layers the attraction energies between macromolecular segments are prevalent if other enthalpic contributions are constant. This result is confirmed by the fact that the most important of these ( caused by sub mersion) should be negative if one considers the hydrophobic groups i n the macromolecules. The nearly linear dependence of ΔΗ on 1 / A (see Figure 3) further confirms that the spreading enthalpy depends largely on the attraction energy between macromolecular segments. The above discussion suggests a two-dimensional spiral on the sur face, with attractive energy between polymeric segments, which agrees satisfactorily with the a-helical form. This has been demonstrated by others for this and other macromolecules (25, 26, 30, 31, 32, 33). To con firm further the α-helical form on the surface, we recorded multiple reflection IR spectra at surface pressures both less than and greater than the plateau pressure ( Figure 4, part 1 ). The band characteristic of the m

2

8

8

8


2

351

352

MONOLAYERS

Ρ0ΒΑ

Figure 4. (1 ) MIR spectra of surface films of ΡβΒΑ from solvent I transferred on germanium prism (a) at surface pressure less than the plateau pressure, (b) at surface pressure greater than plateau pressure (2) MIR spectra of surface films of ΡβΒΑ from solvent II transferred on germa nium prism at high surface pressure

β form is missing, and the bands of the a or coil form are present (32). Since the preceding data show the existence of a macromolecular form more rigid than the coil with attractive energy between the segments at the interface, the presence of the α-helical form appears to be confirmed on the surface as the bulk phase (34). The pressure plateau with solvent I is attributed to either a first-order transition of the two-dimensional phase or to collapse. W i t h polypeptidic monolayers, surface pressure plateau has often been interpreted (30, 31, 33) as a collapse and, more precisely, as a passage of the a form from the monolayer to the bilayer. W e observed after every compression an obvious decrease of surface pressure with time which was characteristic of collapse. The identity of the I R spectra of monolayers transferred at pressures lower and greater than the plateau pressure may confirm that the plateau is not the result of a phase transition connected with a change in the macromolecular conformation. Even the rather low, negative surface entropy of about —0.15 e r g / c m K at the plateau agrees with the observation of others for bilayer formation (30, 31). Finally, if we con sider the collapse at equilibrium and calculate the transition enthalpy from the classic formula In 7Γ1/ΤΓ —ΔΗ/β[1/ΤΊ 1 / Γ ] , we obtain nearly 60 ergs/cm . This agrees favorably with the work required to remove the unit area of monolayer from the water surface as calculated 2

2

=

—

2

2


27.

G A B R I E L L I A N D puGGELLi

Macromolecular

Conformation

353

by Malcolm (30, 31) for other polymers. Therefore, the pressure plateau may be caused by collapse, and the curves of the isotherms at pressures greater than this must be compression curves of collapsed monolayers which are not represented by equations which are valid only for mono molecular films. Thus α-helical monolayers can be obtained from solvent I on the surface; in this case, the stable conformation in the three-dimensional spreading phase is not modified by the interface (35, 36). Therefore the polymer-support interactions are not as great as the polymer-polymer interactions, as confirmed by the small variation of the first degree coeffi cient of the equations of the two-dimensional state with temperature (Table I ) . W i t h regard to the monolayers from solvent II, particular care must be taken i n spreading. Because these isotherms at various surface con centrations have high reproducibility, the difference between the curves obtained with solvent I and these are not attributed to loss of the polymer from the interface to the substrate. Even i n this case we derived the two-dimensional state equation which corresponded to the mean isotherm and, therefore, is valid for the three temperatures. F r o m it we deduced the same parameters described earlier for spreading solvent I ( Table I ). The h'miting area of about 18 A /monomeric unit is less than that of the a form; this agrees with the findings of others (33) for polymethyl glutamate attributed to the β form. Naturally, this is not a criterion for characterizing the macromolecular form found on the surface. The values of z' and ω are the same size as those for spreading solutions I and may only signify that at the interface a much more flexible macromolecular form, like the random coil, w i l l not be found; however, these values cannot be used to distinguish the two helical forms. The value of f shows that the macromolecule is submerged to the same degree as an a form. The negative value of η /ζ indicates that the form at the interface is characterized by inter- or intramolecular attractive energies and there fore is probably not the random-coil type. 2

m

2

Since these spreading isotherms are independent of temperature, the coefficient of the first degree term is independent of temperature. Besides confirming the presence on the surface of a form different from that obtained with solvent I, the interaction energies between polymeric seg ments are smaller than in the preceding case. F r o m earlier observations and the value of the first degree coefficient of the two-dimensional state equation (Table I) which is equal to or less than that of the a form (37), it seems improbable that the random coil form exists on the surface. A helical form more extended than the a one—possibly a β form—appears to be more sound.


354

Figure

MONOLAYERS

5.

Spreading

isotherms of PyBG

at 15°,

20°,

and 25°C

on support I

A further, though indirect, confirmation of the existence of the form can be found by comparing the spreading enthalpies of the two forms. In fact, taking the difference between the enthalpy of the form obtained from solvent I and that from solvent II, taking as surface areas the limiting areas of the two forms, and considering the corresponding pressures, one obtains about 3500 cal/mole of monomeric unit; this agrees satisfactorily with the transition enthalpy a —» β i n the bidimensional phase for other compounds (38), taking into account the diversity of the hydrophobic chains. Finally, the multiple reflection IR spectra, obtained on the monomolecular films from solvent II and transferred at constant pressure in the zone of condensed film, are shown i n Figure 4, part 2. The band at 1630 cm" is characteristic of the β form (33, 39). Thus, the surface contains two-dimensional films of β helixes i n the case of pyridine-rich spreading solvents. This agrees with the results of others for different polymers (33). The β helixes for the same polymer i n monolayer are present (40) because the sample had a low molecular weight. Monolayers of P y B G . A l l the P y B G monolayers were obtained using chloroform as the spreading solvent on two different supports: support I 1


27.

GABRiELLi

A N D PUGGELLi

Macwmolecuhr

355

Conformation

was a solution of 0.001N HC1 i n twice-distilled water; support II was a solution of 0.25M dichloroacetic acid ( D C A ) in twice-distilled water. This concentration was the minimum strength necessary to achieve spreading isotherms of P y B G which were independent of D C A concen tration in the support. Figure 5 shows the spreading isotherms on support I at 15°, 20°, and 25°C. Figure 6 shows the same isotherms on support II. M O N O L A Y E R S O N S U P P O R T I. As Figure 5 shows, all the isotherms have a sharp plateau in surface pressure and therefore are made up of two parts—one less than and one greater than the plateau. For the areas below the plateau, we calculated the two-dimensional state equation 7rA-7r from experimental data; from this equation, we calculated the same parameters that were used for the other polymers. These parameters are shown in Table III. According to Table III:

( 1 ) The value of A^, taking into account the experimental conditions to get the -π-A curves, agrees satisfactorily with that found by others and is generally attributed to the a form although it does not constitute a definite criterion for characterizing the macromolecular form found at the interface. (2) The values of z' and ω would indicate the presence of a rigid form even though one must always remember the approximations made in their deduction. These parameters for P y B G have lower values than

0.3

Figure

0.4

6.

Spreading

0.5

0.6

0.7

isotherms of PyBG

0.8

at 15°,

0.9

20°,

and 25°C

1.0

Area

/n£/

on support


II

356

MONOLAYERS

Table III.

Temperature, °C

Parameters of P y B G in the Two-Dimensional State

A (m / mg) 0

z'

2

fm

Subphase I 15 20 25

0.880 0.866 0.858

1.082 1.041 1.019

2.009 2.009 2.008

0.81 0.81 0.77

-1430 -1438 -1350

0.86 0.85 0.85

+ 414 + 1054 + 1072

Subphase I I 15 20 25

1.146 1.140 1.140

0.833 0.792 0.775

2.010 2.010 2.010

° Values of if/z are in cal/mole of monomelic unit.

Table IV.

Thermodynamic Spreading Values of P y B G

A (m /mg) 0.687 0.742 0.756 0.783 0.825

Δ Η s (ergs/cm )

A S s (ergs/cm K )

2

2

2

Subphase I

Subphase II

Subphase I

Subphase II

-0.160 -0.150 -0.165 -0.165 -0.160

-0.22 -0.24 -0.23 -0.24 -0.24

-58.3 -52.9 -56.6 -55.2 -52.0

-75.0 -73.0 -75.0 -76.4 -64.5

Figure

7.

ΔΗ

8

vs. I / A

2

for PyBG

polymer

those for P/2BA. Besides the different rigidity, this arises from the dif ference in molecular weight (8, 9,10). The values of f indicate that in this case the macromolecule is partially submerged i n the substrate. Since the η /z values are all nega tive, they indicate energies of attraction between the macromolcular segments at the interface, characteristic of rigid forms like an a helix. Values of η /z for this polymer are higher than those for ΡβΒΑ because of the longer hydrophobic chain of monomeric unity. W e also calculated the thermodynamic spreading functions (Table I V ) for five surface area values. In this case, the polymer—interface m

2

2


27.

GABRIELLI AND

Macromolecular

PUGGELLI

357

Conformation

system cannot be considered athermic, but AH is negative even though it is positive for ΡβΒΑ. However, we calculated a total AH which i n cludes all enthalpic contributions to the spreading, particularly those caused by submersion i n the aqueous substrate. For P y B G , the submer sion process could be facilitated by the presence of H ions of the support acid, thus resulting in a higher corresponding negative enthalpic contri bution. The independence of AH with respect to 1 / A (Figure 7) fur ther confirms that such AH is not derived mainly from the attractive and repulsive energies of interaction between macromolecular segments; therefore, the negative sign does not exclude the presence of a form characterized by cohesive energies between macromolecular segments. Finally, the multiple reflection IR spectra retaken on monolayers trans ferred above and below the plateau ( Figure 8 ) show the spectrum char acteristic of the a form or coil as found by others ( 32, 33 ). Therefore, excluding the presence of the β form and considering the values of z' and ω, the negative sign of η /ζ, the variation with tempera ture of coefficient B i , and the presence of plateau characteristic of the S

S

+

2

S

S

2

PjfBG

a

1

b i8oo

cm

-1

1600

2

1800

1608

Figure 8. (1) MIR spectra of surface films of PyBG on support I transferred on germanium prism (a) at surface pressure less than the plateau pressure, (b) at surface pressure greater than plateau pressure (2) MIR spectra of surface films of PyBG on support II transferred on germa nium prism at high surface pressure


358

MONOLAYERS

a form, the monomolecular films of P y B G on 0.001N HC1 are probably made up of the a helix which is stable i n the spreading solution; this was found by others (26, 30, 33) and with different methods. In addition, starting with the initial area of plateau, the surface pressure decreases with time after every compression. The multiple reflection I R spectra are identical for films transferred above or below the plateau pressure (Figure 8). The surface entropies corresponding to the plateau pressure of —0.15 e r g / c m K and the value of AH of transition (monomolecular film —> bimolecular film), calculated at about 70 ergs/cm , agree well with the corresponding values necessary for the a form to pass from the mono- to the bilayer (30, 31). Therefore, the plateau represents "collapse," and the isotherm curves at pressures greater than the collapse pressure pertain to films which are not homo geneous monolayers. 2

2

Table V .

Thermodynamic Values of Transition

eu

Δ

Ή-trans,

Cdl/moU of

A (m /mg) 2

trans,

0.687 0.742 0.756 0.783 0.825

2.6 3.8 2.6 3.1 3.4

601 980 729 870 972

Average values

3.0

830

monomeric unit

F I L M O N S U P P O R T II. The isotherms in this case do not have a plateau in surface pressure at any temperature (Figure 6 ) . W e derived from experimental values the ττΑ-π equation and the parameters shown in Table I V , from which we note:

(1) The values of A are less than those found for support I; this may be the first clue to the presence of a macromolecular form different from the a form on the surface. (2) The values of z' and ω are higher and thus indicate a more flexible form than one on support I. (3) The f factor is of the same order and indicates that even here the macromolecules are partially submerged. (4) The η /ζ factor is positive; this means that at a difference of that observed for the form on support I, the predominant energies among the macromolecular segments are repulsive; this is characteristic of forms less orderly than the helixes. (5) Even the value of B which is higher than the corresponding ones for the a form, indicates the more random form of the helix (37). The higher negative values of the spreading enthalpy (Table V ) further confirm the presence of a macromolecular form that has attractive ener gies lower than those of the stable form on support I; the negative enm

2

u


27.

GABRiELLi AND PUGGELLi

Macromolecular

Conformation

359

thalpic contribution of submersion is of about the same order on the two supports. The IR spectra for monomolecular films transferred at a pressure in the range of condensed films are identical to those obtained for sup port I ( Figure 8 ). Thus the β form is not present, but either the a form or the random coil is present though not distinguishable on the basis of these infrared spectra alone as noted by others (32, 33, 41). Since the macromolecular form must be less orderly and different from the a form, the random-coil form seems plausible. W e calculated the α-helix, random coil transition enthalpy from the difference of the spreading enthalpies of the forms on supports I and II (Table V ) ; the average is 830 cal/mole of monomeric unit. Table V also shows the transition entropies calculated as the difference between the spreading entropies of the two forms; the average is 3.0 eu. The AH and the single values agree well with those found by others using different methods (42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53) i n the bulk phase. This result confirms the existence of the coil form i n the D C A support and shows that i n this case the support (the interface) can modify the stable form in the three-dimensional spreading solution; further, it allows us to consider a new method for measuring the heat of helix coil transition. This method is analogous to that used i n the bulk phase, also based on the difference in heat of the solution by Giacometti and Kagemoto (50, 51, 52), it validates the comparison between two-dimensional and tri dimensional solutions. Even the value of AS transition agrees well with that found in the bulk phase (45). Conclusions ( 1 ) For ΡβΒΑ the macromolecular conformation in the two-dimen sional phase is modified by varying the spreading solvent. Thus it is possible to obtain two different macromolecular forms, at the interface, that modify their conformation only i n the bulk phase. ( 2 ) For P y B G the macromolecular conformation is modified if D C A is in the support. Thus it is possible to obtain different macromolecular forms on the surface that modify the conformation only i n the two-dimen sional state. These results are particularly interesting not only in preparing various methods for obtaining conformations of the same macromolecules at an interface but also because they indicate the possibility of comparing energies of macromolecules i n the two-dimensional phase with those in the three-dimensional phase. Literature

Cited

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Macromolecular Conformation in the Two-Dimensional State

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