Colloidal Dispersions and Micellar Behavior - American Chemical

21 Thermodynamic Aspects of the Mixing of Mono -Amidesand Poly-Peptides with Water P. ASSARSSON, Ν. Y. CHEN, and F. R. EIRICH

Downloaded by EAST CAROLINA UNIV on September 26, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0009.ch021

Department of Chemistry, Polytechnic Institute of New

York, Brooklyn, N.Y.

11201

Introduction, Review of Earlier Work, and Experimental The solvation of amides i s a subject of considerable inter est partly because of the wide use of the low molecular weight members as solvents for organic and inorganic compounds, and also because the higher members can be considered as models for the physical properties and solution functions of the peptide bond i n polypeptides and proteins. We studied the interaction of water with p-(vinyl pyrrolidone) PVP) and of alkyl-substituted simple amides several years ago and some of the main results were subsequently published (1). In the following we wish to report on a number of yet unpublished data and, further, on new work on the heats of mixing of three amino acids, glycine (gly), proline (pro), and hydroxyproline (hypro) and some of their polymers and copolymers. The compounds i n question are shown on Figure 1. It i s interesting to note that N-acetamide, with an endothermic heat of solution, i s only p a r t i a l l y soluble i n water. A l l other amides studied are miscible with water i n all proportions, three to four carbons i n the alkyl groups being the upper limit for complete s o l u b i l i t y ( 2 . 3 ) . One may conclude that the breaking of inter amide polar bonds in the bulk phase and the forming of hydrogen bonds between amide dipoles and water are the predomi nant features governing this s o l u b i l i t y pattern. The preference of the amide group i n N-methyl acetamide to associate preferen t i a l l y with water rather than with i t s e l f has been proven by spectroscopic investigation i n the near infrared by following the N-H absorption band i n the 1.5 micron region (4). Also l i k e l y to be pertinent for the solvation pattern i s the fact that the atomic coordinates for the amide bond as investigated by Pauling (5) and confirmed by x-rays, shows the atoms to lie practically planar, and the N-C bond to have a p a r t i a l double bond character. Our substituted, and i n particular the di-substituted, amides were studied over the whole range of concentration i n

288

Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


21. AssABSsoN E T AL.

Mixing Mono-Amides and Poly-Peptides xvith Water

289

water by measuring the absolute viscosities of the solutions, their density and volume changes, the heats of mixing and the heat capacities by micro-calorimetry, and the phase diagrams by further following the freezing point depressions. Supplementary infrared and NMR spectroscopic data were also obtained, but w i l l not be discussed i n this context. Figures 2, 3, and 4 show some representative data obtained for the substituted amides. In general, di-substitution leads to higher exothermic heats of mixing than mono-substitution, while increasing length of the carbon a l k y l chains causes the heat of mixing to become less negative. We concluded from these data that a definite molecular interaction exists between the peptide dipole and water. The common feature among the amides and pyrrolidones inves tigated, see Figures 1-4, i s the evidence, from thermodynamic and viscosity data, that two molecules of water are being bound per^NGO-group. The logical sites for this association through hydrogen bonding are the unshared electron pairs of the carbonyl oxygen, a postulate which i s i n line with a l l spectroscopic studies and NMR data on the reactivity of the amide group (£)· Even the f u l l y substituted amides appear to be able, to some degree, to hold a third water molecule i n the immediate solva tion sphere by less energetic interaction with the unshared elec trons of the nitrogen. The fact that one does not find evidence of a third water molecule associated with the N-substituted pyrrolidones i s probably caused by the special geometry of the ring which leads also to the greater densities of these compounds. It i s not possible, on the basis of known bond angles and bond distances of the water molecule, to say whether the two water molecules which are postulated to be hydrogen-bonded to the carbonyl might join into a ring structure. Heat Capacities and Thermodynamic Functions a Mono-Amides. We would like to report now b r i e f l y on the heat capacities and p a r t i a l excess thermodynamic functions of water i n the same substituted amides obtained from freezing point depressions ( 2 £ F ) calorimetry ( Δ Η ). Two typical diagrams obtained are shown i n Figures 5.anS 6. It i s seen that the free energies change very l i t t l e up to about 85 mole % of water (15 mole % of the amide), whereas the heats of mixing become increasingly negative over the same range. We have not followed the heats of mixing over the whole range, but qualita tive evidence indicates that they remain negative. It i s inter esting that the entropy changes are also negative, so that some amount of the binding which was evidenced by the phase diagrams, or at least ordering, of water, i s l i k e l y to extend into the higher dilutions of the amides i n water. Of particular interest are the data for the changes i n heat capacities which we obtained over the whole range of compositions, xceefl


COLLOIDAL

DISPERSIONS A N D

MICELLAR

PRO

CH

H N — C—C0 H H ?

CH

2

2

2

2

HN—C H di—N- formonide

COoH

di-N— ocetomide

HN

C H

C0 H 2

di-N-propionote

^1,2

R

0 3·Ι

" \ /

3>l N

- C


R

N- rnethyl-ocetcmide R . . . . C, , C R

\

Νι-ί R

7

4:i X

CH -CH 2

3

3

N- methyl— pyrrolidone

Ν - methyl-coproloctorn

2

0

II N-C

/

CH

\

L

c=o

J

Figure 1.


BEHAVIOR

ASSARSSON E T A L .


21.

Mixing Mono-Amides and Poly-Peptides with Water

VISCOSITY AT 25°C PHASE DIAGRAM

ΔΗ MIX

DENSITY AT 25°C HEAT OF MIXING

80

Figure 3.

291

The a) (top) N-methylacetamide and the b) (bottom) water systems

i.OO

Ν,Ν-dimethylacetamide-


C O L L O I D A L DISPERSIONS A N D M I C E L L A R


292


BEHAVIOR


ASSARSSON E T A L .


Ν - methyl - acetamide

.35

.90

65

.80

X,

ΙΗ,ΟΪ

Figure 6.

pp

CAL

Partial excess functions of wa ter in NMA at 30°C

e

/M0LE- C

(H 0) 2

Figure 7.

Heat capacities of amide-wa ter systems at25°C



294

C O L L O I D A L DISPERSIONS A N D M I C E L L A R B E H A V I O R

see Figure 7. We interpret the many interesting features of the plots as follows: we make the assumption that the heat capacities of a mixture of molecules (partly due to the intramolecular degrees of freedom and partly to the vibrational and translational degrees of freedom of the molecules i n the mixtures as a whole) are additive unless the external degrees of freedom are changed by non-ideal grouping or pairing of the mixing molecules* The straight lines connecting the heat capacity values of the pure amides, on the l e f t side, with the heat capacity of pure water, on the right hand side, are drawn to represent such an ideality. In fact, N-Methyl acetamide, not shown here, exhibits such a linear relation up to 67 mole % of water. Strong devia tions and changes i n directions of the real heat capacity curves from this " i d e a l i t y " should, therefore, indicate non-average changes i n the arrangement and packing of the molecules i n the mixture. Positive deviations i n the heat capacity should stem from a rise i n the number of molecules which are capable of i n creasing the numbers of their intermolecular degrees of freedom. In the mixtures under investigation we deduce that primarily the inter-molecular degrees of freedom of vibration and rotation, but not of translation, are increased, since we find that the densi ties of the mixtures rise less, while the viscosities r i s e more, than ideally. Specifically, we see on Figure 7 that the f i r s t introduction of water up to a molar ratio of 1:4 into the partly polar, partly hydrocarbon, moiety of the pure amides changes the liequid struc ture only to a minor degree. From this point, the number of the molecules per volume acquiring previously inactive degrees of freedom, i.e. amides as well as the introduced water, rises sub stantially up to a mole ratio of about 1:1. From there, the heat capacity s t i l l stays i n excess of additivity, though s l i g h t l y declining, u n t i l a mole ratio of 3 molecules of water to 2 amides i s reached. We interpret the deviations up to this 3:2 ratio as being the result f i r s t of a breakup of amide association by the water, followed by hydrogen bonding up to 1 molecule of water per amide. At s t i l l higher amounts of water and consequent Η-bonding, the excess heat capacity declines further, with a striking jump around the 20:1 ratio of water to amide. This signifies most l i k e l y the beginning of the formation of the regular water struc ture or, coming from the side of pure water, the completion of the breakdown of the water structure by the amide (7). In most studies of amide or peptide solutions i n water i n the literature i t i s this range of concentrations which has been covered. Mix tures near the 1:1 ratio have not been studied. Yet, i n some biological systems i n which we know that bound water i s an essential part of the native conformations, this may be the very range of importance. Another aspect which we would like to emphasize i s that the coincidence of breaks, or maxima, i n the mixing functions for a variety of substituted amides at 67% of


21.

ASSARSSON E T A L .


295


water, indicates a returning form of interaction between the amide group and water, somewhat modified by the substiuents on the nitrogen or on the alpha carbon, consisting of coupling 2 water molecules per substituted peptide bond. b Poly-Amides. In unsubstituted amino acids, and i n p a r t i cular i n polypeptides, the hydrogen of the amide group w i l l , of course, participate i n bonding to other peptide groups and to water. We have extended our studies, at f i r s t , to the three mentioned amino acids gly, pro, and hypro, and to some of their homo-oligomers and polymer and copolymers. N-substituted gly cines were also studied by means of viscosity, density, and freezing point measurements. While this work i s s t i l l i n pro gress, we want to report here on some heat of mixing data obtained by a calorimeter with a precision of better than 1/100 of a degree. In conjunction with the heat capacity of the instru ment, this amounts to a precision of about 0.2 c a l . Because of the rather low s o l u b i l i t y of some of the oligomers and polymers, low number average molecular weights from about 4,000 to 9,000 were used so that the polymers would dissolve i n a sufficiently short period of time commensurate with the heat insulation of the instrument, and i n sufficient amounts to yield data within the limits of our precision. The monomer concentrations up to about 4%, and of the polymers up to approximately 1% i n water were studied. Based on an average amino acid molecular weight, the solutions were therefore approximately 1/1000 molar i n amino acid residues. The polymer molecular weights were determined by osmometry and the composition of the copolymers by optical rota tion analysis. Among other basic d i f f i c u l t i e s encountered were the unknown degrees of c r y s t a l l i n i t y of the monomeric samples undergoing solution, (the polymers were found to be non-crystalline by x-ray and birefringence analysis), which caused endothermic solution enthalpies which varied with the state of c r y s t a l l i n i t y . We instituted therefore dilution studies i n which we extrapolate to zero water, a method which yields also the heats of fusion for which we can not find data i n the literature. At this moment we want to report qualitatively that the heats of mixing of the simple amino acids are less negative than those of the d i substituted amides, and that heat of fusion for the three amino acids gly, pro, hypro are of the order of 3,000-4,000 c a l . Figures 8 and 9 show the heats of mixing of two of the homopolymers, pro and hypro. The polymer of gly i s too insoluble to obtain data. The Figures show also curves of copolymers of pro line and glycine, of hydroxyproline and glycine, of proline and hydroxyproline, and a terpolymer of gly-pro-hypro at the approxi mate mole ratio of 3:4:5. The most important results are that a l l the heats of mixing are negative, substantially more so than those expected for the monomers once they w i l l be f u l l y evaluated. From data not here presented there i s l i t t l e effect





BEHAVIOR

21.

ASSARSSON E T A L .


297

of molecular weights i n our molecular weight range. The heats of mixing for these unsubstituted polypeptide groups are also substantially more negative than those for the N-substituted peptides.


Discussion By and large, during the introduction of dry, unsubstituted, polypeptides into water three factors w i l l affect the heat of mixing: the disruption of the inter-peptide forces, the disrup tion of the water structure, and the rebuilding of a different water structure around the solute. In the light of our e a r l i e r solvation studies on peptides the exchange of bulk hydrogen bonding to heterogeneous Η-bonding, i . e . , from the Α-A to the A-B type, should not make a major difference. The increased negativity of the enthalpies should thus be attributed to the restructuring of water i n the sense of iceberg formation which is well known to be the cause of the negative heats of mixing of the short chain alcohols (§)· The finding that the heat of mixing of polypro i s substan t i a l l y more negative than that of polyhypro i s surprising i n view of the hydrogen bonding to be expected between water and the hydroxy groups of the hypro. Again, one might explain this i n terms of the rough equivalence of exchanging Α-A by A-E hydrogen bond, and i n terms of the more extensive hydrophobic bonding and structuring of water around polyproline. Another p o s s i b i l i t y would be differences i n the polypeptide conformations (9,10). A more t r i v i a l factor may be that not a l l of the protective acetyl groups on hypro were removed. Looking at the copolymers between gly and pro, Figure 8, we see that the heats of mixing are a l l less negative than for polypro. The close likeness of the proportions of the monomer feed i n the reaction mixture to the monomer residue ratio i n the polymer indicates an approximately random distribution, or at least the formation of blocks or of periodicities of similar length. Thus, the decrease i n the negative heats of mixing for increasing gly proportions i n the glypro copolymer indicates less hydrophobic bonding to this polypeptide. In polyglycine, as stated, the absence of nonpolar groups together with high chain regularity leads to a high heat of fusion which prevents polymer solution. For the prohypro copolymers, Figure 9, the heats of mixing l i e approximately between the curves for the homopolymers, so that no new factors seem to enter i n the solvation behavior of these copolymers. Unexpectedly, though, the copolymer of glyhypro shows a more negative enthalpy than polyhypro, i n contrast to the effect of the gly-component i n the gly-pro copolymer.


298


BEHAVIOR


Summary Our data on the thermodynamics of mixing of N-substituted amides with water confirm our earlier conclusion (1) of a strong binding of two water molecules to di-N-substituted amides, with a third more l i g h t l y bonded. This indicates that the carbonyl group i s capable of holding two water molecules by H-bonding, the third presumably being attached to the nitrogen. Applying this conclusion to N-substituted amides or peptides, these should also be solvated by an inner shell of three water mole cules. In dilute solution, according to our heat capacity data, these simple amides affect approximately 15 water molecules i n an outer zone. Since, i n the competition between inter-peptide and peptide-water forces, the latter appear to be substantially favored, the water molecules remain bonded up to the 1:1 molar ratio. For unsubstituted polymeric peptides, the regular array of hydrogen bonds along the polymeric molecules favors the interpep tide Η-bonding, as indicated by the low s o l u b i l i t i e s of many unsubstituted polypeptides. For the polypeptides of the pro-, hypro-, and gly- series here studied, we find great exothermicity and therefore more extensive hydrogen bonding and formation of water structures than those accompanying the solution of simpler peptides and amides. Literature Cited 1 Assarsson, P. and E i r i c h , F. R., i n "Molecular Association i n Biological and Related Systems", Adv. Chem. Ser. No.84, American Chemical Society, Washington, D. C., 1968. 2 Wood, F., Ramsden, D. Κ., and King, G., Nature, (1966) 212, (5062), 606 3 Needham, Τ. Ε., J r . , Diss. Abst. Inter. B., (1970) 31 (6) 3303 4 Klotz, J . and Franzen, J . , J. Am. Chem. Soc. (1960), 82, 5241 5 Pauling, L., Proc. Nat. Acad. S c i . U.S., (1932), 18, 293 6 Costain, C. C. and Dowling, J . Μ., J . Chem. Phys. (1960), 32, 158 7 Klotz, I. M. and Farnham, S. Β., Biochem., (1968) 7 (11) 3879 8 Franks, F., Ann. Ν. Y. Acad. S c i . , (1965), 125, 277 9 Forsythe, Κ. H. and Hopfinger, A. J . , Macromolecule, (1973), 6(3) 423 10 Segal, D. Μ., J . Mol. Biol., (1969), 43(3) 497


Colloidal Dispersions and Micellar Behavior - American Chemical

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