0
HYDROGEN BONDING I N HIGH POLYMERS AND INCLUSION COMPOUNDS' MAURICE I. HUOGINS Research Laboratories, Eastman Kodak Company Rochester, New York
HYDROGEN always has a valence of one, in the sense that it has only one electron which i t can use in the formation of chemical compounds, by electron transfer of electron sharing. Nevertheless, a hydrogen atom bonded to an electronegative atom, such as fluorine, oxygen, or nitrogen, has an attraction for another electronegative atom. This attraction is in part due to the fact that the F-H, O-H, or N-H bond has a considerable degree of polarity, hence the hydrogen has an "effective" negative charge. There is doubtless also some resonance with a structure in which the hydrogen atom is bonded covalently to the second electronegative atom. For example,
Such systems are described as containing hydrogen bonds or hyd~ogenbridges. When the writer (1) and Latimer and Rodebnsh (2) presented the hydrogen-bond concept and used it to explain certain peculiar chemical phenomena, it was slow to gain acceptance. Since then, an immense body of experimental evidence in its favor has accumulated and the existence and importance of these bonds are generally recognized. The great importance of hydrogen bonds results partly from the fact that they exist in a very large number of systems of interest and importance-practically all those which contain polar groups, such as hydroxyl, carboxyl, and amino groups-contributing to their stability and other chemical and physical properties. Their importance is especially great because the hydrogen-bond energy (3, 4) (that is, the net energy decrease when they are formed or the net increase when they are broken) is of the order of 5 kcal./mole, which is roughly one-tenth the usual bond energy for ordinary single bonds and ten times the average kinetic energy per mole a t room temperatures. Hydrogen bonds are thus quite stable t o collisions between molecules at ordinary temperatures, but are readily broken or interchanged a t somewhat elevated temperatures and in chemical reactions. Except in some cases in which Coulomb attractions between ions exist, hydrogen bond forces are stronger than those of any other type between molecules and so are of great im1 This paper is based an lectures by the author before several universities and scientific 80cietieS in Japan, while he was a Fulbright Visiting Professor at Osaka and Kyoto Universities, 195556. A Japanese translation is being published in the journal, Kagaku to Kogyo.
portance in determining intermolecular structures, in solids and in liquids (including solutions). They are likewise of great importance in determining intramolecular structures, in molecules in which there would otherwise be flexibility and randomness of structure, as a result of easy rotation about single bonds. METHODS
OF STUDY
Since some of the more important methods of studying hydrogen bonds have recently been ably described and discussed in THIS JOURNAL by Gorman (6, 6), only a brief summary of the subject will be given here. X-ray diffraction methods have been most useful in showing the presence of hydrogen bonds in many types of crystalline compounds. Where such bonds exist, two electronegative atoms, one known to be bonded to a hydrogen, are found to be much closer together than would be expected if the attractive forces were only those of the van der Waals "dispersion" type. I n fact, relative distances between the centers of the two negative atoms, for a given hydrogen-bond type (such as O-H. . .O), can be taken as rough measures of the hydrogen-bond strength. The shorter the distance, the stronger is the hydrogen bond. Recently, refined X-ray diffraction studies have led to determinations of the positions of the hydrogen atoms also (7). (Strictly speaking, they give the effective centers of the electron clouds, which may differ appreciably, in the case of hydrogen, from the locations of the atomic nuclei.) In some instances, also, neutrondiffraction and nuclear magnetic-resonance studies have led to the location of the hydrogen atoms. Another important method of studying hydrogen bonds is by means of infrared absorption spectra. Certain bands, known to be characteristic of the vibrations of the oxygen and hydrogen a t o m s in hydroxyl groups, for example, are displaced consider* ably from their normal positions in the spectrum if the hydroxyl groups form hydrogen bonds. The frequency ~ s ~ e t c h ~frrguency np shifts are measures of riguleI the changes in the forces H ~ d r o z e nbond length "a. stretohtendine to hold the hving frequency, for certain types of drogen atoms in thiir hydrogen bonds. (Nakamoto. Margoshes, and aundle(II).) equilibrium positions. JOURNAL OF CHEMICAL EDUCATION
siderations. A considerable amount of knowledge has been accumulated from these theoretical studies and from the results of experimental researches, with regard to the effect of various environmental factors (such as the kinds of atoms attached to the electronegative atoms of the hydrogen bonds) on the bond properties.
AFFECTINOTHE STRENGTH OF HYDROGEN
FACTORS BONDS (3)
A-H.
Schematic representation of the structure of a KHIPO~oryatsl, within s domain in which the hydrogen bonds are so oriented 8%to interact oooperatidy.
For a given hydrogen-bond type (e.g., 0-H. . .O), the stronger the bond is, the shorter is the hydrogenbond length (0. . .0) and the smaller is the frequency of the band related to the "stretching" vibrations (8-11) (see Fig. 1). Hydrogen-bond energies can be deduced from appropriate thermochemical measurements, for changes involving the breaking of these bonds (5). Aggregation resulting from hydrogen-bond formation can be studied by thermodynamic activity measurements on solutions or gases (3). Studies of ferroelectric properties of crystals such as K H 9 0 4 give information about cooperative interactions between hydrogen bonds (18, IS) (see Fig. 2). Specific heat measurements can be related to the symmetrical or unsymmetrical nature of hydrogen bonds (14). Hydrogen ion conductivity in water solutions can be related to the ease of changing from an 0-H. . .O configuration to an 0 . . .H-0 configuration (16, 16). In some solutions, sudden changes in the types of hydrogen-bonding environment around the component molecules or ions (i.e., in the structa (17) types) occur a t certain concentrations. As a consequence, the curves representing properties such as refractive index and dielectric constant show sharp breaks a t these compositions (Fig. 3). The locations of these breaks can be used to determine, or to limit, the types of hydrogen-bonding present in each composition range. Many more methods of stitdying hydrogen bonds might be cited. Many attempts (16, 19-82) have been made, with partial success, to compute the energies and other properties of hydrogen bonds from theoretical con-
. .B
as a general formula for a hydrogen bond, then (other things being equal) the more polar the A-H bond the stronger will be the hydrogen bond produced. Also, the greater the effective negative charge on the B atom, the stronger will be the hydrogen bond. One way of making the A-H bond more polar is to make A an acceptor for another hydrogen bond. If B is attached to a hydrogen by a covalent bond, the effective negative charge on B will be increased by the formation of another hydrogen bond with this B-H. Thus, . . .A-H. . .B-H . . is stronger than A-H . .B-H. In agreement with this reasoning, the solid state structures of substances containing hydroxyl groups are invariably such as to make each hydroxyl oxygen both a donor and an acceptor of a hydrogen atom; it constitutes an end of two hydrogen bonds a t the same time. To achieve this result, the hydroxyl groups link together by hydrogen-bonding to form either rings or chains of such great length that they can be considered infinite for all practical purposes. The stability of such chains and rings is enhanced by synchronous oscillations of the hydrogen-bonding hydrogen atoms and the valence electron systems therein (2s). The same arguments apply if the hydrogen bonds in a ring or chain are connected, indirectly, through a conjugated system of atoms. A carboxylic acid, for example, can donate its hydroxyl hydrogen atom to one hydrogen bond and, at the same time, its carbonyl oxygen atom can be an acceptor for a hydrogen bond from a hydroxyl group of another carboxyl. Changes in electronegativity of the hydroxyl oxygen are transmitted, through the mobile electron system of the carboxyl group to the carbonyl oxygen, and vice versa. The CO.NH groups in polypeptides are likewise both donors and acceptors of hydrogen bonds, and these bonds are strengthened by uniting them together into rings or long chains. SYMMETRICAL AND UNSYMMETRICAL HYDROGEN BONDS
vsriation of refrmtive index (n) with composition, for solutions of dimethyl forrnamide (A) and phenol (B)in carbon tetrachloride. The break in the curve at the equirnolar point can be attributed to astrone tendency to form 1 : i oomp~eres, b y hydrogen-bonding. (Huggins (In, from data by Arshid. Giles. MeLure. Ogilvie, and Rose (Is).)
VOLUME 34, NO. 10, OCTOBER, 1951
Several lines of evidence (14, 24) agree in showing that the hydrogen bond in the HF2- ion is symmetrical. The hydrogen is oscillating about a mean position midway between the two fluorine centers. The potential energy curve for the motion of the hydrogen along the fluorine-fluorine centerline has a single minimum, but is presumably much flatter than the corresponding curve for the H F molecule. Evidence has been presented (25, 26, 27) for the existence of symmetrical 0-H-0 bridges in nickel dimethylglyoxime,
but there is no strong evidence, known to the author, for a symmetrical O-H-0 bridge in any other substance. Normally, the potential energy curve doubtless has two minima, with an energy barrier between. Theoretical calculations show that this barrier cannot be very high and must be influenced greatly by the factors mentioned. Increasing the effective negative charges on the two oxygen atoms and conjugation with other hydrogen bonds both tend to decrease the size of the energy hump, facilitating a jump of the hydrogen from one side of the hump to the other (Fig. 4).
Projection of a portion of the network of hydrogen-bonded hydraquinane moleoules in i t s olathrs*te complex with methanol or any of various other small molecules. The filled circles denote carboo atoms: the smaller open oiroles, oxygen atoms; and the lhrger open airoles, the methanol lor othed molecules. Hydrogen atoms arc not shown.
INCLUSION COMPOUNDS (18, 99, 30)
Certain types of molecules, containing hydrogenbonding groups, interact in such a way as to form threedimensional frameworks containing holes (or, in some cases, channels) large enough to contain other molecules. Even though these added molecules are not held by chemical bond forces or by hydrogenbonding, van der Waals attractions suffice to keep them;witbin the framework. Molecular compounds formed in this way have been designated as "EinHydroquinone, ~ - C G H ~ ( O H for ) ~example, , forms a hydrogen-bond framework into which go molecules of a n i of the following varieties ($1, 22): A, Kr, H2S, SOz, HCOaH, HCN, HCI, HBr, CHZOH, and CHaCN (Fig. 5). Larger molecules will not fit into the holes and so cannot be included. Urea, OC(NH&, forms a framework structure with
Theoretiosl potential-energy curvea for O H 0 hydrogen bonds, assuming dtderent 0.. .0 distsnees and uniform effective negative charzes on the oxygen atoms. A similar set of curves reaults from the auumption of uniform 0...0 distanoea, with different effective charges on the oxyeen atoms. (Huggins (IB).)
482
Figure B Projection (aliphtly idealized1 of a portion of the network of hydrogenbonded urea molecules in a urea complex with a normal parsffin hydrooarban. The projeotion of the zinasr hvdrncarban chain is s h o w in the center.
JOURNAL OF CHEMICAL EDUCATION
N-H. . .O hydrogen bonds connecting the urea molecules (33). This structure contains channels which are sufficiently large to accommodate normal paraffin hydrocarbon molecules (Fig. 6). Branched-chain paraffin or ring hydrocarbons will not fit. This fact has been utilized in the development of a commercial method for separation of branched and normal hydrocarbons from their mixtures. In the presence of hydrocarbon molecules of sufficiently small molecular size, water will freeze, a t a temperature of a few degrees above 0" C., to a structure of cubic symmetry, in which the water molecules are hydrogen-bonded together in much the same way as in ordinary ice, but with regularly distributed holes, filled by the small hydrocarbon molecules (34, 35).
ing the sheets, but no detailed hydrogen-bond structure bas been published. When ceUulose absorbs water, the water molecules enter into the structure between the cellulose molecules, undoubtedly participating in the hydrogen-bond system. Schematically, one can represent this as a change from a system like this:
HYDROGEN BONDING IN HlGH POLYMERS AND INCLUSION COMPOUNDS
Poly(viny1 alcohol). The structures and many of the properties of many types of high polymers, both natural and synthetic, depend largely on hydrogen bond formation. One example is poly(viny1 alcohol), [CH2CH(OH)],, which contains crystalline regions in which the molecular chains are nearly fully extended zigzags (36, 37), presumably connected by hydrogen bonds in such a way that each hydroxyl oxygen atom forms hydrogen bonds to two others. There are two possible orientations of the C-H and
t o one like this:
\
C-0
bonds in each 'CH(OH) group, relative to the
, ,
plane of the two C-C bonds. As poly(viny1 alcohol) is customarily made, there is a random distribution of these two orientations within each molecule. Because of this randomness, a perfectly regular stmcture, within the crystalline regions of poly(viny1 alcohol), is not possible. Several alternatives, with regard to the details of the atomic arrangement, have been proposed (38). It seems likely that X-ray diffraction studies (39) now in progress will soon settle the points of disagreement. Cellulose. Cellulose molecules are known to consist of long strings of 8-glucose residues (see Fig. 7), with the glucose rings connected together by oxygen bridges. Arrangements of csrbon and orygen atoms in the molecules of 8- snd
B
rn
riwr. 7
=-glucose. Locations of the hydrogen atoms are not shown. The 1 and 4 oxygen atoms, which bridge between the rings in cellulose and atarch, are those at the extreme right and extreme left, reegeotively, in each case.
In the crystalline regions of native cellulose, these strings are nearly as fully extended as is possible, consistent with the maintenance of the customary bond angles and distances. The main features of the arrangement of the atoms are known, but not the details (40). One would expect the intermolecular forces to be almost exclusively of the hydrogen-bond type, and the X-ray data are in agreement with this expectation. It has been suggested, reasonably, that the molecular chains are tightly hydrogen-bonded together into sheets or layers, with fewer or weaker hydrogen bonds connectVOLUME 34, NO. 10, OCTOBER, 1957
The actual arrangements in space are, of course, more complex. Becanse of the rigidity of the main part of the glucose residue framework, it is impossible, in pure cellulose, for all of the hydroxyl groups to be in the best positions and with the best orientations for strong hydrogen-bonding. The introduction of water molecules introduces additional flexibility and relieves the strain where it is greatest. A few water molecules are therefore very tightly held. As more are added, however, the forces holding them become weaker. This is indicated by water vapor sorption studies (41). Similar remarks should be applicable to water sorption by other polymers in which intermolecular hydrogen-bonding is important. Starch. Starch is known to be composed of two types of molecules. One, amylose, consists of linear chains of a-glucose units, connected a t the 1 and 4 positions by oxygen bridges. The other type, amylopectin, is similar, except that there are frequent branches. Although some facts have been learned about the nature of the structure a t the branches, the degree of branching, etc., an adequate discussion of these matters would be lengthy and will not be attempted here.
Amylase starch, in the native state, exists in two different types of crystalline structure, A and B, giving different X-ray diffraction Fjgula 8 patterns. The X-ray Arrangement of ring atoms in the data from each of these "boat" form of sir-membered ring have been interpreted compounds. (42) in terms of a unit cell, through which pass four extended molecular chains. If this is correct, the extension per two glucose residues is 10.6 A., even greater than the corresponding extension (10.3 A.) in cellulose. Such an extension would be impossible, if the glucose residues had the puckered arrangement of ring atoms found in crystalline or-glucose itself. It is possible (45), however, if the rings have the "boat" form of structure (see Fig. 8), with carbon atom 3 and oxygen 5 in the median plane of the boat. Another possibility, apparently in conformity with the X-ray data, is that there are only two molecular chains passing through each unit' each being coiled with four glucose residues per turn, in a hydrogen bonded structure similar to that discussed in the next paragraph, The optical data seem to favor the extended-chain alternative, although no adequate comparison of the two has been made. Another structural modification of amylose is the V form, produced by precipitation from solution. The X-ray data have been interpreted (44, 45) as indicating a helical structure with six glucose residues per turn. It is possible to build a model of this sort, with good hydrogen-bonding (Fig. 9). There is no intramolecular requirement that the number of residues per turn be exactly integral. In the model made by the writer, the or-glucose residues have the puckered-ring form.
,
.
Figu.e 9 Suggested pattern of bonds and hydrogen bonds in the V modification 01 amylase starch. The planar arrangement ~icturedir to be thoueht of as bent around s cylinder, in such a way as to produce a continuous helical chain molecule, with adiaaent turns connected by hydrogen bonds.
Certain other substances can be enclosed within these helixes (44, 46, 46). (Compare the inclusion compounds discussed earlier.) These include iodine and certain alcohols, acids, and nitro compounds. An obvious requirement is that they must be of such size and shape as to fit within the space available. There is probably also some specific dipolar or hydrogenbonding interaction with groups which do not enter into the intramolecular framework or whose affinities are not sufficientlywell satisfied within that framework.
o=c
Figore 10. Hydrogen-bonding in nylon 6 6 lBunn and Garner (47).) Figure 11. Hydrogen-bonding in nylon B (polyoaprolhctam) (Holmes, Bunn and Smith (48)) Figure 12. Proposed pattern of intermoleeuiar hydrogen-bonding in nylon 16, in s direction such as A , . .. A in Figure 13. The hydrogen A bonding in s B . . .. B direation is aimilar, involving all NH and CO groups not hydrogen-borded in the A , . . A direotions.
.
Fiw..
484
10
Figure I1
Figure 12
JOURNAL OF CHEMICAL EDUCATION
, I
B, '
..
-.
,' B -'
.A
-'A Fig-
13
schematic representation of proposed hydrogen-bonding aystem in nylon 16,as ~ r o j e o t ~on d a plane normal to the chain axes.
The V modification seems to be formed only when molecules appropriate for inclusion are available. Polyamides. X-ray diffraction data from various polyamides of the nylon type show that, in the crystalline regions, the molecular chains are practically fully extended zigzags. The evidence also indicates very extensive N-H . .O hydrogen-bonding, probably involving all the NH and CO groups in the crystalline regions. In nylon 66, poly(hexamethy1eneadipamide), for instance, the hydrogen-bonding binds the chains together into sheets, in which the pattern of bonds and hydrogen bonds is as indicated in Figure 10 (47). In nylon 6, polycaprolactam, it is impossible to have rectilinear hydrogen bonds connecting all of the NH and CO groups, if the molecular chain is even approximately fully extended and if all the chains run in the same direction. If alternate chains run in opposite directions, however, good hydrogen-bonding and extended chains are both possible. (See Fig. 11.) Such a structure bas, infact, been found (48). Fornylonl6, poly(methy1ene adipamide), no struct u x with extended zigzag chains and rectilinear hydrogen bonds binding the molecules into sheets is possible. Either (1) there is some coiling or twisting of
the molecular chains to give maximum hydrogen-honding, (2) only half of the NH and CO groups participate in hydrogen-bonding (49, 60), or (3) each molecule is hydrogen-bonded to four others, rather than two, to give a 3-dimensional network, as shown schematically in Fignres 12 and 13. The last alternative seems most likely to he the correct one. Extended-Chain Structures (51). A recent X-ray diffraction study of Bombys mom' silk (53) confirms earlier hypotheses that i t consists of extended polypeptide chains (55,54) and that these chains are connected in sheets by N-H. . .O hydrogen bonds (66, 66), with alternate chains oppositely oriented (Fig. 14; compare nylon 6, Fig. 11) and with the sheets packed together in such a way as to give good packing of the R groups between them. About half of the residues are glycine residues (gly); the other half are alanine residues (ala) plus some residues having larger side groups. The larger side groups in each sheet are all on the same side. This permits the layers t o stack efficiently in pairs. In Tussah silk, there is a much larger proportion of alanine. A structure in which hydrogen-bonded sheets are paired, with only gly-gly contacts between the two sheets of each pair, is impossible. Otherwise, the structure (67) is essentially like that of Bombys mom' silk. Poly-L-alanine, like keratin and many other synthetic polypeptides, has been observed with two different types of structure (68), or and P. The P form gives X-ray photographs closely resembling those from
0
5P
F~."F* 15
Hydroasn-banding in silk (6s. 66. 66.67). Adjacent chains run in opposite direotions.
Projection, on a plane normal t o the w r e w axis, of the structure
for polyglydne I1 by Crick and Rich
(60).
VOLUME 34, NO. 10, OCTOBER, 1957
485
Tussah silk. Its structure is undoubtedly similar, Collagen. Many models have been proposed for the although simpler in that all R groups are alike. structure of collagen (the protein of connective tissue). The B-forms of polyglycine and other synthetic polyAll but one, however, are either in definite contradicpeptides are apparently also composed of sheets of tion with experimental facts or permit only half 01 extended polypeptide chains (58), joined by hydrogen the NH groups in each polypeptide chain to be hydrobonds. From the identity distances in the direction of gen-bonded to CO groups in the same or other chains. the fiber axis, it seems likely that polyglycine, a t least, The only model which seems, to the writer, to be has (like silk) an antiparallel arrangement of the satisfactory is one recently proposed by him (68), in chains within each sheet. For a parallel arrangement, which each polypeptide chain is helically coiled, 30 a somewhat shorter identity distance would he exresidues per 3 turns and per 28.6 A. translation, with pected (55). the residues in groups of three. In the idealized strucAlthough polyglycine normally has an extended ture, each group of three residues contains one glycine structure of the @-type, another form, giving X-ray residue, one of proline or hydroxyproline, and one of data incompatible with such a structure or with an another type. The pattern of bonds and hydrogen a-type structure, has also been observed (58,59). For bonds is as indicated, for two 3-residue groups, in the this form, Crick and Rich (60) have proposed a reaformula diagram, sonable structure (Fig. 15). The polypeptide ........................ ......................... chains are extended, hut not fully so, each chain . . H H H H 0 H H H H" ,j being twisted around a 3-fold screw axls m -~-C-N-CC-N-C~N-CC-N-CC~N-C-NC-, such a way that the NH groups and carbonyl H H R 0 / \O H H R 0/ \0 H oxygen atoms can form hydrogen bonds with oxwen atoms and NH erouos, res~ectivelv, ......................... ..................... .. in ......... : the Bix surrounding mol&ul& chains. and in Figure 17. The spiral is left-handed for lev0 Keratin, the protein of hair, which has been stretched orientations at the asymmetric carbon atoms. in steam @-keratin) gives X-ray photographs similar to those from silk, but with fewer and more diffuse All of the peptide NH groups are hydrogen-bonded to carbonyl oxygeus of other peptide groups. The diffraction spots, indicating less regularity of structure rings formed by these hydrogen bonds contain 10 atoms (61). The observed pseudo identity distance in the each. As far as can be determined from the model, fiber direction is about 6.6 A,, which is somewhat less the N-H and C=O bond orientations are in agreethan the 6.8-7.0 A. distances in B m b y z mori and ment with the infrared evidence. Tussah silk and in the synthetic polypeptides having the extended (6) type of structure. This shorter identity distance suggests that the structure is one in which all (or a t least many) pairs of adjacent molecular chains are oriented in the same direction (55) (Fig. 16). This probably results from a parallel-chain arrangement in ekeratin, the natural unstretcbed form. Alpha Polypeptides (51). The availability of synthetic polypeptides having crystalline structures from which good X-ray diffraction data can be obtained has made it possible to limit greatly their possible molecular structures (58). Although extended B-type structures are sometimes obtained, most of the samples which have been studied give X-ray and infraritd data H-N ..H-N \ ..H-N \ agreeing with a spiral molecular arrangement, with hydrogen bonds connecting consecutive turns, as predicted (55) for a-keratin. Poly(y-methyl-L-glutaFkure 16. Proposed hydrogen-bond pattern for @-keratin. Adiaoent mate), for example, probably consists of molecules havchains run in the same direotion. Some distortion and shortening of the aigraj chains would be expected, in order to make the N-H .. .O bonds ing a spiral structure in which the hydrogen bonds more nearly rectilinear (66). form 13-atom rings (62,6S) as indicated by the formula.
i \/
'
-WNH-CHR-CO)rN0 .......................H
An 11-atom ring spiral (62, 64, 65) -N+CHR-CO-NH)rCHR-C H ..............................
does not seem to be definitely ruled out, but is less likely. Alpha Keratin. Keratin in the unstretcbed alpha form gives X-ray photographs (61, 66) which resemble, in their main features, thhse from the alpha forms of the synthetic polypeptides. It seems certain that a-keratin consists primarily of helical molecules, probably of the 13-atom ring type, in a roughly closepacked arrangement (67).
i \/
,
Fimre 17. Diagrammatic representation of the pattern of bonds and hydrogen bonda in slightly more than the paeudo unit of the atruoture proposed lor oollagen (68). The moleoular structure is derived b y coiling the structureshown hsre around an axis normal to the plane of the projection. in such a way 8s to produce a continuous spiral polypeptide chain.
The analytical data show that close t o one-third of the total number of residues are glycine residues, -NH-CH-CO-, in agreement with the model. The proline and hydroxyproline residues together constitute only about 22%, but it seems reasonable to suppose that the residues which yield aspartic acid and glutamic acid on hydrolysis, when in native collagen, contain rings which act structurauy much like proline and hydroxyproline rings. Such rings might be produced by hydrogen-bonding or by condensation reactions: JOURNAL OF CHEMICAL EDUCATION
locomotion may involve the same sort of process. Memory may be related to the setting up of standing wave patterns of oriented hydrogen bonds in the brain, and thought may involve changes in such wave patterns. CONCLUSION
I t is perhaps significant that the sum of the numbers of residues of these four kinds is very near to one-third of the total. In the arrangement described, the proline and hydroxyproline rings (and also the glu and asp side chains, if they have the ring structures suggested) extend outward from the rest of the structure. The hydroxyl groups of the hydroxyproline residues are thus apprcpriately situated for interchain hydrogen-bonding. This is in agreement with the experiments and deductions of Gustavson (69). The model described requires further checking and elaboration, of course. It is presented, tentatively, as the model which, in the writer's opinion, is most likely to approximate the true structure of collagen. OTHER BIOLOGICAL SYSTEMS
In recent years, it has become evident that hydrogen bonds play important roles in many hiological systems and processes. A few speculative examples may he cited.. Hydrogen bonds between purine and pyrimidine rines hold toeether oairs of helical molecular chains in the nucleic acid structure, according to the model of Watson and Crick (70). The transmission of a nerve impulse may consist essentially of the reversal of pclarity of the hydrogen bonds in chains of such bonds, conjugated together (71) (Fig. 18). Muscle contraction and expansion may consist primarily of chemically controlled coiling and uncoiling of polypeptide chainschanging intermolecular hydrogen bonds into intramolecular hydrogen bonds, and vice versa. Flagella
-
-
I
R
I
1
R
R Figure 18
Illustrating a possible mode of oonduotion of sn impulse in a nerve fiber (711.
VOLUME 34, NO. 10, OCTOBER. 1957
Enough examples have been cited to illustrate the importance of hydrogen-bonding in determining or modifying the structures of chemical substances and biological systems and in affecting properties which depend on these structures. Many more examples could be given. There seems to be good reason to believe that hydrogen bonds will become increasingly important in many fields of science: chemistry, physics, mineralogy, biology, etc. LITERATURE CITED ( 1 ) HUGGINS,M. L., Undergraduste courae thesis, University of California (1919). See G. N. LEWIS,"Valence and the Structure of Atoms and Molecules," Chemical Catalog Co., New York, 1923, p. 100. J. Am. Chem. ( 2 ) LATIMER,W. M., AND W. H. RODEBUSH, Soc., 42, 1419 (1920). M. L., J. Org. C h m . , 1,407 (1936). ( 3 ) HUGGINS, (4) SuzUK1, K , S. ONISHI,T. KorDE, AND S. SEKI,BuI~.Chem. Soe. Japan, 29, 127 (1956). M., J . CHEM.EDUC.,33, 468 (1956). ( 5 ) GORMAN, ( 6 ) Ibid., 34, 304 (1957). , ( 7 ) For example, TOMIIE,Y., G. H. Roo, AND I. N I ~ unpublished research a t Omka University, Japan. ( 8 ) T s n m ~M., , Bull. Chem. Soe. Japan,25, 60 (1952). ( 9 ) KUHN,L. P., J. Am Chem. Soc., 74,2492 (1952). ( 1 0 ) M~zusHrMa,S., "Structure of Molecules and Internal RGtation," Academic Press, New York, 1954, pp. 1 3 1 3 3 . AND R. E. RUNDLE, J. ( 1 1 ) Nnsimrom, K., M. MARGOSAES, Am. Chem. Soc., 77,6480 (1955). J . C., J. Chem. Phys., 9 , 1 6 (1941). ( 1 2 ) SLATER, T., Pwg. Theor. Phys., 7 , 275 (1952). ( 1 3 ) NAGAMIYA, J. Am. Chem. Soc., 7 1 , ( 1 4 ) PITZER,K . S., AND E. F. WESTRUM, I 9.4.. 0 (19411). . ,----,-
HUGGINS, M. L., J. Am. Chem. Soc., 53,3190 (1931). HuGGrNs, M. L.,J. Phys. Chem., 40, 723 (1936). HUGGINS, M. L., Bull. C h . Sor. Japan,28, 606 (1955). ARSHID,F. M., C. H. GILES,E. C. MCLWE, A. OGILVIE, AND T. J. ROSE,J. Chem Soc. (London), 1955.67. NGRDMAN, C. E., A N D W. N. LIPSCOMB,J. Chem. Phys., 21, 2077 (1953). BAKER, JR., A. N., J. Chem. Phys., 22, 1625 (1954). OSHIDA,I., Y. OOSHI~A, AND R. MIYASAKA, J. Phys. Soc. Japan, 10, 849 (1955). LIPPINCOTT, E.R., A N D R. SCHROEDER, J. Chem. PhW, 23, 1099 (1955). H u c o ~ ~M. s , L., Nature, 139, 550 (1937). P m . Phy8 SOC., ( 2 4 ) CorS, G. L., *ND H. W. THOMPSON. A210, 206 (1951). ( 2 5 ) GODYCKI,L. E., R. E RUNDLE,R. C. VOTER,AND C. V. BANKS, J. Chem. Phys., 19, 1205 (1951). ( 2 6 ) RUNDLE,R. E., A N D M. PAUSOL, J. Chem. Phys. 20 1487 (1952). ( 2 7 ) GODYCKI, L. E., A N D R. E. RUNDLE, Ada Cryst., 6, 487 (1953). . . ( 2 8 ) WELLS,A. F., in "Structure and Properties of Solid Surfaces," R. COMERAND C. S. S ~ m n Editors, , Univ. of Chicago Preew, Chicago, Illinois, 1953, Chap. VII. ( 2 9 ) CMMER, F., "Ein~chlussverbindungen,"Springer-Verlag, Berlin, Gettingen, Heidelberg, 1954. ( 3 0 ) HiicseL, W . , "Theoretical Principles of Organic Chemistry," Elsevier Publishing C a , New York, Vol. I, 1955, pp. 162-69. ( 3 1 ) PALIN,D. E., AND H. M. POWELL, J. Chem. Soc. (London), 1947, 208. ( 3 2 ) POWELL,H. M . , J. Chem. Soe. (London), 1950, 298, 300. ( 3 3 ) SMITH,A. E., Acla Cryst., 5 , 224 (1952).
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