The Increase in Denaturation Temperature Following Cross-linking of Collagen is Caused by Dehydration of the Fibres
Post on 19-Oct-2016
nCross-linking of Collagen is Caused by Dehydration of
Christopher A. Miles1
Allen J. Bailey1
1Collagen Research GroupUniversity of Bristol, LangfordBristol BS40 5DU, UK
2MATI Russian StateTechnological University3, Orshanskaya str, Moscow121552, Russian Federation
tendon, explained the new data also. In this mechanism, the configura-
in connective tissues, such as tendon, bone and skin an effect is well known to be induced by lathyriticagents that inhibit the oxidative-deamination of
doi:10.1016/j.jmb.2004.12.001is essential for proper tissue function. In the absencetional entropy of the unfolding molecule is reduced by its confinement inthe fibre lattice, which shrinks on cross-linking.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: collagen; DSC; cross-link; stability; polymer-in-a-box*Corresponding author
The covalent cross-linking of collagen molecules
of these intermolecular cross-links, the molecules inthe fibre are capable of sliding relative to each other;consequently, the fibre is weak and extensible. Such0022-2836/$ - see front matter q 2004 E
Abbreviations used: MDA, malonhexamethylene diisocyanate; DSC,calorimetry.E-mail address of the correspond
firstname.lastname@example.org*, Nicholas C. Avery1, Victor V. Rodin2 and
Differential scanning calorimetry (DSC) was used to study the thermalstability of native and synthetically cross-linked rat-tail tendon at differentlevels of hydration, and the results compared with native rat-tail tendon.Three cross-linking agents of different length between functional groupswere used: malondialdehyde (MDA), glutaraldehyde and hexamethylenediisocyanate (HMDC). Each yielded the same linear relation between thereciprocal of the denaturation temperature in Kelvin, Tmax, and the watervolume fraction, 3 (1/TmaxZ0.0007313C0.002451) up to a critical hydrationlevel, the volume fraction of water in the fully hydrated fibre. Thereafter,water was in excess, Tmax was constant and the fibre remained unchanged,no matter how much excess water was added. This Tmax value and thecorresponding intrafibrillar volume fraction of water were as follows:84.1 8C and 0.48 for glutaraldehyde treated fibres, 74.1 8C and 0.59 forHMDC treated fibres, 69.3 8C and 0.64 for MDA treated fibres, and 65.1 8Cand 0.69 for untreated native fibres. Borohydride reduction of the nativeenzymic aldimines did not increase the denaturation temperature of thefibres. As all samples yielded the same temperature at the same hydration,the temperature could not be affected by the nature of the cross-link otherthan through its effect on hydration. Cross-linking therefore causeddehydration of the fibres by drawing the collagen molecules closertogether and it was the reduced hydration that caused the increasedtemperature stability. The cross-linking studied here only reduced thequantity of water between the molecules and did not affect the waterin intimate contact with, or bound to, the molecule itself. The enthalpyof denaturation was therefore unaffected by cross-linking. Thus, thepolymer-in-a-box mechanism of stabilization, previously proposed toexplain the effect of dehydration on the thermal properties of nativethe FibresThe Increase in Denaturatiolsevier Ltd. All rights reserve
dialdehyde; HMDC,differential scanning
ing author:Temperature Following
J. Mol. Biol. (2005) 346, 551556specific lysine residues in the first step of cross-linkformation by one or more isoforms of the enzymelysyl oxidase.1,2 Impaired cross-link formation isalso seen in the connective tissues of oim mice, amodel for a variant of Osteogenesis imperfecta inwhich the a2 chain is absent resulting in thehomotrimer [a1(I)]3 forming the functional fibre.
Figure 1. The denaturation temperature of collagen inrat-tail tendon measured at different hydrations andcross-linked with different agents: (a) glutaraldehyde, (b)HMDC, (c) MDA. Squares represent the denaturationtemperatures of the native tendon before cross-linking.Circles and triangles denote cross-linked tendons fromdifferent rats. The continuous lines represent theequations quoted previously for native tendon.15 Thickline (intermediate hydration): 1/TmaxZ0.0007313C0.002451; thin line (high hydration): 1/TmaxZ0.002956.The dotted lines are the means of fully hydrated cross-linked fibres. All scans at 10 8C/minute.Oim collagen fibres are abnormally hydrated, withlarger than normal interaxial separations of thecollagen molecules, and this is a possible cause ofthe reduced cross-linking.3 Increased cross-linkconcentrations and a change in the quality andtype of the cross-linking and tissue hydration areobserved with maturation, and a further increaseoccurs with ageing of mature tissue by non-enzymicglycation cross-linking (for a review see Bailey et al.4).The lysine-aldehyde derived cross-links formed
in the early stages of life simply connect onemolecule head to tail to another. The concentrationsof these immature cross-links increase initially, butare then replaced by mature cross-links formedby making a further cross-link between an imma-ture cross-link and a third residue.4 These maturecross-links may therefore connect two or threemolecules depending on whether the third residueis on a different molecule or on a different alphachain within the same molecule. Cross-linking to athird molecule must occur transversely with themolecules in register, which would account forfurther increase in mechanical strength.The accumulation of increased concentrations of
mature cross-links with age is associated with amarked increased thermal stability5 that may bemonitored by calorimetry6,7 or hydrothermal iso-metric tension measurement.8 Analysis of skin fromsubjects with diabetes showed increased tempera-ture stability over controls in humans,9 reflecting anaccelerated ageing. A similar effect has beenobserved in experimental diabetes in rats10,11 andthis increase in denaturation temperature can befollowed by cross-linking with glucose in vitro,which also increases the thermal denaturationtemperature.12 Historically in vitro cross-linkingwith a variety of agents has been used to stabilizecollagen, for example hides for leather13 andtendons for high tensile biomaterials,14 but themechanism underlying the increased denaturationtemperatures at the molecular level is unknown.At least two factors may be considered as
potentially important. The first mechanism is aloss of chain entropy brought about by the reducednumber of molecular configurations available dueto molecules being increasingly attached covalently.The second is dehydration. Cross-links mightdehydrate the fibre by closer binding of themolecules. Indeed, dehydration caused by subject-ing the fibre to a lowwater activity environment hasbeen shown to result in very substantial increases inthe temperature stability of collagen fibres.15,16
Here we have attempted to differentiate betweenthe mechanisms experimentally, based on thesepreviously studied effects of hydration (at a fixedlevel of cross-linking) on the thermal stability ofrat-tail tendon.15 We have measured the thermalstability at different levels of hydration in rat-tailtendon cross-linked to different degrees and com-pared the results with those obtained in
552the previous work. We thereby deduce that dehy-dration is the dominant mechanism increasing thethermal stability of cross-linked collagen fibres.Thermal Stability of Cross-linked Collagen FibresChemical cross-linking
When samples of rat-tail tendon, cross-linked
with the solid line in each case. Thus, the
continuous lines in these Figures are the least-
particular environment, fibres with different cross-linking will equilibrate to different intrafibrillarfluid contents and therefore have different tem-perature stabilities (see Figure 1).A similar effect is seen during the ageing of the
Figure 2. The denaturation enthalpy of collagen in rattail tendon fibres measured at different hydrations andcross-linked with different agents: (a) glutaraldehyde,(b) HMDC, (c) MDA. Squares denote the native tendonbefore cross-linking. Circles, triangles and crosses arecross-linked tendons from different rats. The continuousline is the enthalpy of native rat-tail tendon at highhydration recorded previously,15 58.6 J gK1, now found toalso equate with those of artificially cross-linked collagenat high hydrations.squares lines reported previously15 for native rat-tail tendon and clearly fit the present data, showingthat the temperature stability of cross-linked fibresis determined only by the hydration of the fibresand not by any other property of the cross-linkitself.The enthalpy of denaturation of the fibres is also
shown (Figure 2), revealing that the enthalpy of thefully swollen cross-linked fibres and of fibreshydrated beyond a critical level is constant andthis value is the same as reported for native rat-tailtendon. Below a given level of hydration, theenthalpy falls in both cross-linked fibres and nativefibres.
Stabilization of native enzymic cross-links
Native rat-tail tendon with and without boro-hydride reduction of the enzymic aldimine cross-link yielded the same denaturation temperature(63.5 8C and 64.7 8C) and enthalpy (55.3 J/g and59.8 J/g), respectively, when measured in water.
The experiment has clearly demonstrated thatthe increased temperature stability of collagen incross-linked fibres is determined mainly by theintrafibrillar water content. Cross-linking causesdehydration of the fibres and it is the reducedhydration that causes the increased temperaturestability. Equally the results show that fibres cross-linked by different agents yield exactly the sametemperature and enthalpy of denaturation pro-vided they are measured at the same intrafibrillarwater content. Cross-linking affects the denatura-glutaraldehyde treated fibre possessed the highesttemperature stability with TmaxZ84.1 8C and thelowest volume fraction of water of 0.48, and wasconsequently the most closely packed. The volumefraction of water in the HMDC and MDA cross-linked fibres in excess water were 0.59 and 0.64,respectively, and the corresponding temperatures74.1 8C and 69.3 8C. It is emphasized that thewith bifunctional agents of different length, werescanned by differential scanning calorimetry (DSC),the temperature of the characteristic collagendenaturation endotherm changed with hydrationas shown in Figure 1. In all cases there was a linearrelation between 1/Tmax and the water volumefraction up to a critical hydration level (the volumefraction of water in the fully hydrated fibre).Thereafter, water was in excess and Tmax wasconstant and the fibre remained unchanged nomatter how much excess water was added. Thislevel differed between the different treatments, andis represented by the intersection of the dashed line
Thermal Stability of Cross-linked Collagen Fibrestion temperature of fibres fully saturated with waterbecause it reduces the extent to which fibres canswell in a given environment. More generally, in a553collagen fibre as the cross-links mature. Thetrivalent structures of the pyridinoline and pyrrolemature cross-links (e.g. Figures 6 and 7 of Bailey
554closer together than the bivalent immature cross-links. This would explain the correlation betweenthe temperature stability of calf tendon and pyri-dinoline or pyrrole concentrations.5 Further, ageingof mature fibres is believed to occur through non-enzymic glycation4 when further increases indenaturation temperature are observed.12
Increasing the stability of the mature aldiminebond by borohydride reduction does not affect thelength of the bond between the twomolecules in thenative fibre. Thus, the fact that reduction had noappreciable effect on the temperature of denatura-tion of rat-tail tendon compared to non-reducedtendon provides evidence that the transition is notinitiated by a thermally induced breakage of across-link. The data also show that any change inlength/tension relationship of the cross-link onreplacing a carboncarbon double bond by a singlebond, did not have an appreciable effect on thetemperature stability and therefore we infer noappreciable change in the intrafibrillar hydration inbuffered saline.We have found previously that the reciprocal of
the denaturation temperature (in Kelvin) is linearlydependent upon the volume fraction of intrafibrillarfluid.15,16 This has been confirmed and reinforcedby the current work. The present data showing thatthe previously determined relation, defined byleast-squares analysis of measurement of naturallycross-linked tail tendons, also holds when thetendons have been cross-linked synthetically usingexternal agents.Thus the polymer-in-a-box mechanism of
stabilization,15 previously proposed to explain thenative tendon data, explains the new data also.Briefly in this mechanism, the rate of unfolding isdepressed by the proximity of the surroundingcollagen molecules in the fibre. These molecules actlike the walls of a box to reduce the number ofpossible molecular configurations and therefore theentropy of the unfolding molecule, thus reducingthe rate of unfolding. For example, we havepreviously suggested that collagen unfolding pro-ceeds via an activated state in which the inter-chain bonds in a thermally labile region have to bebroken before the whole molecule can unzip.17 Theentropy of this activated state is reduced by theconfinement of the molecules in the box.18 Conse-quently the Gibbs free energy of the activated stateis increased and therefore the probability ofactivation and the rate of collagen denaturationare reduced (as analysed).15 Cross-linking thereforestabilizes the collagen molecules in a fibre byreducing the separation of the molecules, i.e. byreducing the lateral dimensions of the box.The temperature stability of the collagen
molecule in a fibre is composed of two terms.
(a) The intrinsic temperature stability of theet al.4) would clearly draw the collagen moleculesmolecule itself, without the stabilizing inter-actions of surrounding collagen molecules. Thisis the intercept with the vertical line, water
crosthelinkevelop within the cross-link itself by distortionthe unstressed molecular conformation.hough natural cross-links, and the syntheticarouto destigate the apparent extension/tension charac-stics of specific cross-links. The actual forcesolved are potentially complex and such experi-ts might allow the effect of individual com-ents to be investigated. We envisage formple, that adding a cross-link could changecharge and disrupt the hydration networknd the molecule, as well as allowing a tensionvolume fractionZ1. It is also the stability of themolecule in dilute solution.
(b) The second term is the temperature stabilitygained by the collagencollagen interactions,i.e. the stability gained from being confined inthe box. This is given by the slope ! (1Kthevolume fraction of water). The volume fractionof water determines the dimensions of the boxand the slope is determined by specific statis-tical characteristics of the collagen chains.15
The stability of the triple helix is governed bymany factors in addition to the cross-linkingstudied here, notably its primary structure19 par-ticularly its hydroxyproline content,20 pH,21 and thepresence of salts. Ions affect the denaturationtemperature of collagen in solution22 and in fibres23
and the effect depends on ion concentration and onthe identity of the ions involved.24 Thus, a particu-lar salt solution at a particular pH value may have astabilizing or a destabilizing effect on the moleculeitself, and it may swell or shrink the fibre. Theresults of these studies indicate that analysis ofthe temperature stability of fibres would be clarifiedby separating the effects of confinement of themolecule in the fibre lattice from the intrinsic effectson the molecule itself. Here we have thereforeanalysed the collagen in the presence of pure water.Within the limits of experimental error, cross-
linking affected neither the intrinsic temperaturestability nor the slope of the relation between thereciprocal of the denaturation temperature andwater content. Hence the cross-linking must havehad a negligible effect on the intrinsic stability of themolecule and a negligible effect on the relevantchain statistics.The major effect was caused by the cross-links
reducing the axial separation of the molecules. Atan equilibrium separation, the net force on themolecule is zero, i.e. the sum of the attractive forcesmust equal the sum of those of repulsion.25 Otherthings being equal, we therefore deduce that the netresult of adding a cross-link is to increase theattractive force or reduce the repulsive force. This isequivalent to the cross-links sustaining a tension,which in principle might be calculated by measur-ing the pressure required to dehydrate the fibre bythe same amount as the cross-linking. In such amanner, we speculate that it should be possible toinv
Thermal Stability of Cross-linked Collagen Fibress-links we have studied appear to dehydratecollagen fibre we do not envisage that all cross-s will necessarily sustain a tension. We also
paper has brought into focus, and betweenhydration and mechanical properties (obvious to
anyone who has watched a collagen fibre dry andchange from a wet flexible thread to a dry stiffbristle) implies that there is a relation betweencross-linking and mechanical properties over andabove that which is normally discussed. The resultsof this paper have therefore generated a question.How far are themechanical properties of connectivetissue caused by the collagen cross-linking per seand howmuch by the dehydration the cross-linkinginduces? In theory this question might be answeredby experiments analogous to those here, by com-paring the mechanical properties of syntheticallycross-linked and native fibres over a range ofhydrations.To summarize, when the collagen in rat-tail
tendon is cross-linked with glutaraldehyde, hexa-methylene diisocyanate, or malondialdehyde itstemperature stability is increased solely by dehy-dration of the fibre. The dehydration is broughtabout by the cross-links drawing the collagenmolecules closer together. The stabilization isconsistent with the polymer-in-a-box mechanism,previously suggested to explain the increase intemperature stability brought about by dehy-dration. The cross-linking studied here onlyreduced the quantity of water between themolecules and did not affect the water in intimatecontact with or bound to the molecule itself. Theenthalpy of denaturation was therefore unaffectedby the cross-linking treatments.
Materials and Methods
Tails were excised from the carcasses of five to six weekold rats and frozen atK20 8C until required. On removalfrom the freezer, tails were thawed and the tendonsremoved, cleaned of all visible contaminants and washedin distilled water.
The cross-linking was carried out with three differentcompounds in which the reactive groups were separatedforesee the possibility that a synthetic cross-linkcould theoretically be in compression. This wouldtend to rehydrate the fibre, and have novelmechanical consequences.The primary function of native cross-linking is to
impart desired mechanical characteristics on thecollagen fibres in connective tissue. Covalent linksbetween the molecules will undoubtedly reduce theextensibility of the fibre by preventing longitudinalslippage of one molecule against another, andincrease stiffness by developing a lateral networkof linkages. However, the association betweencross-linking and hydration, which the present
Thermal Stability of Cross-linked Collagen Fibresto different extents: malondialdehyde (1!CH2), glutar-aldehyde (3!CH2) and hexamethylene diisocyanate (6!CH2). Borohydride reduction was used to stabilize thepre-existing aldimine cross-link without altering itslength.
Cross-linking with glutaraldehyde
Tendons were placed in 0.5% aqueous solutions ofglutaraldehyde at 4 8C for 24 hours. After removal andwashing in copious amounts of distilled water, the cross-linked tendons were kept in water at 4 8C.
Cross-linking with hexamethylene diisocyanate(HMDC)
The method used by Rault et al.26 was adopted.Tendons were placed in 1% (w/v) HMDC (Sigma) inwater buffered to pH 5.5 and frozen at K19 8C for 22hours. Samples were then thawed and washed in copiousquantities of water and kept in water at 4 8C.
Cross-linking with malondialdehyde (MDA)
Tendons were placed in 150 mM MDA (a gift from DrD. A. Slatter, Biochemistry Department, CambridgeUniversity) at pH 7.5 and left for different lengths oftime at 20 8C. Following treatment, the tendon wasthoroughly washed and stored in water at 4 8C.27
Pre-existing cross-link reduction with borohydride
Equal masses of blot dried rat-tail tendon were placedin two separate containers with 5 ml of fresh phosphate-buffered saline (PBS). Sodium borohydride equivalent to1% of the blot-dried tendon mass was dissolved in 100 mlof 0.01 M NaOH and added to one container. 100 ml of0.01 M NaOH alone was added to the second, controlcontainer. Both open containers were left for one hour atroom temperature in the fume hood. The tendons werethen removed with forceps from each incubation mixture,separately rinsed twice with 5 ml of distilled water andplaced in 5 ml of 0.01 M HCl for approximately 30seconds to stop any residual reaction. Each sample wasthen rinsed twice more with excess distilled water andfinally rinsed with a single change of PBS before beingplaced in 5 ml of fresh PBS.
Sample preparation at different hydrations andcalorimetry of fibres
Control and cross-linked tendons in water were dabbeddry, weighed in open aluminum DSC pans and left todehydrate on the balance for different lengths of time inorder to obtain a range of hydrations. After the chosentime pans were hermetically sealed, weighed (G0.01 mg)and left to equilibrate at 4 8C. After at least one dayof equilibration in the sealed pans, samples werere-weighed to ensure that there had been no leakageand scanned in a PerkinElmer DSC-7, fitted withIntracooler, at 10 8C/minute from 5 8C to an appropriatespecified temperature using an empty pan as reference.Following scanning the pans were pierced and placed inan oven at 105 8C overnight and re-weighed for dry mass(md) and water mass (mw) determinations. The volumefraction of water, 3, was calculated using the relation:
where rw and rd, the densities of water and dry tendon,
were taken as 1.00!103 kg mK3 and 1.38!103 kg mK3,
Calorimetry of solutions
chemical, and biomechanical analysis of human skin
D. J., Reeve, T. & Turtle, J. R. (1983). The thermal
Diabetologia, 24, 282285.
556 Thermal Stability of Cross-linked Collagen Fibresfrom individuals with diabetes mellitus. Anat. Rec.259, 327333.
10. Andreassen, T. T., Seyer-Hansen, K. & Bailey, A. J.(1981). Thermal stability, mechanical properties andreducible cross-links of rat-tail tendon in experimen-tal diabetes. Biochim. Biophys. Acta, 677, 313317.
11. Yue, D. K., McLennan, S., Delbridge, L., Handelsman,Collagen solutions were stirred and degassed for eightminutes using a Thermovac apparatus (Microcal Inc.,Northampton, MA) and scanned in a VP-DSC (Microcal)from 10 8C to 60 8C at 1 8C/minute. Numerical analysis ofthe data was undertaken with Microcal software using acubic interpolation for the baseline.
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The Increase in Denaturation Temperature Following Cross-linking of Collagen is Caused by Dehydration of the FibresIntroductionResultsChemical cross-linkingStabilization of native enzymic cross-links
DiscussionMaterials and MethodsRat-tail tendonCross-linking procedureCross-linking with glutaraldehydeCross-linking with hexamethylene diisocyanate (HMDC)Cross-linking with malondialdehyde (MDA)Pre-existing cross-link reduction with borohydrideSample preparation at different hydrations and calorimetry of fibresCalorimetry of solutions