Conformational Analysis of Dipeptide-Derived Polyisocyanides

Transcription

1 Conformational Analysis of Dipeptide-Derived Polyisocyanides JEROEN J. L. M. CORNELISSEN, 1 W. SANDER GRASWINCKEL, 1 ALAN E. ROWAN, 1 NICO A. J. M. SOMMERDIJK, 2 ROELAND J. M. NOLTE 1,2 1 Department of Organic Chemistry, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands 2 Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands Received 20 January 2003; accepted 17 March 2003 ABSTRACT: The conformational properties of polymers derived from isocyanodipeptides have been investigated with a combination of model calculations, X-ray diffraction, and circular dichroism spectroscopy. Depending on the configuration of the side chains, defined arrays of hydrogen bonds along the polymeric backbone are formed. This leads to a well-defined conformation as, for example, expressed in the formation of lyotropic liquid-crystalline phases and increased helical stability. Upon the disruption of the hydrogen bonds by a strong acid, a less well-defined macromolecular conformation is observed Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: , 2003 Keywords: biomimetic; chiral; conformational analysis; modeling; polyisocyanides INTRODUCTION The function of biopolymers, such as proteins, RNA, and DNA, is strongly related to their secondary structure. Similar correlations have been found between the physical properties of synthetic polymers and the conformations of their macromolecular chains. 1 In particular, helix formation has recently received considerable interest because it can be used to control the organization of synthetic macromolecules at different hierarchical levels. 2 6 Polyisocyanides are a member of this class of helical polymers and have the exceptional feature This article includes Supplementary Material available from the authors upon request or via the Internet at /11/v html. Correspondence to: J. J. L. M. Cornelissen ( jeroenc@sci.kun.nl) Correspondence to: N. A. J. M. Sommerdijk ( n.sommerdijk@tue.nl) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, (2003) 2003 Wiley Periodicals, Inc. that every carbon atom in the polymer backbone is provided with a substituent. On the basis of molecular models, Millich 7 proposed a 4 1 helical conformation for the main chain in polyisocyanides. Experimental evidence for the helical structure was first provided by Nolte and coworkers; 8,9 with chromatography they were able to resolve the polymer of achiral tert-butyl isocyanide into the two optical antipodes. The optical activity in this case was solely originating from the conformation of the polyisocyanide backbone, which was stable even at elevated temperatures. By the application of Ni(II) as a catalyst, a large number of polyisocyanides have been prepared using different approaches to create an excess of either left- or right-handed helices. 10 The conformation of polyisocyanides with side chains sterically less demanding than those present in poly(tert-butyl isocyanide), however, is still under debate. Computational studies have indicated that the 4 1 helix, particularly when bulky substituents are present, is the most favorable conformation. More recently, this con- 1725

2 1726 CORNELISSEN ET AL. Chart 1 formation was recognized as a local minimum, whereas the so-called syndio conformation was considered to be the absolute minimum. 15 Experimental investigations toward the conformation of polyisocyanides are limited. Green et al. 16 showed that sterically unencumbered polyisocyanides have a limited persistence length of approximately 3 nm. In the case of polyphenylisocyanide, a combination of gel permeation chromatography, light scattering, and X-ray diffraction revealed that the native polyisocyanide has a rigid-rod character, but the polymer slowly precipitates from solution as a random coil. 17 These data suggest that the helix inversion barrier in polyphenylisocyanide is not sufficiently high to create a stable helical conformation at room temperature, and as a result of the nickel(ii)-catalyzed merry-go-round polymerization mechanism, 10 the 4 1 helix is initially formed as the kinetic product. 17 In many biopolymers, a combination of (noncovalent) interactions is required to maintain a stable secondary structure in solution. In the field of helical polymers, examples of the combined use of such interactions are rare, and in the reported cases, mainly steric interactions have been used to realize a high helix inversion barrier. 4,18 20 Recently, we have shown that the presence of well-defined hydrogen-bonding arrays between the side chains of peptide-based polyisocyanides can lead to stabilization of the helix structure of these polymers. 21 In this article, we describe in more detail the physical properties of polyisocyanides prepared from alanyl alanine-based and alanyl glycine-based monomers (Chart 1). 22 The conformations of these polymers are discussed in connection with their ability to form extended arrays of hydrogen bonds that stabilize their secondary structure. 2 6,21 The physical properties of peptide-based polyisocyanides previously re-

3 DIPEPTIDE-DERIVED POLYISOCYANIDES 1727 Figure 1. (A) Single-crystal X-ray structure of L,L- IAA showing three stacked molecules and the intermolecular hydrogen bonds 21 and (B) schematic representation of the hydrogen-bonding arrays between the side chains in L,L-PIAA. ported by our group (Chart 1) are reinterpreted, particularly in relation to the presence of hydrogen bonds between their side chains. RESULTS AND DISCUSSION Characterization of the Polymer Structure For the polyisocyanopeptides derived from L-alanyl-L-alanine (L,L-PIAA), L-alanyl-D-alanine (L,D- PIAA), and glycyl-l-alanine (L-PIGA), NMR and IR spectroscopic investigations indicate that hydrogen bonds are present between the amide groups in the polymer side chains; for the L-alanyl-glycine-based polymer (L-PIAG), no indication for the presence of these hydrogen bonds was found. 22 In these studies, the single-crystal X-ray structure of L-isocyanoalanyl-L-alanine methyl ester (L,L-IAA) served as a reference point. 21 For this monomer, the presence of an intermolecular hydrogen-bonding array (Fig. 1) resulted in a characteristic NOH stretching vibration in the solid-state IR spectrum ( NH 3279 cm 1 ), whereas no hydrogen bonding was observed in a chloroform solution. The IR spectra of the polymers all showed NOH stretching vibrations in the range of cm 1, both in solution and in the solid state, implying that they possess a structure in which the side chains have a hydrogen-bonding arrangement similar to that found in the crystal structure of L,L-IAA and, importantly, that this arrangement is preserved in solution. Further confirmation of the presence of hydrogen bonds came from 1 H NMR spectra. All the resonances assigned to the amide protons of the isocyanopeptide moieties were significantly shifted downfield by ppm upon polymerization, and this is indicative of very strong hydrogen bonding. In neither IR nor 1 H NMR spectra were any signals detected corresponding to amide groups not participating in hydrogen bonds. The absence of the hydrogen-bonding array in the case of L-PIAG is tentatively explained by a diminished preorganizing capacity of the glycine group in the polymerization reaction. 22 The increased rotational freedom of the glycine segment in comparison with that of an alanine one might account for this difference. As a result of the hydrogen bonds between the side chains, these polyisocyanopeptides are very rigid, and individual macromolecules could be visualized by atomic force microscopy. 23 The latter method was used to accurately measure their contour lengths, from which the absolute molecular weights could be deduced. 21,22 The rigid character of these polymers is also reflected in the formation of a nematic liquid-crystalline (LC) phase in a concentrated ( 10% w/w) solution of L,L-PIAA in CHCl 3, as was indicated by the birefringence of this solution visualized between crossed polarizers (Fig. 2). 21 The observed characteristic fingerprint texture points to a cholesteric arrangement of the macromolecules, 24 as frequently observed for concentrated solutions of stiff single-handed helical polymers. 25 Figure 2. Optical micrograph between crossed polarizers of a 10% (w/w) solution of L,L-PIAA in CHCl 3.In the diluted solution, the optical rotation amounted to [ ] D For the LC phase, a pitch of 17 m and an optical rotation of [ ] D 44,000 were observed 24 h after sample preparation. These values eventually became 9.2 m and [ ] D 72,000, respectively, after 40 h of standing (see the Experimental section for molecular weight data).

4 1728 CORNELISSEN ET AL. Figure 3. PXRD patterns of (A) L,L-PIAA as a cast film, (B) L,L-PIAA as a powder, (C) L,L-PIAA as a cast film after treatment with TFA, (D) L,D-PIAA as a cast film, (E) L-PIGA as a cast film, and (F) L-PIGA as a cast film after the heating of the polymer at 60 C for 1hin CHCl 3. Powder X-Ray Diffraction (PXRD) The diameters of the macromolecular rods in the solid state were determined for L,L-PIAA, L,D- PIAA, and L-PIGA with PXRD. Diffraction patterns displaying sharp signals were observed (Fig. 3), which pointed to a regular crystallization of well-defined macromolecular rods. 26 The differences observed in the peak intensities and peak widths between patterns obtained from powders and from films spun from CHCl 3 solutions may be attributed to local differences in the ordering of the rods and/or the presence of solvent molecules. Macromolecular diameters were calculated from the diffraction patterns obtained form both types of specimens. The recorded reflections fitted best with an orthorhombic arrangement of the polymers (see the supplementary material). With this model, the diameters were determined to be 15.9 Å for both L,L-PIAA and L,D-PIAA and 16.6 Å for L-PIGA. Model Calculations Two different macromolecular conformations have been proposed for polyisocyanides, 10 the 4 1 helix 7 14 and the so-called syndio conformation (Fig. 4, top and middle). Clericuzio et al. 15 calculated that the latter conformation is thermodynamically more favorable. In the syndio conformation, two adjacent imine groups are in one plane, which makes an angle of approximately 90 with the plane of the next two imine groups. When initially a 4 1 helix was assumed for both L,L-PIAA and L,D-PIAA, calculations suggested a final structure in which the macromolecular rods had a calculated diameter of Å, a helical pitch of 4.6 Å, and an average spacing between the side chains n and n 4 of 4.7 Å. In these calculations, the polyisocyanides were treated as a polymeric spring of which the extension and compression elongated or shortened the polymer backbone and consequently narrowed or widened its diameter (Fig. 4, bottom). The calculated sidechain spacings of 4.7 Å resulting from this iterative procedure match very well with the spacing of Å between the amide groups found in the crystal structure of the monomer L,L-IAA. The diameters obtained for the macromolecular rods are in excellent agreement with those determined experimentally by PXRD (15.9 Å). 27 For L-PIGA, the diameter was calculated to be Å, again in good agreement with the value of 16.6 Å determined experimentally (discussed previously). Calculations on L,L-PIAA and L,D-PIAA were also performed with the syndio model, in which the Figure 4. Models representing the 4 1 helical conformation of a 20-residue polyisocyanide (top) and the syndio conformation of a 12-residue polyisocyanide (middle). A schematic representation of the relationship between the relative length and diameter of a helix is also shown (bottom).

5 DIPEPTIDE-DERIVED POLYISOCYANIDES 1729 Figure 5. CD and UV vis spectra in CHCl 3 of (A) L,L-PIAA, (B) L,D-PIAA, (C) L-PIGA, and (D) L-PIAG. NOCOCON dihedral angles were kept close to 180 to maintain the conjugation of the adjacent imine groups (Fig. 4, middle). This, however, led to an average spacing between side chains n and n 4 of 5.5 Å, which is too large for the formation of well-defined hydrogen bonds; therefore, the syndio conformation was discarded as being of relevance in these polyisocyanopeptides. Circular Dichroism (CD) Spectroscopy The helical structure of polyisocyanides can be studied by CD spectroscopy, which monitors the n * transitions of the backbone imine functions residing in the wavelength range of nm. 28 The direct determination of the macromolecular helix sense of polyisocyanopeptides with this technique is, however, hampered by sidechain contributions to the Cotton effects originating from the backbone n * transitions. Nevertheless, by a comparison of the CD spectrum of an L-alanine-based polyisocyanide containing a socalled spectator group (i.e., a diazochromophore) with the CD spectra of other L-alanine-based polyisocyanides, a right-handed (P) helical backbone geometry has been assigned to these polymers. 29 For the polyisocyanodipeptides in which hydrogen-bonding arrays between side chains n and n 4 are present, that is, L-PIGA, L,L-PIAA, and L,D-PIAA, the CD spectra display a single strong Cotton effect centered around 315 nm (Fig. 5). For L-PIAG, which does not have well-defined hydrogen-bonding arrays, the band at 315 nm is absent,

6 1730 CORNELISSEN ET AL. and instead a couplet with lower intensity can be observed. The difference between these spectra and the ones obtained either experimentally 10 or theoretically (by calculations) 15 for aliphatic polyisocyanides may be attributed tentatively to an effect of the side chains on the n * transitions of the CAN chromophores. Because of the hydrogen bonds, the amide carbonyls within one particular array all point in the same direction (Fig. 1). This will locally result in the formation of a large permanent dipole, which will strongly influence the n * transitions of the nearby imine groups. Although the electronic transitions of the amide functionalities cannot be observed directly because they are masked by the solvent absorptions, our present hypothesis is that the ordering of these amides is reflected in the imine n * transitions. Unfolding the Secondary Structure of the Polymers Because the helical structure of the polyisocyanopeptides appears to be directly related to the presence of well-defined hydrogen-bonding arrays along the polymer chain, it was anticipated that the disruption of these arrays would lead to distinct changes in the secondary structure and properties of these polymers. The strength of hydrogen bonds in solution is strongly dependent on the type of solvent and usually decreases with increasing temperature. As the well-defined hydrogen-bonding arrays between side chains n and n 4 in the polymers are reflected in the intense Cotton effects, CD spectroscopy was used to study the effects of changes in these two parameters on the polymers. The addition of polar hydrogen-bonding solvents such as methanol and dimethyl sulfoxide to PIAA solutions in CHCl 3 did not lead to any noticeable changes in the CD spectra, and neither did the addition of a solution of urea in CHCl 3. For L,L-PIAA, it was found that the addition of an excess of trifluoroacetic acid (TFA) was required to disrupt the hydrogen-bonding arrays, leading to a decrease in the Cotton effect at 312 nm and eventually resulting in a weak negative signal [Fig. 6(A)]. In the case of L,D-PIAA, this decrease was so fast that no intermediate state could be detected. The stepwise addition of smaller amounts of acid allowed the recording of the intermediate spectra [Fig. 6(B)], which showed that the strong signal at 307 nm vanished and two new signals appeared, that is, a positive peak with max 361 nm and a negative signal with max 275 nm Also, in the ultraviolet visible (UV vis) spectra, significant changes were observed; for L,L-PIAA, the band at 308 nm broadened and shifted to longer wavelengths. Upon the addition of TFA, a similar redshift was observed in the UV vis spectrum of L,D-PIAA; however, in this case, a more narrow absorption band remained, suggesting that this polymer retained a regular structure. This is supported by the higher intensity of the preserved CD signals and the presence of an isodichroic point at 335 nm, which indicates that the breaking of the hydrogen bonds induces a transition to another discrete conformation The structural changes induced by TFA on L-PIGA were so fast even in the presence of small amounts of acid that it was impossible to obtain reproducible intermediate spectra; only the complete loss of the Cotton effect could be observed. The absence of a defined hydrogenbonding array in L-PIAG was further confirmed by the absence of any observable change in the chiroptical properties of this polymer upon the addition of TFA. In the IR spectra, the acidification was accompanied by a small shift of the amide NH vibration to higher wave numbers. In a representative example of L,L-PIAA, this shift is 23 cm 1, and it is accompanied by the appearance of a shoulder at 3395 cm 1, which suggests a weakened or less well-defined type of hydrogen-bonding pattern. With increasing temperature, similar changes in the CD spectra were observed for these polymers, with the exception of L-PIAG. The latter observation is in line with the absence of hydrogen-bonding arrays in this particular polymer. In the case of L-PIGA, the intensity of the Cotton band at 321 nm [Fig. 7(A)] decreased significantly faster with increasing temperature in comparison with the reduction of the same bands in the PIAAs [Fig. 6(C,D)]. The absence of sterically demanding groups next to the imine moieties in the polymeric backbone of this polymer could explain this effect. After the heating of L-PIGA for 1 h at 60 C in CHCl 3, IR studies revealed that hydrogen bonds were only partially retained, resulting in a pattern similar to that found for L-PIAG. 22 The observed nonlinear decrease of the Cotton effect centered around 321 nm and the increase of the absorption band at 346 nm [see Fig. 7(B)] indicate that the unwinding of the helical backbone is a cooperative process. For the PIAAs, partial reversibility was found in this cooperative

7 DIPEPTIDE-DERIVED POLYISOCYANIDES 1731 Figure 6. (A) Changes in CD and UV vis spectra with the addition of 18.2% (v/v) TFA to a 2.6 mm solution of L,L-PIAA in CHCl 3 [CD spectra: (a) before addition, (b) 1 min after addition, (c) 6 min after addition, and (d) 17 min after addition; UV vis spectra: (e) before addition, (f) 3 min after addition, (g) 14 min after addition, and (h) 35 min after addition], (B) changes in CD and UV vis spectra of L,D-PIAA (initial concentration 2.2 mm) with the addition of TFA [(a) 0, (b) 2.2, (c) 6.3, (d) 10.0, and (e) 13.5% v/v], (C) CD spectra of L,L-PIAA at different temperatures, and (D) CD spectra of L,D-PIAA at different temperatures. unwinding (not shown) when the temperature was lowered after it had been raised first stepwise to 55 C; this suggested a higher helix inversion barrier of these polymers in comparison with L- PIGA. This implies that the breaking of the hydrogen bonds in L,L-PIAA does not immediately lead to complete unfolding of the helical backbone; however, after a period of cooling, the welldefined starting situation will not be regained, possibly for entropic reasons. PXRD patterns were recorded for samples of L,L-PIAA and L-PIGA in which the hydrogenbonding arrays had been disrupted (discussed previously). In contrast to the situation in which these hydrogen-bonding arrays were still present, only broad signals were found in these samples [Fig. 3(C,F)]. From the observed reflections, polymer diameters of 16.3 and 17.2 Å were calculated for L,L-PIAA and L-PIGA, respectively, under the assumption that the poly-

8 1732 CORNELISSEN ET AL. rigid-rod polyphenylene. 34 The CD spectrum obtained for acidified L,L-PIAA is indeed very similar to that of poly(l-isocyanoalanine ethyl ester) (L-PIA). The same resemblance is observed between these spectra and the CD spectrum of L- PIAG [Fig. 5(D)], for which no defined arrays of hydrogen bonds were found. 22 For L-PIGA, the absence of a substituent next to the imine moiety leads to a complete loss of excess helical sense; this suggests that in this situation the inversion barrier is sufficiently low to allow reversal and equilibration of the helices once hydrogen bonds are no longer present. Figure 7. (A) CD and UV vis spectra of L-PIGA with increasing temperature (the arrows indicate the directions of the changes) and (B) relative intensities of selected CD and UV vis bands as a function of temperature. mers were still present as macromolecular rods with hexagonal ordering. These results show that the secondary structure in these polyisocyanodipeptides strongly depends on the presence of well-defined arrays of hydrogen bonds. After disruption of the hydrogen bonds in the side chains, the rotation around the carbon carbon bonds in the polymeric backbone is only hindered by the steric bulk of these side chains. The PXRD patterns obtained for these polymers indicate that even in the absence of such arrays, the macromolecules still behave like rodlike structures with a high degree of organization in comparison with polyphenylisocyanide 17 and Effects of the Side-Chain Configuration For the polyisocyanopeptides previously discussed, the presence of a hydrogen-bonding array between the amide groups is in all cases (i.e., L,L-PIAA, L,D-PIAA, and L-PIGA) accompanied by a strong positive Cotton effect around 315 nm attributed to a right-handed (P) helix, whereas for the polyisocyanides in which these arrays were absent (i.e., L-PIAG and L,D-PIAA after TFA treatment), CD spectra were obtained lacking this signal. For polyisocyanopeptides derived from alanyl serine 35 and alanyl histidine 36,37 prepared in the past by our group, similar differences in CD spectra were observed, including differences between the two stereoisomers of poly(isocyanoalanyl serine methyl ester), L,L-PIAS 36,37 and L,D- PIAS, and between the two stereoisomers of poly- (isocyanoalanyl histidine methyl ester), L,L-PIAH and D,L-PIAH (Fig. 8). The CD spectra of L,L-PIAS and L,L-PIAH show a strong Cotton effect around 310 nm in analogy to those of L,L-PIAA, L,D-PIAA, and L-PIGA, whereas such a Cotton effect is absent in the spectra of L,D-PIAS and D,L-PIAH, which show more of a resemblance to the spectrum obtained for L,D-PIAA after treatment with TFA and the spectrum of L-PIAG. 38 Although the precise structures of these polymers were never elucidated, we tentatively attribute these differences to the presence of hydrogen-bonding arrays in the former two polymers and the absence of such arrays in the latter two. For the alanyl serine-derived polymer, this explanation is supported by the differences in the intrinsic viscosities ([ ] 4.1 vs [ ] 0.35) and observed polymerization times ( 30 vs 240 min) determined for L,L-PIAS and L,D-PIAS, respectively; this suggests that the first polymer has a better defined conformation, that is, including the hydrogen-bonding arrays, than the second. 39 Similar differences

9 DIPEPTIDE-DERIVED POLYISOCYANIDES 1733 Figure 8. CD spectra of (A) L,L-PIAS, 36,37 (B) L,D-PIAS, 36,37 (C) L,L-PIAH, 35 (D) D,L-PIAH, 35 and (E) poly(l-isocyanoalanyl-l-o-acetyl histidinol) [D,L-PIAH(OAc)]. 36,37 were found between the intrinsic viscosities and polymerization times of L,L-PIAA and L-PIGA. The differences in the polymerization times between polymers that form such hydrogen-bonding arrays and polymers that do not, in tandem with the fact that the Ni(II)-catalyzed polymerization reaction is kinetically controlled, suggest that the preorganization of the monomers with respect to the growing chain is an important factor in determining the polymer structure. 22 In this scenario, the interaction between the growing chain and the incoming monomer will depend on both the steric bulk and the configuration of the monomeric unit and will be reflected in the polymerization time and in the properties of the resultant polymers (i.e., the absence of hydrogen bonding in L-PIAG). CONCLUSIONS Polyisocyanides derived from dipeptides can have, depending on the precise configuration of the constituting amino acids, a highly defined

10 1734 CORNELISSEN ET AL. conformation maintained by hydrogen-bonding arrays formed between amide functionalities in the polymer side chains. From a combination of spectroscopic data, powder diffraction, and model calculations, an extended 4 1 helical conformation is proposed for these polymers in which hydrogen bonds are present between side chains n and n 4. On the basis of the data obtained, it is expected that the integrity of these macromolecular helices is largely determined by a delicate interplay between steric and hydrogen-bonding interactions between the monomer and the growing polymer chain. For L,L-PIAA and L,D-PIAA, the helix inversion barrier is large compared with that of L-PIGA, which is caused by a combined effect of the sterically demanding substituents and the stabilization by the hydrogen bonds. For these polymers, LC properties have been observed that support the idea that these macromolecules have a well-defined structure. Similar to the denaturation of proteins, the disruption of the hydrogen bonds by an increase in the temperature or by a treatment of the polymers with a strong acid leads to a less well-defined macromolecular conformation. The presence of hydrogen-bonding arrays along the polymeric backbone is reflected in the intense Cotton effects that are present around 315 nm in the CD spectra. This technique, therefore, is a powerful tool for studying the conformation of these peptidederived polyisocyanides. Indeed, upon the disruption of the hydrogen bonds, drastic changes in the CD spectra are found. Because the Ni(II)-catalyzed polymerization of isocyanides is a kinetically controlled process, it is likely that all polymers grown from L-alaninebased monomers have the same stereochemistry. On the basis of this kinetic control and the observed properties of the polymers, including the short polymerization time, high intrinsic viscosity, and intense Cotton effects, it is proposed that the L-alanine-derived peptide monomers fit better in the developing polymer helix than monomers with different configurational properties because they can form internal hydrogen-bonding arrays. 40 As a result of the combination of steric interactions and hydrogen bonding, subtle differences in the side-chain configuration can have a pronounced effect on the formation of the polymers, as demonstrated for L,D-PIAA, which can be formed even without the help of nickel(ii) ions, that is, by H catalysis. 21 The accessibility of a large number of natural and unnatural amino acids opens the possibility of designing and synthesizing a wide array of well-defined polyisocyanopeptides and related block copolymers. The arrangement of the side groups, resembling the -sheet peptide motif, provides a stable and robust scaffold to which a variety of functional groups can be attached, such as metal catalysts 41 and nonlinear optical chromophores. EXPERIMENTAL General Methods and Materials All chemicals were commercial products and were used as received. 1 H NMR spectra were recorded on Bruker AC-100, Bruker WM-200, and Bruker AC-300 instruments at 297 K. Chemical shifts are reported in parts per million relative to tetramethylsilane ( 0.00 ppm) as an internal standard. Fourier transform infrared spectra were recorded on a Bio-Rad FTS 25 instrument. UV vis spectra were measured on a Varian Cary 50 concentrated spectrophotometer, and CD spectra were measured on a Jasco 810 instrument. X-ray powder diffractograms were collected on a Philips PW1710 diffractometer equipped with a Cu LFF X-ray tube operating at 40 kv and 55 ma. Samples were measured on a silicon wafer between 3 and 60, with a step width of Optical micrographs between crossed polarizers were obtained on a Jeneval THMS 600 microscope. Model Calculations Molecular modeling calculations were carried out with the CHARMM 4.4 molecular mechanics force field (with quanta charm charges) and the Quanta modeling package. Calculations were carried out on small oligomers (n 64, 16 turns of the helix) initially possessing a 4 1 helix. The oligomers were preliminarily minimized with molecular dynamics (3ps, 273 K) in vacuo. The resulting minimum energy conformations were subsequently minimized with molecular mechanics, and this resulted in the presented conformations. Because of end-group effects during the dynamic calculations, only the central 8 helical turns were used as a model of the resulting helices. CD Spectroscopy CD spectra were measured at ambient temperature in CHCl 3. Spectra of the samples to which

11 DIPEPTIDE-DERIVED POLYISOCYANIDES 1735 acid was added were recorded in 8/1 (v/v) CHCl 3 / MeOH to prevent precipitation of the polymers. During the temperature-dependent measurements, after each interval, the solution was equilibrated for 10 min. Synthesis The polymers L,L-PIAA [number-average molecular weight (M n ) 186 kg/mol, polydispersity index (PDI) 1.4], L,D-PIAA (M n 221 kg/mol, PDI 1.7), L-PIGA (M n 120 kg/mol, PDI 1.4), and L-PIAG were synthesized as described previously. 22 The Council for Chemical Sciences of the Netherlands Organization for Scientific Research is acknowledged for its financial support of J. J. L. M. Cornelissen. REFERENCES AND NOTES 1. Macromolecules: An Introduction to Polymer Science; Bovey, F. A.; Winslow, F. H., Eds.; Academic: San Diego, Rowan, A. E.; Nolte, R. J. M. Angew Chem Int Ed 1998, 37, Okamoto, Y.; Nakano, T. Chem Rev 1994, 94, Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem Rev 2001, 101, Nomura, R.; Tabei, J.; Masuda, T. J Am Chem Soc 2001, 123, Nomura, R.; Tabei, J.; Nishuira, S.; Masuda, T. Macromolecules 2003, 36, Millich, F. Chem Rev 1972, 72, Nolte, R. J. M.; van Beijnen, A. J. M.; Drenth, W. J Am Chem Soc 1972, 96, van Beijnen, A. J. M.; Nolte, R. J. M.; Drenth, W. Recl Trav Chim Pays-Bas 1980, 90, Nolte, R. J. M. Chem Soc Rev 1994, 23, Kollmar, C.; Hoffmann, R. J Am Chem Soc 1990, 112, Cui, C. X.; Kertesz, M. Chem Phys Lett 1990, 169, Huige, C. J. M. Ph.D. Thesis, University of Utrecht, Huige, C. J. M.; Hezemans, A. M. F.; Nolte, R. J. M.; Drenth, W. Recl Trav Chim Pays-Bas 1993, 112, Clericuzio, M.; Alagona, G.; Chio, G.; Salvadore, P. J Am Chem Soc 1997, 119, Green, M. M.; Gross, R. A.; Schilling, F. C.; Zero, K.; Crobsy, C., III. Macromolecules 1988, 21, Huang, J.-T.; Sun, J.; Euler, W. B.; Rosen, W. J Polym Sci Part A: Polym Chem 1997, 35, Quantitative theoretical and experimental analyses of the helix inversion barrier in polyisocyanates have been reported; see refs. 19 and Young, J. A.; Cook, R. C. Macromolecules 2001, 34, Ute, K.; Fukunishi, Y.; Jha, S. K.; Cheon, K. S.; Munoz, B.; Hatada, K.; Green, M. M. Macromolecules 1999, 32, Cornelissen, J. J. L. M.; Donners, J. J. J. M.; de Gelder, R.; Graswinckel, W. S.; Metselaar, G. A.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001, 293, Cornelissen, J. J. L. M.; Graswinckel, W. S.; Adams, P. J. H. M.; Nachtegaal, G.; Kentgens, A.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. J Polym Sci Part A: Polym Chem 2001, 39, Samori, P.; Ecker, C.; Gössl, I.; de Witte, P. A. J.; Cornelissen, J. J. L. M.; Metselaar, G. A.; Otten, M. B. J.; Rowan, A. E.; Nolte, R. J. M.; Rabe, J. P. Macromolecules 2002, 35, Collings, P. J.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics; Taylor & Francis: London, Green, M. M.; Zanella, S.; Gu, H.; Sato, T.; Gottarelli G.; Jha, S. K.; Spada, G. P.; Schoevaars, A. M.; Feringa, B.; Teramoto, A. J Am Chem Soc 1998, 120, Ballauf, M. Angew Chem 1989, 101, In strict terms, the polymer no longer has a 4 1 helical conformation but instead has a helical conformation. 28. van Beijnen, A. J. M.; Nolte, R. J. M.; Naaktgeboren, A. J.; Zwikker, J. W.; Drenth, W. Macromolecules 1983, 16, Cornelissen, J. J. L. M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Macromol Chem Phys 2002, 203, In addition to the protonation of the amide groups and the subsequent disruption of the hydrogenbonding pattern, the imine groups also can be protonated, and this, in turn, can cause syn anti changes around the CAN bond. However, the changes in the chiroptical properties of L,L-PIAA and L-PIGA induced by acid were irreversible upon neutralization of the solution with a base, and similar, albeit less pronounced, changes in the CD spectra were also found for L,L-PIAA and L,D-PIAA with increasing temperature (see the text). This suggests that these are not likely the result of the protonation of the imines (see refs. 31 and 32). It has been found recently that the acid-induced changes in the chiroptical properties of L,D-PIAA can be reversed completely by titration with a base (see ref. 33). 31. Aharoni, S. M. J Polym Sci Polym Phys Ed 1979, 17, Kamer, P. C. J.; Drenth, W.; Nolte, R. J. M. Polym Prep 1989, 30, 418.

12 1736 CORNELISSEN ET AL. 33. Metselaar, G. A. University of Nijmegen, Unpublished results, Gin, D. L.; Conticello, V. P.; Grubbs, R. H. J Am Chem Soc 1994, 116, Van der Eijk, J. M.; Nolte, R. J. M.; Drenth, W.; Hezemans, A. M. F. Macromolecules 1980, 13, Visser, H. G. J.; Nolte, R. J. M.; Zwikker, J. W.; Drenth, W. J Org Chem 1985, 50, van der Eijk, J. M. Ph.D. Thesis, University of Utrecht, The Netherlands, Yamada, Y.; Kawai, T.; Abe, J.; Iyoda, T. J Polym Sci Part A: Polym Chem 2002, 40, For L,D-PIAA, a high intrinsic viscosity was also measured ([ ] 5.26), whereas for poly(l-isocyanoalanyl ethyl ester), a much lower value ([ ] 0.44) was obtained. In the former case, a polymerization time of less than 5 min was found, in contrast to 7200 min in the latter case. See Cornelissen, J. J. L. M. Pure Appl Chem 2002, 74, A similar relation was recognized by Visser et al., 36 although no relationship with hydrogen-made formation was found at that time. 41. Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature 1994, 372, 659.