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Winner of the IUPAC Prize
for Young Chemists - 2002

Jeroen J. L. M. Cornelissen wins one of the first 4 IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Polymers and Block Copolymers of Isocyanopeptides -Towards Higher Structural Order in Macromolecular Systems."

Current address (at the time of application)

IBM Almaden Research Center
650 Harry Road
San Jose, CA 95120, USA

E-mail: jeroenc@sci.kun.nl

Academic degrees

  • Ph.D. in Organic Chemistry / Polymer Chemistry (Cum Laude), University of Nijmegen, Nov. 2001
  • "Doctoraal" (Masters) in Physical Organic Chemistry, University of Nijmegen, Aug. 1996

Ph.D. Thesis

Title Polymers and Block Copolymers of Isocyanopeptides-Towards Higher Structural Order in Macromolecular Systems
Adviser Prof. Dr. R.J.M. Nolte
Thesis Committee Prof. Dr. Steggerda (chairman), Dept. of Inorganic Chemistry, University of Nijmegen; Dr. N.A.J.M. Sommerdijk (co-adviser), Prof. Dr. E.W. Meijer, Dr. A.P.J.H. Schenning, Lab. of Macromolecular and Organic Chemistry, Eindhoven University of Technology; Dr. P.C.J. Kamer, Dept. of Inorganic Chemistry and Homogeneous Catalysis, University of Amsterdam; Prof. Dr. E. Vlieg, Dept. of Solid State Chemistry, University of Nijmegen; Prof. Dr. C.W. Hilbers, Dept. of Physical Chemistry, University of Nijmegen; Prof. Dr. F.P.J.T. Rutjes, Dept. of Organic Chemistry, University of Nijmegen; and Dr. A.E. Rowan, Dept. of Organic Chemistry,University of Nijmegen.

Essay

Since van 't Hoff and Le Bel independently proposed the concept of the tetrahedral carbon geometry, stereochemistry has been a key element in not only the chemical diciplines, but also in related areas in biology, physics and materials science. On the molecular level stereochemistry provides the parameters by which we understand the properties of a substance, the way information is transferred in biological systems and other processes essential in life. The importance of (controlling) molecular geometry is recently emphasized again by awarding the 2001 Nobel Prize in chemistry to catalytic asymmetric synthesis.

Stereochemistry in man-made polymeric materials, however, remained an unexplored area untill the development of the Ziegler catalysts and the pioneering work of Natta and Pino on stereoregularity and optical activity in polyolefins. More recently the elegant studies by Green and others on optically active helical polymers have shown the versatility of these kind of systems and the analogy they display with (the formation of) structural motifs in proteins. In synthetic macromolecules, however, no transfer of stereochemical or structural information from the monomer level to a defined polymeric architecture and subsequently to an ordered assembly of macromolecules, like is present in biopolymers, has been observed to date. The investigations presented in my thesis, deal with this hierarchical transfer of stereochemical information (i.e. chirality) present in monomeric peptide related isocyanides to higher levels of molecular organization, i.e. in polymers and block copolymers of these isocyanides and in aggregates formed by them. Polyisocyanides have a rather well defined helical conformation and are accessible in an optically active form by a nickel catalyzed polymerization reaction. This macromolecular 41 helix (side chain n is more or less above side chain n + 4) is stable in solution when bulky side groups are present, but slowly unfolds in the case of less stericly demanding side groups.

In the first part of my thesis the synthesis, characterization and conformational studies on polymers of isocyanopeptides are described. Because of the regular helical structure it was proposed that hydrogen bonds could be formed between the amide groups present in the side chains which are more or less above each other. This would result in an increased regularity, stability and rigidity of the macromolecular helix. Using the single crystal X-ray structure of one of the monomers as a reference, infrared (IR) and 1H NMR studies revealed that indeed such a hydrogen bonding pattern is present between the side groups n and n + 4. Due to a sort of preorganizing effect of the monomers a highly organized polymer is formed which is even more rigid than DNA and, depending on the composition of the side arms, stable at elevated temperatures (i.e. 40-50 C) above which they unfold in a co-operative fashion. These conclusions were established by a host of experiments combining Circular Dichroism (CD) spectroscopy, X-ray scattering and model calculations. Because of the rigid character of the polymers it was possible to visualize individual macromolecules by Atomic Force Microscopy (AFM), which also enabled us to determine their absolute molecular weights.

Expanding the size of the side chains in the polyisocyanopeptides from a di- to a tripeptide, results in the formation of polymers with a beta-sheet-like organization of these side groups. The macromolecular architecture obtained in this way mimics the beta-helical structural motif found in proteins and can act as a reference point in the study of amyloid fibers. The straightforward chemical transformation of the penultimate methyl esters of the side groups to carboxylates led to the formation of watersoluble macromolecules. From IR spectroscopy and the slow H/D exchange observed in the 1H NMR experiment is was concluded that hydrogen bonds are still present in water, both in the di- and tripeptide case. This shows the highly efficient side chain directing capacity of the macromolecular helix; achieving ordered hydrogen-bonded architectures in water has appeared to be notoriously difficult in the past. In particular without the protection of the hydrogen bonding groups from the solvent, for example by the creation of a hydrophobic pocket.

Although the polyisocyanopeptides soluble in organic solvents formed cholesteric lyotropic liquid crystalline phases in concentrated solutions, we aimed for the formation of defined asymmetric structures by self-organizing chiral block copolymers in an aqueous environment. For this purpose block copolymers were synthetisized containing a hydrophobic polystyrene segment and a helical, charged polyisocyanide part. Under optimized conditions these superamphiphiles self-assembled in water, like low molecular weight surfactants do, to form a variety of structures (e.g. micelles, rods, plates and vesicles). When the relative ratio of the hydrophilic and hydrophobic parts is small also helical superstructures were observed. The mechanism of formation of these superhelices is dependent on the chemical nature of the side groups, but more interestingly involves the hierarchical transformation of chiral information present in the monomer via the macromolecular conformation to the superhelix formed. In this way providing the first example of the formation of asymmetric architectures by the self-assembly of amphiphilic block copolymers.

In the final part of my thesis the synthesis and aggregation behavior of block copolymers containing a unique combination of structural elements, i.e. a flexible, bulky and apolar dendritic carbosilane segment and a rigid, helical rod-like polyisocyanopeptide block, is described. The solubility parameters of the resultant macromolecules were different from the homopolymers, but to obtain larger ordered aggregates silver salts were added to increase the incompatibility between the component blocks. It was found that depending on the volume fraction of the two polymer segments micellar assemblies were formed. In the presence of silver larger structures were formed which deposited on a surface in a striping pattern consisting of metal rich and poor domains. Remarkably, under the influence of the electron beam in the Transmission Electron Microscope (TEM) the silver salts were reduced and metallic silver nanopatterns were formed.

Because of the defined architecture and the hierarchical organization displayed by the polyiscocyanopeptides described in my thesis, these macromolecules with tunable properties can be considered as synthetic analogs of naturally occuring proteins (e.g. beta-sheet helices) and serve as reference points in the study of complex assembling processes.


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