Die Liquid-Phase-Methode zur Peptidsynthese; Strategie und experimentelle Grundlagen

SENSASS NMR: New NMR techniques for enhancing the sensitivity and the spectral resolution of polymer supported chemicals

The use of polyethylene glycols (PEGs) as organic polymer soluble supports for synthesis has been receiving growing interest. The main advantages of using PEGs as support are related to their non-toxicity, their commercial availability and their solubility properties allowing easy recovery and analysis of compounds linked to the polymer. The NMR characterization of PEG-branched products could however be difficult due to the presence of huge signals of the polymeric support. In order to overcome this problem, we developed new NMR experiments named SENSitivity increAsed and resolution enhanced by Signal Suppression or SENSASS NMR. These experiments implement either semi-selective pulses or Water Gate sequences for reducing signals of the polymer as well as fast pulsing techniques optimizing the recycling delay for enhancing the sensitivity of signals. They have been successfully implemented in classical NMR characterization experiments namely, COSY, HSQC and HMBC experiments.

Convenient Routes to Efficiently N-PEGylated Peptides

Efficient routes to N-terminal PEGylated peptides are described. Alternative supports such as superparamagnetic core-shell nanoparticles as colloidal supports and end-functional poly(styrene) as homogeneous supports improve the available solid-phase supported coupling strategies preserving ease of purification. Poly(ethylene glycol) (PEG)-peptide bioconjugates are obtained in high yield despite the use of nearly stoichiometric PEGylation agents with respect to the supported peptides.

Enzymatically degradable poly(ethylene glycol) based hydrogels for adipose tissue engineering

Adipose tissue engineering requires biomaterials that promote the differentiation of seeded adipocytes. Here, we report on the development and characterization of in situ forming, poly(ethylene glycol) (PEG) based hydrogels for soft tissue augmentation. Branched PEG-amines were modified with collagenase-sensitive peptides and cross-linked with branched PEG-succinimidyl propionates without the use of free-radical initiators (enzymatically degradable hydrogels). Alanine-modified PEG-amines were used for the preparation of non-degradable gels. Depending on the used polymer concentration, the strength of degradable gels after swelling ranged from 1708 to 7412 Pa; the strength of non-degradable hydrogels varied between 1496 and 7686 Pa. Enzyme mediated gel degradation occurred within 10, 16, and 19 days (5%, 10%, and 15% initial polymer content). To evaluate their suitability as scaffold materials for adipose tissue engineering, the hydrogels were functionalized with the laminin-derived adhesion peptide YIGSR, and seeded with 3T3-L1 preadipocytes. Compared to a standard two-dimensional cell culture model, the developed hydrogels significantly enhanced the intracellular triglyceride accumulation of encapsulated adipocytes. Functionalization with YIGSR further enhanced lipid synthesis within differentiating adipocytes. Long-term studies suggested that enzymatically degradable hydrogels furthermore promote the formation of coherent adipose tissue-like structures featuring many mature unilocular fat cells.

Dendrimers and hyperbranched polymers as high-loading supports for organic synthesis

Polymeric supports have become a big necessity for automated synthesis and combinatorial chemistry, yet, the loading capacities of most polystyrene resins are very limited (typically < 1.5 mmol x g(-1)). Dendrimers and hyperbranched polymers have been discussed for this application and now became readily available. These soluble polymers can either be used directly as high-loading supports for substrates, reagents, and catalysts or alternatively in hybrid polymers linked to conventional polystyrene resins.

Formation of specific amino acid sequences during carbodiimide-mediated condensation of amino acids in aqueous solution

Carbodiimide-mediated peptide synthesis in aqueous solution at room temperature has been studied with respect to self-ordering of amino acids. Inasmuch as glutamic acid is readily converted into pyroglutamic acid, the peptides formed by copolymerization of glutamic acid with other amino acids are preferentially pyroglutamyl-peptides. Competition experiments were carried out to determine the influence of the amino acid side chains on the reactivity of amino acids and peptides during the dehydration condensation. The results show that the self-ordering process is controlled by both the activated carboxyl component (amino acid or growing peptide) and the incoming amino acid. A condensation of pyroglutamic acid, alanine and another amino acid component Xxx (Xxx = Gly, Val, Leu or Gly-Tyr) preferentially yielded the dipeptide pyroGlu-Ala, but the formation of the tripeptide pyroGlu-Ala-Ala became strongly reduced because of competing reactions. A simple explanation for the observed selectivities is not at hand. Polypeptides were so far only obtained when they were allowed to precipitate in the reaction system. Evidence for the non-random copolymerization of larger peptides is presented as well.

Studies on the coupling rates in liquid-phase peptide synthesis using competition experiments

Competition experiments were carried out in order to determine relative reaction rates, Vrel, for the peptide-forming step. Using the liquid-phase method of peptide synthesis a model system was developed to investigate the influence of coupling method, amino component, derivatization and solvent upon Vrel. According to a standard procedure, Vrel for all 20 commonly occurring amino acids were established. A strong dependence of Vrel upon steric effects of the coupling components was found. The data obtained may serve as a guideline for optimizing the reaction conditions in peptide synthesis.

Liquid-phase method for peptide synthesis utilizing photolytic cleavage from a new o-nitrobenzoyl polyethylene glycol support

Photolysis as a method for removal of a tert-butyloxycarbonyl-protected peptide possessing a free C-terminal carboxyl group from a polystyrene support in the solid-phase method of peptide synthesis was introduced by Rich, D. H. and Gurwara, S. K.((1975) J. Am. Chem. Soc. 97, 1575-1578). Using this basic concept we prepared a photosensitive 3-nitro-4-bromomethylbenzoyl polyethylene glycol support for use in the liquid-phase method of peptide synthesis. Photolytic cleavage of a protected tetrapeptide possessing a free C-terminal carboxyl group from the polyethylene glycol support resulted in a 98% yield compared with a 69% yield for the photolytic leavage from the polystyrene support. This application of photolysis as a cleavage method in liquid-phase peptide synthesis avoids the low yields and rather drastic conditons needed to remove a peptide attached directly to the poly-ethylene glycol support in the conventional liquid-phase method.