Development of New Tools for the Synthesis of

Abstract

The known as "difficult peptides", as well as those sequences that aggregate in solution, are some of those molecules with high applicability as nanomaterials or even in medicine field. This thesis has addressed to overcome the synthetic, as well as, the peptide manipulation in solution drawbacks associated to this kind of peptides. The strategies proposed and evaluated in the present work have been divided in three chapters classified depending on the strategies evaluated.

The "difficult peptide" RADA-16, which is also a self-assembling peptide, has been described as a valuable sequence for its biomedical applications. In order to reach its synthesis with the maximum purity, three solid-phase peptide strategies (SPPS) were selected. First, the most commonly used, the stepwise methodology; and later the two convergent approaches: the solid-phase and the fragment condensation in solution. The four times repeating Arg-Ala-Asp-Ala sequence comprised in this peptide was crucial for the retro-synthetic design of both convergent strategies. Fragment condensation in solution of two fully protected segments resulted in the most fruitful strategy to synthesize the RADA-16 peptide. Development of a new work-up protocol was essential to achieve the final peptide with a satisfactory purity. Several HPLC conditions of commercial RADA-16 samples were studied allowing to find the most significant parameters to achieve an accurate characterization of this peptide.

In the second chapter, the design of a new backbone amide protecting group, the Mmsb, was developed as a "safety-catch" protector to facilitate the synthesis of "difficult peptides". The solid-phase incorporation of the Mmsb into a sequence was preceded by the synthesis of an alanine protected with the Mmsb. The safety-catch property of this protecting group was confirmed by evaluating its stability to the cleavage conditions, and its subsequent lability after a chemoselective modification.

Three difficult peptides were selected to evaluate the Mmsb strategy, the decaalanine, the RADA-16 and the Aß(1-42). The three peptides were synthesized by SPPS affording improved purities when one Mmsb moiety was present in the sequences, compared with those non-protected peptide syntheses. Moreover, due to the presence of Mmsb containing sequences after the cleavage from the resin, the solubility of the three peptides resulted enhanced compared with the non-Mmsb containing sequences. Two of these peptides were purified by HPLC confirming that thanks to the enhanced solubility this step was possible to be performed.

The last chapter was focused on the strategy to solubilize non-polar molecules/peptides in solution by conjugation of short cationic peptide sequences (known as solubilizing tags). The permanent and temporary conjugation methods were evaluated in this chapter. Initially, various short cationic peptide sequences based on Lys or Arg residues were synthesized and conjugated to a non-polar model to evaluate their effect in terms of polarity as permanent conjugation model. The most polarity enhancer tag structure was linked to a known insoluble drug providing a proof of concept to demonstrate the tag contribution to enhance the solubility and polarity of some drugs.

The temporary conjugation of a non-soluble peptide, through a linker, to a short linear poly-Lys solubilizing tag was evaluated herein. The Mmsb-OH is a linker described in the literature which was used in this study as a connector between the solubilizing tag and the insoluble peptide. Enhancement of solubility of the peptide permitted the accurate peptide characterization. A final step allow to reach the peptide target by detaching it from the solubilizing tag.

A new application of the Mmsb-OH linker was provided in this case as a new C-terminal protecting group. This protector allow to carried out the fragment condensation of two fully protected sequences in solution successfully and, the subsequent protecting groups removal rendered the expected peptide.

En la presente tesis, en base al diseño de metodologías de síntesis que contribuyen a facilitar la obtención de péptidos complejos, se ha participado en el desarrollo de nuevas herramientas para la síntesis de los llamados "péptidos difíciles". Además, facilitando también la purificación de aquellos péptidos que agregan en solución. Dentro del campo de la síntesis de péptidos en fase-sólida (SPPS), este trabajo se encuentra dividido en tres capítulos, clasificados según el tipo de estrategia abordada en cada caso.

En el primer capítulo se han evaluado tres estrategias conocidas para poder obtener el "péptido difícil" RADA-16 con la máxima pureza. El desarrollo de la obtención de este péptido, con elevado interés en la biomedicina, se ha basado en la SPPS, combinada con la síntesis de péptidos en solución. De modo que, mediante la condensación en solución de dos fragmentos de RADA-16, sintetizados previamente en fase-sólida, se ha logrado la secuencia con una pureza razonable. Además, se ha diseñado un método de eliminación de las impurezas principales, así como un protocolo para una rigurosa caracterización mediante HPLC.

En el segundo capítulo se han sintetizado tres "péptidos difíciles" mediante la incorporación, en sus secuencias, de un nuevo grupo protector para las amidas. La síntesis del grupo y su incorporación se han desarrollado con resultados satisfactorios. La mejora en la síntesis de las secuencias que contenían este grupo han permitido la obtención de los péptidos con mayor pureza, comparado con aquéllos que no contenían este grupo. La presencia del grupo protector en los péptidos ha hecho aumentar su solubilidad en solución y por tanto ha facilitado también su caracterización y purificación.

En el tercer capítulo se han diseñado péptidos iónicos de cadena corta que, conjugados a una molécula/péptido apolar, han hecho aumentar su solubilidad y, consecuentemente, su manipulación en solución. De manera permanente y temporal, esta unión se ha realizado para dos casos, demostrándose una solubilidad mejorada que ha permitido una adecuada caracterización. Un espaciador bifuncional, descrito previamente en la literatura, se ha utilizado para la conjugación temporal, y además se ha desarrollado como nuevo grupo protector de ácido carboxílico C-terminal para péptidos.