Supplement to Chimica Oggi/CHEMISTRY TODAY Vol 26 nr 4 • Focus on Peptides 29 ABSTRACT The most popular way to synthesize peptides is via the solid- phase approach, mostly on a research scale, although progress is being made in large-scale production. The most evident example is Fuzeon, a commercial anti-HIV peptide, which is produced in multi-kilograms using a solid support for the synthesis of the fragments. Success in solid-phase peptide synthesis is heavily determined by the solid support. In this review we focus on the evolution of the solid support from the totally polystyrene-based resin used by Merrifield to the most sophisticated ones currently available on the market. These new resins offer access to previously inaccessible compounds as well as the possibility to be used in diverse applications but without losing stability. Moreover, these new supports are easy to handle. The final chapter of the review highlights the complex sequences that are difficult to achieve and the reasons for this. It then concludes by explaining the approaches that have been followed to synthesize such “difficult” peptides. INTRODUCTION From the first dipeptide synthesis in 1901 until now, the field of peptide synthesis has experienced considerable growth as a result of several new developments: i) The solid-suppor t has been one of the main contributors to chemical peptide synthesis. Merrifield developed this ingenious and neat approach, which consists of using polystyrene (PS) resin as a solid matrix where the peptide chain is grown by being covalently attached at one end to the functionalized support. The solid support allows for simple stepwise syntheses, thereby avoiding a large number of individual steps. In addition, it allows a shortening of the time required and also the synthesis of previously inaccessible peptides. ii) Pr otecting gr oups , which allow a cleaner synthesis, the construction of selective disulfide bridges, and orthogonal groups, which permit new synthetic strategies and even the disruption of highly aggregated sequences. iii) The handle/linker (1), which allows cleavage of the peptide from the resin in diverse conditions and even modification of the carbonyl terminus at one’s convenience. iv) New coupling r eagents and additives for more efficient amide bonds, thereby permitting shorter times, better yields and non-racemization, among others. The solid support is made of diverse materials with improved intrinsic properties that allow more reagent diffusion, swelling in a wider range of solvents, uniform beads and an amphiphilic character. The solid support is precisely the focus of this review. MOST USEFUL RESINS FOR PEPTIDE SYNTHESIS (2) A solid-phase approach is the most popular way to synthesize peptides on small and large scales for research purposes. This is partly because this approach facilitates the production of medium-short and simple sequences with a minimum of work. Figure 1 shows how a peptide is usually constructed: it consists mostly of introducing a first amino acid conveniently protected, usually by C-terminus, to the solid matrix. The deprotection of the temporal group is then carried out, usually at the N-terminal, as Solid supports for the synthesis of peptides From the first resin used to the most sophisticated in the market well as coupling of the consecutive amino acid. This repetitive process allows the stepwise attachment of the side-chain- protected sequence directly on the support, thereby avoiding several steps simply by ready filtration. Once the total peptide has been achieved, it is cleaved and permanent protecting groups are removed. This is followed by precipitation, characterization, purification and folding, when required (3). The success of solid-phase synthesis is strongly related to the support and its performance. The requirements to use polymeric support in this kind of synthesis include: i) uniform beads; ii) stability in variation of temperature; iii) mobile, well-solvated and reagent-accessible sites; iv) good swelling in a broad range of solvents, if applied; v) functionalized beads permitting covalent coupling of the first compound and vi) acceptable loadings (4). Finally, the resin should be chemically, mechanically and physically stable to allow ready filtration and high chemical conditions. Currently there is no general rule to decide on the most convenient solid support. However, it is important to consider the type of chemistry to be carried out during the synthesis, resin–reagent compatibility, swelling–solvent ratio, and the length and sequence of the desired product. When synthesizing a peptide it is also relevant to take into account several questions related to the nature of the resin, such as its uniform bead size, cross-linking, and loading. In the case of the synthesis of “difficult” peptide sequences, it is crucial to select an appropriate Fayna Garcia Martin Fernando Albericio Figure 1. Solid-phase peptide synthesis (SPPS) scheme.