The outburst of microbial resistance to antibiotics creates the need for new sources of active compounds for the treatment of pathogenic microorganisms. Marine microalgae are of particular interest in this context because they have developed tolerance and defense strategies to resist the exposure to pathogenic bacteria, viruses, and fungi in the aquatic environment. Although antimicrobial activities have been reported for some microalgae, natural algal bioactive peptides have not been described yet. In this work, acid extracts from the microalga Tetraselmis suecica with antibacterial activity were analyzed, and de novo sequences of peptides were determined. Synthetic peptides and their alanine and lysine analogs allowed identifying key residues and increasing their antibacterial activity. Additionally, it was determined that the localization of positive charges within the peptide sequence influences the secondary structure with tendency to form an alpha helical structure.

Peptide synthesis is an area with a wide field of application, from biomedicine to nanotechnology, that offers the option of simultaneously synthesizing a large number of sequences for the purpose of preliminary screening, which is a powerful tool. Nevertheless, standard protocols generate large volumes of solvent waste. Here, we present a protocol for the multiple Fmoc solid-phase peptide synthesis in tea bags, where reagent recycling steps are included. Fifty-two peptides with wide amino acid composition and seven to twenty amino acid residues in length were synthesized in less than three weeks. A clustering analysis was performed, grouping the peptides by physicochemical features. Although a relationship between the overall yield and the physicochemical features of the sequences was not established, the process showed good performance despite sequence diversity. The recycling system allowed to reduce N, N-dimethylformamide usage by 25–30% and reduce the deprotection reagent usage by 50%. This protocol has been optimized for the simultaneous synthesis of a large number of peptide sequences. Additionally, a reagent recycling system was included in the procedure, which turns the process into a framework of circular economy, without affecting the quality of the products obtained.

Used in solid-phase peptide synthesis (SPPS) for peptides with an acid termination, the 2-chlorotrityl chloride (2-CTC) resin is highly susceptible to moisture, leading to reduced resin loading and lower synthetic yields. It is therefore recommended that the resin be activated with thionyl chloride (SOCl₂) before peptide assembly. Here we present an optimized procedure for resin activation that minimizes the use of SOCl₂ as the activation reagent and reduces the activation time. Additionally, we demonstrate the feasibility of reusing the 2-CTC resin when following the activation protocol, achieving comparable results to the first usage of the resin. Moreover, we achieved different degrees of resin activation by varying the amount of SOCl₂. For instance, the use of 2% SOCl₂ in anhydrous dichloromethane (DCM) allowed up to 44% activation of the resin, thereby making it suitable for the synthesis of longer peptides. Alternatively, employing 25% SOCl₂ in anhydrous DCM resulted in up to 80% activation with a reaction time of only 5 min in both cases.

One approach to enhance the bioavailability and half-life of peptides in vivo is through N-methylation of one or more of the amino acids within the peptide sequence. However, commercially available Fmoc-N-Me-AA-OHs are limited and often expensive. In this study, a solid-phase synthesis method for Fmoc-N-Me-AA-OH was developed using a 2-chlorotrityl chloride (2-CTC) resin as a temporary protective group for the carboxylic acid strategy. Two strategies for the alkylation step were compared, employing either dimethyl sulfate or methyl iodide in the Biron−Kessler method. In this work we tested the protocol with two amino acids: Fmoc-Thr(tBu)-OH and Fmoc-βAla-OH. The first one is an alpha amino acid, very hindered and with the amine group directly influenced by the electronic effects of the carboxy group, whereas in Fmoc-βAla-OH, the presence of a methylene group weakens this influence due to the intervening carbon atoms. The desired amino acids, Fmoc-N-Me-Thr(tBu)-OH and Fmoc-N-Me-βAla-OH, were synthesized by both strategies with high yield and purity.