polymers Article Synthesis and Characterization of a Bioconjugate Based on Oleic Acid and L-Cysteine Marco Vizcarra-Pacheco 1 , María Ley-Flores 2 , Ana Mizrahim Matrecitos-Burruel 3 , Ricardo López-Esparza 1 , Daniel Fernández-Quiroz 4 , Armando Lucero-Acuña 1,4 and Paul Zavala-Rivera 1,4, *   Citation: Vizcarra-Pacheco, M.; Ley-Flores, M.; Matrecitos-Burruel, A.M.; López-Esparza, R.; Fernández- Quiroz, D.; Lucero-Acuña, A.; Zavala-Rivera, P. Synthesis and Characterization of a Bioconjugate Based on Oleic Acid and L-Cysteine. Polymers 2021, 13, 1791. https:// doi.org/10.3390/polym13111791 Academic Editors: TriDung Ngo and Aman Ullah Received: 13 March 2021 Accepted: 25 May 2021 Published: 29 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Departamento de Física, Universidad de Sonora, Blvd. Luis Encinas y Rosales S/N, Colonia Centro, 83000 Hermosillo, Sonora, Mexico; marco.vizcarra@unison.mx (M.V.-P.); ricardo.lopez@unison.mx (R.L.-E.); armando.lucero@unison.mx (A.L.-A.) 2 Pritzker School of Molecular Engineering, The University of Chicago, Chicago, IL 60637, USA; mleyf@uchicago.edu 3 Departamento de Investigación en Polímeros y Materiales, Universidad de Sonora, Av. Colosio S/N, Colonia Centro, 83000 Hermosillo, Sonora, Mexico; a213220985@unison.mx 4 Departamento de Ingeniería Química y Metalurgia, Universidad de Sonora, Av. Colosio S/N, Colonia Centro, 83000 Hermosillo, Sonora, Mexico; daniel.fernandez@unison.mx * Correspondence: paul.zavala@unison.mx Abstract: One of the main challenges facing materials science today is the synthesis of new biodegrad- able and biocompatible materials capable of improving existing ones. This work focused on the synthesis of new biomaterials from the bioconjugation of oleic acid with L-cysteine using carbodi- imide. The resulting reaction leads to amide bonds between the carboxylic acid of oleic acid and the primary amine of L-cysteine. The formation of the bioconjugate was corroborated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and nuclear magnetic resonance (NMR). In these techniques, the development of new materials with marked differences with the precursors was confirmed. Furthermore, NMR has elucidated a surfactant structure, with a hydrophilic part and a hydrophobic section. Ultraviolet-visible spectroscopy (UV-Vis) was used to determine the critical micellar concentration (CMC) of the bioconjugate. Subsequently, light diffraction (DLS) was used to analyze the size of the resulting self-assembled structures. Finally, transmission electron microscopy (TEM) was obtained, where the shape and size of the self-assembled structures were appreciated. Keywords: surfactant; oleic acid; L-cysteine; bioconjugation; biomolecule 1. Introduction This work focused on the synthesis of new biomaterials from the bioconjugation of oleic acid with L-cysteine using carbodiimide. The process of chemically joining two or more molecules through a covalent bond with the help of a crosslinking agent is called bioconjugation. These agents have reactive ends for specific functional groups (primary amines, sulfhydryls, etc.) [1]. The demand for biocompatible and biodegradable mate- rials for medical applications has increased the research of new compounds that satisfy those properties. Within these new compounds, some surfactants could meet these char- acteristics [2,3]. Surfactants have a characteristic molecular structure comprising a group (generally a long alkyl chain) with little attraction for the solvent (hydrophobic, if the solvent is water), together with another group (ionic or non-ionic) that shows a strong at- traction of the solvent (hydrophilic in aqueous systems). The degree of surface activity and the type of application depends on the hydrophilic/lipophilic balance (HLB) characteristics of this amphiphilic structure [4]. The surfactant molecules of renewable raw materials that mimic natural lipoamino acids are one of the preferred options for food, pharmaceutical, and cosmetic applications [5]. These surfactants can be obtained from molecules that mimic natural amphiphilic structures. Based on these considerations, and as part of our study objective, a surfactant was synthesized from oleic acid and cysteine (lipoamino acids). The Polymers 2021, 13, 1791. https://doi.org/10.3390/polym13111791 https://www.mdpi.com/journal/polymers