RESEARCH ARTICLE Raman spectroscopy probing of redox states and mechanism of flavin coenzyme Achut P. Silwal | H. Peter Lu Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA Correspondence H. Peter Lu, Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA. Email: hplu@bgsu.edu Funding information Ohio Eminent Scholar Endowment Abstract Redox states of flavin mononucleotide (FMN) play important and regulating roles in living systems. To understand the involvement and contribution of FMN coenzyme in different biological processes, probing and characterizing of the associated FMN redox states using powerful experimental approaches are fundamental and crucial. In this study, we have generated a number of typ- ical FMN redox states in Britton–Robinson buffer at different pH environments by applying electric potentials. The electric potential and pH‐dependent events of protonation, deprotonation, and electron transfer process of FMN are probed and characterized by surface‐enhanced Raman spectroscopy (SERS) or reso- nance SERS (SERRS) using silica coated silver nanoparticles (AgNP@SiO 2 ) as SERS substrate. In addition to experimental SERS analysis, we, using density functional theory, computationally calculated Raman spectra to identify the spectral signatures of the FMN redox‐state sensitive Raman modes. Here, we have specifically probed, analyzed, and characterized signature Raman modes of different redox states of FMN coenzyme including FMNH 2 •+ (1508‐ 1510 cm -1 ), FMNH 2 (1512‐1514 cm -1 ) , FMN 2-• (1498 cm -1 ), and FMN 3- (1492 cm -1 ) and proposed the redox reaction schemes of FMN in different experimental conditions. KEYWORDS electrochemical spectroscopy, flavin coenzyme, pH‐dependent redox mechanism, SERS 1 | INTRODUCTION The family of flavoproteins commonly contains flavin coenzymes such as flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) as the prosthetic group. Flavoproteins catalyze many redox reactions in biological systems where FMN and FAD have key roles for their functions. Flavoproteins are involved in a broad range of biological processes including bioluminescence, photosynthesis, DNA repair, apoptosis, and elimination of reactive oxygen species. [1–4] The reactive oxygen spe- cies has been associated to the induction and complica- tions of diabetes mellitus, age‐related eye disease, and neurodegenerative diseases such as Parkinson's disease. Malfunctioning of flavoproteins is also related to oxida- tive stress and the damage of extensive range of molecu- lar species such as lipids, proteins, and nucleic acids, initiation, and development of cancer, as well as the side‐effects of radiation and chemotherapy. Thus, flavo- proteins and flavin coenzymes are extensively studied and highly significant in protein science. In addition, fla- vin coenzymes catalyze electron‐transfer reactions in diverse ways on flavoproteins, which is involved in catal- ysis of both one and two electrons transfer process. [5,6] Sometimes, they are involved in the catalysis of electron transfer reactions between two‐electron donor and one‐ Received: 25 January 2018 Revised: 9 March 2018 Accepted: 12 March 2018 DOI: 10.1002/jrs.5379 J Raman Spectrosc. 2018;1–12. Copyright © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs 1