Research Article Correlation between the NMR Chemical Shifts and Thiolate Protonation Constants of Cysteamine, Homocysteine, and Penicillamine Juliana Ferreira de Santana , 1 Arash Mirzahosseini , 1,2 and B´ ela Nosz ´ al 1,2 1 Department of Pharmaceutical Chemistry, Semmelweis University, Budapest, Hungary 2 Research Group of Drugs of Abuse and Doping Agents, Hungarian Academy of Sciences, Budapest, Hungary Correspondence should be addressed to B´ ela Nosz´ al; noszal.bela@pharma.semmelweis-univ.hu Received 24 March 2022; Revised 7 June 2022; Accepted 11 July 2022; Published 4 August 2022 Academic Editor: Davidson Sajan Copyright © 2022 Juliana Ferreira de Santana et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1 H and 13 C NMR measurements were carried out to explore anticipated correlations between chemical shifts versus thiolate basicities and redox potentials of cysteamine, homocysteine, penicillamine, and their homodisulfides. All correlations were analyzed and statistically evaluated. e closest correlation was observed for the αCH nuclei concerning 1 H and 13 C NMR data. Since neither site-specific basicities nor site-specific redox potentials can be directly measured by any means in peptides and proteins containing several thiol and/or disulfide units, these data provide a simple method and predictive power to estimate the aforementioned site-specific physicochemical parameters for analogous sulfur-containing moieties in related biopolymers. 1. Introduction Oxidants in biological systems, commonly known as reactive oxygen species (ROS) or reactive nitrogen species (RNS), are produced chiefly in the mitochondria during the normal cellular metabolism. In the cytosol and plasma membrane, certain enzymes such as NADPH oxidase and cytochrome P450 oxidase are able to produce ROS/RNS as well [1]. ROS have an important role against infectious agents and in cel- lular signaling systems, although their effects seem to be beneficial only at low and highly regulated concentration [2]. At higher concentrations, these species evolve oxidative stress and can become toxic. During evolution, cells have adapted to counter the detrimental effects of ROS using small antioxi- dant molecules and detoxifying enzymes [3]. However, when the antioxidant processes are not sufficient, free radicals in various tissues will lead to organ damage and in the long term will act as risk factors of serious illnesses, such as cancer, arthritis, and various neurodegenerative diseases [4]. In biological systems, the major defensive process against oxidative stress is the transition of the thiol (-SH) groups into disulfides (-S-S-), ensuring thus the redox ho- meostasis. e thiol-containing cysteine (CysSH or Cys) is a principal chemical entity targeted by oxidizing species in the redox signaling routes [5]. e two main low molecular weight redox couples in human plasma are cysteine/cystine (CysSSCys) and glutathione (GSH)/glutathione disulfide (GSSG) [6]. Redox transitions are known to actually take place via the thiolate (-S - ) form, which has not only reducing, but also, proton-binding propensities, and the involvement of the perturbing acid-base processes is therefore inevitably necessary. Apart from perturbing redox and NMR phe- nomena, protonation states within a molecule are known to have an effect on other spectroscopic properties as well [7–11]. is work is focused on extending the co-dependent relationship observed between the NMR chemical shifts of the aforementioned thiols and their acid-base characteristics to the following compounds: cysteamine (CysASH)/cyst- amine (CysASSCysA); homocysteine (hCysSH)/homocys- tine (hCysSShCys); penicillamine (PenSH)/penicillamine disulfide (PenSSPen). e studied compounds are pre- sented in Figure 1, where the reduced form is always in the Hindawi Journal of Spectroscopy Volume 2022, Article ID 9491360, 8 pages https://doi.org/10.1155/2022/9491360