Electrochemical and UV/VIS Study of L-Histidine and Its Complexes with Cobalt and Nickel

In this work oxido-reduction properties of L-histidine were studied by cyclic and differential pulse voltammetry in a pH range from pH 4–10. The results have shown that L-histidine is not electroactive in a pH range from pH 4 to pH 9, while oxidation peak of L-histidine’s imidazole ring at pH 10 was detected with differential pulse voltammetry. It has been found that the oxidation peak current of L-histidine increased with the increase of its concentration in a solution. Adsorption of oxidation product of L-histidine on a glassy carbon electrode surface was also detected. Voltammetric techniques (cyclic and differential pulse voltammetry) and UV/VIS spectroscopy were used to study formation of L-histidine complexes with transition metals. It has been found that L-histidine formed octahedral complex with metal ion (Co2+ and Ni2+) in a 1 : 1 ratio. This work is licensed under a Creative Commons Attribution 4.0 International License

Antioxidant Activity and Polyphenols of Aronia in Comparison to other Berry Species

Total polyphenols, total anthocyanins and antioxidant activity of various berries: [aronia (Aronia melanocarpa), blackberry (Rubus fruticosus), red raspberry (Rubus idaeus), and strawberry (Fragaria anannassa)] have been evaluated in this study by using Folin-Ciocalteau, pH-differential, and DPPH method. Amount of anthocyanins ranged from 232 to 4341 mg kg-1 in strawberry and aronia, respectively. Total polyphenol content varied from 1005 mg kg-1 in strawberry to 10637 mg kg-1 in aronia. Aronia contains the highest amounts of polyphenols and anthocyanins among the berries studied. It shows the highest antioxidant activity, as well. Therefore, individual polyphenolic compounds of aronia were studied further in more details. The amount of fl avonols (quercetin, myricetin, kaempferol) and anthocyanins in aronia was determined by using HPLC method. Anthocyanins found in aronia were derivatives of cyanidin of which the most abundant were cyanidin-3- galactoside (68.9%) and cyanidin-3-arabinoside (24.5%). Quercetin was the main flavonol, with 93.07 % in total flavonol amount. The level of kaempferol was low (6.93%) while myricetin was not identified

Dihidrokalkoni u starim kultivarima jabuka iz Hrvatske

Dihydrochalcones are the only subgroup of polyphenols that is specific to apples. They showed many beneficial bioactivities. In this work, the aim was to analyze dihydrochalcones in old apple varieties from Croatia to see if varieties differ amongst themselves according to dihydrochalcone percentage. Dihydrochalcones were extracted from the peel and flesh of five old apple varieties (Lještarka, Ljetna rebrača, Slavonska srčika, Zimnjara, Adamova zvijezda) and from wild apples (crabapples). Reverse-phase high performance liquid chromatography was used to identify dihydrochalcones (phloretin-2’-xyloglucoside and phloretin-2’-glucoside in the flesh; phloretin-2’- glucoside in the peel) and to express their percentage in relation to all other polyphenol subclasses. Dihydrochalcones occupied from 3 to 12.4% in the flesh and from 0.8 to 10.8% in the peel. Slavonska srčika and Adamova zvijezda showed higher dihydrochalcone percent in the flesh and peel. Principal component analysis also confirmed that Slavonska srčika and Adamova zvijezda differed from other varieties by higher dihydrochalcone percent. The pattern in the dihydrochalcone distribution was not shown. Although dihydrochalcones do not occupy a high portion in the polyphenol content, they are still an important feature of old varieties

ELECTROCHEMICAL DETECTION OF VITAMIN C IN REAL SAMPLES

Electrochemical properties of vitamin C (also known as L-ascorbic acid) have been studied by cyclic and differential pulse voltammetry in the model systems in order to develop simple and suitable method for vitamin C detection in real samples. The results indicated that vitamin C oxidation is a quasi-reversible and diffusion-controlled process, as well that the oxidation product of ascorbic acid, dehydroascorbic acid is adsorbed on the glassy carbon electrode surface. Calibration curve was constructed and the linear response was obtained in a concentration range from 0.0025 mol dm-3 to 1.0 mol dm-3. Vitamin C was successfully determined in real samples (fruit juices and food supplement) with cyclic voltammetry within concentration range from 0.034 mol dm-3 to 0.340 mol dm-3. In addition, antioxidant activity of vitamin C in real samples was determined using DPPH assay with a good linear correlation obtained between the cyclic voltammetry results and the results evaluated by DPPH assay of the samples

Green Approach in Synthesis of Bio-Inspired Materials

In this Special Issue, we focus on biomineralization/pathological biomineralization systems and the synthesis of bioinspired materials. This Special Issue promotes the current trend of “green chemistry”, and, as such, in all published papers, only aqueous solutions and eco-friendly additives were used for the production of well-characterized bioinspired materials. This Special Issue contains four papers: three original research papers and one review. Here, a brief introduction of these papers is provided

Electrochemical and Antioxidant Properties of (+)-Catechin, Quercetin and Rutin

The electrochemical properties of three structurally different flavonoids ((+)-catechin, quercetin and rutin) on a glassy carbon electrode were studied at different pH by cyclic, differential pulse and square-wave voltammetry. It was determined that in all investigated compounds oxidation of catechol 3\u27,4\u27-dihydroxy group on the B-ring (the first oxidation peak) occurs. This process is reversible, pH dependent and includes the transfer of 2e− and 2H+. The products of electrochemical oxidation of all investigated flavonoids (especially of quercetin) strongly adsorb on the electrode surface. In terms of quantification, the absorbance value, proportional to the remaining ABTS+• concentration, is measured after a fixed reaction time. Results are expressed as Trolox equivalents (TEAC value), that is, as the concentration of Trolox solution (mmol dm−3) with an antioxidant capacity equivalent to that found for 1.0 mmol dm−3 of the substance under investigation. It was found that the TEAC values depend on the concentration of the investigated compounds. The activity sequence of the investigated, structurally different, flavonoids follows the sequence: quercetin > (+)-catechin > rutin

Kinetika inhibicije DPPH radikala i antiradikalna aktivnost polifenola iz plodova aronije i bazge

Chokeberry and elderberry fruits are rich sources of polyphenols, the compounds with significant antiradical and chelating properties. They are studied intensively because of beneficial effects on human health. In this study, three fractions enriched with different classes of polyphenols were isolated from chokeberry and elderberry to study their antiradical activity by DPPH test. The first fraction contained flavonols and phenolic acids, the second anthocyanins, and third proanthocyanidins. The polyphenol content was determined by reversed-phase high performance liquid chromatography. Anthocyanins were found to be the main polyphenolic components of these berries with the highest portion in total antiradical activity, followed by proanthocyanidins and phenolic acids and flavonols. Furthermore, proanthocyanidins were found to be more effective in free DPPH• radical scavenging than flavonols, phenolic acids and anthocyanins. A biphasic reaction was observed in reaction between DPPH• radicals and polyphenols, at “fast” and “slow” scavenging rate. All classes of polyphenols showed strong antiradical activity in the first fast period, while in the slow period antiradical activity of polyphenols decreased. This kind of biphasic reaction could be important in biological activity of chokeberry and elderberry. This study is a contribution to a better understanding of antiradical activity of polyphenols from chokeberry and elderberry.Plodovi aronije i bazge bogat su izvor polifenola, spojeva sa značajnim antiradikalnim osobinama i osobinama stvaranja helata. Istražuju se intenzivno zbog pozitivnog utjecaja na ljudsko zdravlje. U ovom istraživanju izolirane su tri frakcije iz aronije i bazge obogaćene različitim skupinama polifenola da bi se istražila njihova antiradikalna aktivnost pomoću DPPH testa. Prva frakcija sadržavala je flavonole i fenolne kiseline, druga antocijanine, a treća proantocijanidine. Količina polifenola određena je reverzno-faznom tekućinskom kromatografijom visoke djelotvornosti. Pronađeno je da su antocijanini glavne polifenolne komponente ovog voća s najvećim udjelom u ukupnoj antiradikalnoj aktivnosti, a slijede ih proantocijanidini te fenolne kiseline i flavonoli. Pokazalo se također da su proantocijanidini efektivniji u hvatanju slobodnih DPPH• radikala od flavonola, fenolnih kiselina i antocijanidina. Opažena je bifazna reakcija u reakciji između DPPH• radikala i polifenola sa „brzim” i „sporim” razdobljem hvatanja. Sve skupine polifenola pokazale su snažnu antiradikalnu aktivnost za vrijeme prvog brzog perioda, dok je antiradikalna aktivnost opadala za vrijeme sporog razdoblja. Ovakva bifazna reakcija mogla bi biti važna u biološkom djelovanju aronije i bazge. Ovo istraživanje predstavlja doprinos boljem razumijevanju antiradikalne aktivnosti polifenola iz plodova aronije i bazge

Electrochemical and Spectrophotometric Determination of Total Polyphenol Content in Croatian Apple Varieties

Oxidation mechanism of quercetin in three different electrolytes (KCl, NaCl and LiCl) was studied by cyclic and differential pulse voltammetry. The results have shown that quercetin oxidation is reversible and diffusion controlled process in all three investigated electrolytes. Calibration diagram of quercetin was obtained in KCl, so this electrolyte was selected for total polyphenol content determination in apple peel extracts. Total polyphenol content was analysed by differential pulse voltammetry (dpv) and spectrophotometric Folin-Ciocalteu method (FC) in several ancient Croatian apple varieties and a wild apple variety grown in Croatia by using quercetin as a standard. Very high correlation for TP content obtained by two methods was observed since the TPdpv/TPFC ratio was around 1 and a correlation coefficient (R2) for TPdpv vs. TPFC was 0.98. The highest total polyphenol content was found in Slavonska srčika variety. This work is licensed under a Creative Commons Attribution 4.0 International License

Antioxidant activity of polyphenols from blueberry and straweberry

Da bi se istražila antioksidacijska aktivnost polifenola borovnice i jagode, ekstrahirane su dvije frakcije polifenola iz ovog voća. Jedna je sadržavala flavonole i fenolne kiseline, a druga antocijanine. Antioksidacijska aktivnost polifenola određena je DPPH testom, a količina polifenola HPLC metodom. Pokazalo se da se u borovnici nalaze visoke količine derivata kafeinske kiseline i kvercetina te antocijanina dok je u jagodi sadržaj polifenola manji. U obje vrste voća je doprinos antocijanina antioksidacijskoj aktivnosti veći nego doprinos flavonola i fenolnih kiselina. Antioksidacijska aktivnost antocijanina borovnice 19 puta je jača u usporedbi s antioksidacijskom aktivnošću flavonola i fenolnih kiselina borovnice. Antocijanini jagode 14 puta su jači antioksidansi nego flavonoli i fenolne kiseline jagode. Rezultati ovog rada pružaju bolji uvid u antioksidacijsko djelovanje borovnice i jagode.In order to investigate antioxidant activity of polyphenols from blueberries and strawberries, two fractions of polyphenols were extracted from these fruits. Antioxidant activity of polyphenols was determined by DPPH test, and the polyphenols content by HPLC method. It was shown that blueberries contained high amount of caffeic acid derivatives, quercetin derivatives and anthocyanins, whereas the amount of polyphenols in strawberries was lower. In both berry species, the contribution of anthocyanins to antioxidant activity was higher than the contribution of flavonols and phenolic acids. Antioxidant activity of blueberry anthocyanins was 19 times higher than antioxidant activity of flavonols and phenolic acids in blueberries. Strawberry anthocyanins were 14 times stronger antioxidants than flavonols and phenolic acids from strawberry. The results of this study provide a better insight into antioxidant activity of blueberries and strawberries

A study of the electrochemical reactivity of titanium under cathodic polarization by means of combined feedback and redox competition modes of scanning electrochemical microscopy

Sensors and Actuators B. Chemical, 320 (2020) 128339. https://doi.org/10.1016/j.snb.2020.128339.The effect of cathodic polarization on the electrochemical behavior of the thin titanium dioxide film formed by anodic pretreatment over pure commercial titanium metal for biomaterial application was investigated in situ using scanning electrochemical microscopy (SECM). Quantitative information on the electron transfer rates (keff) at the titanium surface was obtained using the feedback operation of SECM using ferrocene-methanol (FcMeOH) as electrochemical mediator. An increase of keff values with the increase of the negative polarization was detected, a feature that correlates well with the decrease of titanium oxide resistance with increasing cathodic polarization observed using electrochemical impedance spectroscopy (EIS). In addition, SECM operation in the redox competition mode proved that hydrogen was absorbed in the surface oxide film leading to changes in conductivity and electrochemical reactivit