The Formation of Radical Anion of Vitamin K 1 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 11 3133 DOI 10.5012/bkcs.2010.31.11.3133 Electrochemical and Spectroelectrochemical Behaviors of Vitamin K 1 /Lipid Modified Electrodes and the Formation of Radical Anion in Aqueous Media † † JeeEun Yang, ‡ Jang-Hee Yoon, Mi-Sook Won, * and Yoon-Bo Shim ‡,* Busan Center, Korea Basic Science Institute, Busan 609-735, Korea. * E-mail: mswon@kbsi.re.kr ‡ Department of Chemistry, Pusan National University, Busan 609-735, Korea. * E-mail: ybshim@pusan.ac.kr Received July 28, 2010, Accepted September 17, 2010 The electrochemical properties of the liposoluble vitamin K1 adsorbed on bare and lipid coated glassy carbon electrodes (GCEs) were studied in unbuffered and well buffered aqueous media. The reduction products of vitamin K1 were charac- terized by employing cyclic voltammetry and the in situ UV-visible spectroelectrochemical technique. The radical species of vitamin K1 cannot be observed at the bare GCEs in well buffered media. The formation of the anion radical of vitamin K1 was observed in unbuffered solutions above pH 5.9 or at the lipid coated GCE in a well-buffered solution. UV-visible absorption bands of neutral vitamin K1 were observed at 260 nm and 330 nm, and a band corresponding to the anion radical species was observed at 450 nm. The derivative cyclic voltabsorptometric (DCVA) curves obtained for electrochemical reduction of vitamin K1 confirmed the presence of both neutral and anion radical species. The anion radical of vitamin K1 formed at the hydrophobic conditions with phosphatidylcholine (PC) lipid coated electrode was stable enough to be observed in the spectroelectrochemical experiments. Key Words: Vitamin K1, Electron transfer reaction, In situ UV-visible, Anion radical, Lipid Introduction Elucidating the electron transfer mechanism of redox pro- cesses of riboflavin, quinones, cytochrome and NADH 1-4 in the cell membrane is essential because they generated ATP by the electron transfer through the cell membrane. Of these, quinones are one of the most important and well-studied examples of orga- nic redox couples. 5-6 They are well-represented examples of the electron transfer processes of the biological species in the photo- synthetic reaction center and in mitochondrial ATP synthesis. 7 We have also studied the electrochemical behavior and anion radical detection of benzoquinone. 8-10 Vitamin K has the chemical structure based on 2-methyl-1,4- naphthoquinone derivatives with an aliphatic side chain in the 3-position and it is involved in cellular respiration and in oxida- tive phosphorylation 11 as electron carriers. In addition, vitamin K functions as a blood clotting cofactor 12 and participates in bone mass increase. 13 For these reasons, knowledge of the redox and the electron transport properties of vitamin K are important for a better understanding of their behavior in biological environ- ments. To observe electron transport properties of vitamin K, studies have been carried out employing polarography 14 and cyclic voltammetry 15-16 in aqueous and non aqueous media. 17-18 The electrochemical behavior of vitamin K has been investigat- ed using various modified electrodes, such as gold 19 and plati- num electrodes modified with cysteamine, cystamine 20 n-al- kanethiols, 21 mercury-coated carbon fiber microelectrode, 18 in- terdigitized array microelectrode, 22 etc. Few reports, however, have been reported for the electrochemical properties of vita- min K in lipid membrane systems. Although electrochemical behavior of vitamin K was investigated in various conditions, there is no report for the formation of anion radical as an inter- † This paper is dedicated to Professor Hasuck Kim for his outstanding contribution to electrochemistry and analytical chemistry. mediate in aqueous media by electrochemical techniques. In the present study, the electrochemical behavior of vita- min K 1 (VK 1 ) adsorbed on a bare and VK 1 /PC coated elec- trodes was investigated in unbuffered and well buffered aqueous media. The stability of vitamin K 1 anion radical as an inter- mediate species was studied in unbuffered aqueous media and at the lipid coated GCE in well-buffered solutions. The lipid used in this work, PC, had both ammonium and phosphoric acid as a head group. The effects of the pH and composition of the electrolyte solution on the redox reaction of VK1 was ev- aluated by employing cyclic voltammetry and in situ UV-visible spectroelectrochemical techniques. Experimental Reagents and solutions. Vitamin K1 (VK1) and phosphatidyl- choline (PC) were purchased from Sigma-Aldrich Co (USA) and Avanti Polar Lipids Inc, respectively. VK 1 /PC solutions were dissolved in a chloroform solution to an adequate con- centration. All aqueous solutions were prepared in doubly dis- tilled water obtained from a Milli-Q water-purifying system (18 MΩ cm). Well-buffered aqueous solutions in various pHs were prepared using the mixtures of HCl/potassium chloride and citric acid/NaH 2 PO 4 . The unbuffered aqueous solutions were prepared by adjusting a 0.1 M H2SO4 solution to the de- sired pH by adding proper amounts of a 0.1 M NaOH solution. Instruments. Before each experiment, the GC electrode was polished with 0.1 μm alumina powder to a mirror finish and rinsed with ethanol. The electrode potentials were measured with respect to an Ag/AgCl (sat’d KCl) electrode. The counter electrode was a Pt wire. The temperature of the solution was 25 o C. Cyclic voltammetry (CV) was performed using a poten- tiostat/galvanostat, Kosentech Model PT-2 (S. Korea). The in situ UV-visible spectra were obtained from the assembly of a