Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University , Singapore 637371.
J Phys Chem B. 2013 Nov 7;117(44):13755-66. doi: 10.1021/jp4069619. Epub 2013 Oct 23.
The electrochemical reduction mechanisms of flavin mononucleotide (FMN) in buffered aqueous solutions at pH 3-11 and unbuffered aqueous solutions at pH 2-11 were examined in detail using variable-scan-rate cyclic voltammetry (ν = 0.1-20 V s(-1)), controlled-potential bulk electrolysis, UV-vis spectroscopy, and rotating-disk-electrode voltammetry. In buffered solutions at pH 3-5, FMN undergoes a two-electron/two-proton (2e(-)/2H(+)) reduction to form FMNH2 at all scan rates. When the buffered pH is increased to 7-9, FMN undergoes a 2e(-) reduction to form FMN(2-), which initially undergoes hydrogen bonding with water molecules, followed by protonation to form FMNH(-). At a low voltammetric scan rate of 0.1 V s(-1), the protonation reaction has sufficient time to take place. However, at a higher scan rate of 20 V s(-1), the proton-transfer reaction is outrun, and upon reversal of the scan direction, less of the FMNH(-) is available for oxidation, causing its oxidation peak to decrease in magnitude. In unbuffered aqueous solutions, three major voltammetric waves were observed in different pH ranges. At low pH in unbuffered solutions, where [H(+)] ≥ [FMN], (FMN)H(-) undergoes a 2e(-)/2H(+) reduction to form (FMNH2)H(-) (wave 1), similar to the mechanism in buffered aqueous solutions at low pH. At midrange pH values (unbuffered), where pH ≤ pKa of the phosphate group and [FMN] ≥ [H(+)], (FMN)H(-) undergoes a 2e(-) reduction to form (FMN(2-))H(-) (wave 2), similar to the mechanism in buffered aqueous solutions at high pH. At high pH (unbuffered), where pH ≥ pKa = 6.2 of the phosphate group, the phosphate group loses its second proton to be fully deprotonated, forming (FMN)(2-), and this species undergoes a 2e(-) reduction to form (FMN(2-))(2-) (wave 3).
在 pH 值为 3-11 的缓冲水溶液和 pH 值为 2-11 的未缓冲水溶液中，详细研究了黄素单核苷酸 (FMN) 的电化学还原机制。使用变扫描速率循环伏安法 (ν = 0.1-20 V s(-1))、恒电位批量电解、紫外可见光谱和旋转圆盘电极伏安法进行了研究。在 pH 值为 3-5 的缓冲溶液中，FMN 在所有扫描速率下经历两电子/两质子 (2e(-)/2H(+)) 还原生成 FMNH2。当缓冲 pH 值增加到 7-9 时，FMN 经历 2e(-)还原生成 FMN(2-)，其最初与水分子形成氢键，然后质子化形成 FMNH(-)。在低伏安扫描速率为 0.1 V s(-1)时，质子化反应有足够的时间进行。然而，在高扫描速率为 20 V s(-1)时，质子转移反应速度过快，在扫描方向反转时，可用于氧化的 FMNH(-)较少，导致其氧化峰幅度减小。在未缓冲的水溶液中，在不同的 pH 值范围内观察到三个主要的伏安波。在未缓冲溶液的低 pH 值下，其中 [H(+)] ≥ [FMN]，(FMN)H(-)经历两电子/两质子还原生成 (FMNH2)H(-)（波 1），类似于低 pH 值下缓冲水溶液中的机制。在中间 pH 值范围内（未缓冲），其中 pH 值 ≤ 磷酸基团的 pKa 值且 [FMN] ≥ [H(+)]，(FMN)H(-)经历两电子还原生成 (FMN(2-))H(-)（波 2），类似于高 pH 值下缓冲水溶液中的机制。在高 pH 值（未缓冲）下，其中 pH 值 ≥ 磷酸基团的 pKa = 6.2，磷酸基团失去第二个质子而完全去质子化，形成 (FMN)(2-)，该物种经历两电子还原生成 (FMN(2-))(2-)（波 3）。
