Potential Dependence of the Conformations of Nicotinamide Adenine Dinucleotide on Gold Electrode Determined by FT−Near-IR−SERS

Abstract

High-quality Fourier transform surface-enhanced Raman scattering (FT−SERS) of a dilute solution of nicotinamide adenine dinucleotide (NAD) was recorded on a roughened Au electrode. It was found that the detection limit could be lower as much as 3 orders of magnitude by using a near-infrared laser as the exciting source and an in-situ roughening technique. The SERS of NAD shows a strong potential dependence in the non-Faradaic regions. In regions of positive electrode potential, only the bands responsible for the adenine and nicotinamide moieties can be observed. In contrast, with a negative shift of the potential, several additional strong bands representing the ribose and phosphate moieties are also evident. Based upon the SERS behaviors of NAD in the regions of different potential, an adsorption mechanism for dilute NAD on the gold electrode was proposed. With this mechanism, the stacked NAD molecule is considered to be opened to some extent on an electric charged electrode. Specifically, under sufficiently negative potential, the NAD molecule appears to exist in a well-extended state on the gold electrode, leading to the tight adsorption of the entire NAD molecule on the electrode.

Structural features of the adenosine conjugate in means of vibrational spectroscopy and DFT

“…The appearance of this mode suggests that ribose is in intimate contact with the metal surface through the OH groups, and rather weak interactions between the pentose ring and the surface that has been previously reported [20][21][22][23]. It is the most likely that other bands arising from ν(OH) of ribose are hindered by the broad band at around 3300 cm , which are attributed chiefly to the bending modes of COH and CCH, and the stretching modes of CC and CO bonds according to our calculations (see Table 2).…”

“…Until now, vibrational interpretation of this class of compounds has been carried out by using characteristic frequency groups only [13][14][15]. Also the possible mechanism of adsorption of all the compounds studied here on different silver and gold surfaces were proposed [13,[16][17][18][19][20][21][22][23][24][25].…”

“…The allocation of the SERS bands to the normal vibrations was done referring to DFT calculations mentioned-above. Also, previously reported assignments of SERS bands reported for adenine [16][17][18], adenosine [19][20][21][22], D-ribose [23] and benzo-5-crown ether [13,24,25] . Below 1150 cm -1 there is a number of very weakly enhanced bands.…”

“…All In addition, it has been previously suggested [16][17][18][19][20][21][22] that the SERS band at ca. 1330 cm -1 was attributed to the stretching vibration of the C 5 -N 7 bond.…”

“…Also, all the SER spectra of adenine and adenosine exhibit a strong band near 730 cm -1 , which has been attributed to the "breathing" mode of the adenine ring. This vibration is much stronger enhanced for vertical orientation of adenine than for flat (parallel) position of this ring [16][17][18][19][20][21][22]. Since, as shown in Fig.…”