The Spectra of Lidocaine

Lidocaine drug is topically applied to relive pain, burning and itching from skin inflammations. This local anesthetic and antiarrhythmic drug is also injected as a local anesthetic or dental anesthetic for minor surgery. It is also available as oral gels for tooth pain relief. Figure 1 shows the 90MHz proton spectrum for a solution of lidocaine in CDCl₃, resolving all of the resonances. The ¹H-¹H proton coupling is observed only for the ethyl group.

Figure 1. Proton spectrum of lidocaine in CDCl₃

¹³C Spectra of Lidocaine

Figure 2 presents two ¹³C spectra of lidocaine solution in CDCl₃ (top) and H₂O (bottom). The acquisition time was roughly 2min for each spectrum, resolving all of the peaks. The top spectrum clearly shows even the solvent triplet at 77ppm. Most of the peaks except the peaks related to the benzene ring in the two spectra are similar.

Figure 2. C13 spectra of lidocaine in CDCl₃ (top) and H₂O (bottom)

The impact of hydrogen bonding can be observed in the bottom spectrum. Although the evidence is ambiguous, the peaks can be assigned as follows: C_2,3:136.3ppm, C₁:132.9ppm, C_4,5,6:128.9ppm. The assignment is based on hydrogen bond theory due to deshielding caused by hydrogen bonding of H₂O to the carbonyl and NH (9).

Distortionless Enhancement via Polarization Transfer (DEPT)

The ¹³C DEPT experiment is the ideal polarization transfer sequence for spectral editing. An EFT spectrometer can differentiate between quaternary, methyne, methylene and methyl groups in a number of compounds utilizing a set of three DEPT experiments. The DEPT45, DEPT90 and DEPT135 spectra are acquired as a single data array by the 13C DEPT experiment used in the EFT spectrometers. The NUTS software program automatically processes the acquired spectra. Single experiments are also available such as the DEPT135 and APT (Attached Proton Test).

Figure 3 shows the C13 DEPT spectra of lidocaine in CDCl₃, confirming the peak assignments made in the aforementioned C13 spectra. Signals are generated from carbons without a directly bonded proton in all three experiments, leading to the quenching of the carbonyl and the substituted sites on the benzene ring. The two peaks in the proximity of 130ppm are corroborated to be the CH groups. The two resonances near 50-60ppm are pointing down in the DEPT135, representing the CH₂ groups. The peaks on the far right represent the CH₃ groups.

Figure 3. C13 DEPT spectra of lidocaine in CDCl₃

Heteronuclear Correlation (HETCOR)

Unlike the COSY experiment, the Heteronuclear Correlation (HETCOR) experiment focuses on coupling between two different nuclei types. Using the HETCOR experiment, proton resonances are correlated with those of directly attached carbon-13 or other nuclei. The proton is represented by the axes of the contour plot.

Other X-nucleus chemical shift ranges and signals take place at coordinates relative to the shifts of the bonded pairs of nuclei. This experiment detects X-nucleus signals rather than those of the protons. Long range coupling information (couplings between X-nucleus and proton through 2-3 bonds) can be obtained by modifying the delays in HETCOR.

The superimposed HETCOR spectra for lidocaine dissolved in H₂O (green) and CDCl₃ (maroon) are presented in Figure 4, showing the variation in the two spectra that was evident in the two C13 spectra.

The results of the HETCOR further corroborate the assignments made in the C13 spectrum of a lidocaine solution in H₂O. The resonances at 136.3ppm and 132.9ppm reveal the absence of directly bonded protons related to those two peaks.

Figure 4. C13-H1 HETCOR of lidocaine in CDCl₃ (maroon) and H₂O (green)

About Anasazi Instruments

Anasazi Instruments has been providing high quality, rugged, easy-to-use 60 and 90 MHz NMR spectrometers and upgrades to the educational and industrial markets. These instruments have been successfully implemented at hundreds or institutions ranging from large companies and top-tier universities to community colleges throughout North and South America.

In research environments, the Eft is a cost-effective workhorse for synthetic and analytical laboratories. These permanent magnet based FT-NMR spectrometers have applications in industrial labs for quality testing or as a "walk-up" NMR resource. Crucial to the success of the Eft is that, over the lifetime of the instrument, the total annual cost is fixed, whereas for a supercon-based NMR, annual costs increase.

In education, the Eft gives thousands of undergraduates the hands-on opportunity to learn to acquire and analyze FT-NMR data. Additionally, the wide appeal of the Eft spectrometer is due to the ease of obtaining high quality NMR spectra on an instrument that does not required cryogens and has minimal maintenance requirements.

This information has been sourced, reviewed and adapted from materials provided by Anasazi Instruments.

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