Benchtop 1H NMR spectroscopy as a practical tool for characterization of chemically ex-changing systems in Ascorbic acid

Year 2021, , 35 - 47, 20.03.2021

Sevil Cikrikci

https://doi.org/10.28979/jarnas.890343

Abstract

Nuclear magnetic resonance (NMR) spectra and its action on characterization of pH dependent systems are among the most useful tools in spectroscopic techniques. Nevertheless, practical considerations often limit the application of High field NMR owing to its huge dimensions, expense and time-consuming property. On the other hand, benchtop devices could be another option with portable function and permanent magnet mechanism (1–2 T) but with several disadvantages such as reduced sensitivity and low resolution. In this study, proton exchange rates depending on pH of a selected molecule, were monitored by both 800 MHz High-Field and 60 MHz Bench-top H1 NMR to be compared. L-Ascorbic acid (AA) which is an important substrate for body and biological reac-tions was chosen as a target model molecule. This study supported the use of NMR for pH dependent chemical changes due to protonation. According to spectrum results and previous literature studies, shifting behaviour of Peak 1 (assigned to C4-H13) towards lower frequencies upon pH increase, gave us its feasibility for pKa determi-nation. While High-field NMR spectroscopy demonstrated the compositional analysis of AA and proton exchanges showing couplings between protons, Benchtop NMR was displayed as a limited tool owing to broadened lines of splitting peaks. Even so, shifting tendency of Peak 1 was still obtained quantitatively and promoted the feasibility of Benchtop NMR for characterization of a model molecule which was L-ascorbic acid. In brief, this study sup-ported the effect of magnetic field strength on characterization of pH dependent chemical exchange regimes for vitamins.

Keywords

ascorbic acid, benchtop NMR, pH, proton transfer

Thanks

S.C. would like to thank Postdoctoral Scholar Danila Barskiy for his help in acquiring Benchtop 1H NMR spectra and Dr. Jeff Pelton for his technical help in High Field NMR measurements. The author is also very thankful to Prof. Alexander Pines for these all laboratory facilities in UC Berkeley.

Farklı pH ortamlarında hazırlanmış L-Askorbik Asit çözeltilerinin taşınabilir 1H NMR spektroskopisi ile pratik analizi

Abstract

Nükleer manyetik rezonans (NMR) spektrumları ve bu spektrumların pH'a bağlı sistemlerin karakterizasyonunda kullanımı, spektroskopik tekniklerdeki kullanışlı araçlar arasındadır. Ancak, yüksek manyetik alanlı NMR aletleri, büyük boyutlu ve yüksek maliyetli oluşu gibi dezavantajlara sahiptir . Öte yandan, düşük manyetik alana sahip ancak taşınabilir NMR cihazları ise bu dezavantajları ortadan kaldırmakta ancak azaltılmış hassasiyet ve düşük çözünürlük gibi çeşitli kısıtlamalara sahip olmaktadır. Bu çalışmada, askorbik asit, vitamin çözeltisi hazırlamak amacıyla seçilmiş olup hazırlanan solüsyonların pH'ına bağlı proton değişim oranları, karşılaştırılmak üzere ürünler 800 MHz Yüksek Alan ve 60 MHz Masa Üstü (Benchtop) H1 NMR ile izlenmiştir. Vücut ve biyolojik reaksiyonlar için önemli bir substrat olan L-Askorbik asit (AA) hedef model molekül olarak seçilmiştir. Bu çalışma, pH'a bağlı proton transferi gibi kimyasal değişiklikler için günümüz NMR aletleri kullanımını desteklemiştir. Spektrum sonuçlarına ve önceki literatür çalışmalarına göre, Pik 1'in (C4-H13'e atanan) pH artışı ile daha düşük frekanslara kayma davranışı bize pKa tayini için NMR spektrumunun kullanımını doğrulamıştır. Yüksek alan NMR spektroskopisi, AA yapısı ve AA çözeltilerinde pH'a bağlı meydana gelen proton değişimlerinin incelenmesinde molekül karakterizasyonunu başarılı şekilde sağlarken, 60 MHz Benchtop NMR bu uygulamada sınırlı kalmıştır. Buna rağmen, pik 1'in düşük frekanslara kayma eğilimi hala kantitatif olarak elde edilmiştir. Sonuç olarak, bu çalışma, manyetik alan gücü bu çalışmadaki cihazdan daha yüksek, yeni jenerasyon Benchtop NMR aletlerinin benzer başka moleküllerde daha yüksek performansla ileriki çalışmalarda kullanımını desteklemiştir.

Keywords

Askorbik asit, masaüstü NMR, pH, proton transferi

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