Aplicação da espectroscopia de ressonância magnética nuclear para estudos de difusão molecular em líquidos: a técnica DOSY

Abstract

Recebido em 12/9/01; aceito em 22/2/02 USING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TO STUDY MOLECULAR DIFFUSION IN LIQUIDS: THE DOSY TECHNIQUE. Diffusion coefficients provide uniquely detailed and easily interpreted information on molecular organization and phase structure. They are quite sensitive to structural changes, and to binding and association phenomena, in particular for liquid colloidal or macromolecular systems. This paper describes the principles of diffusion measurements in liquids by pulsed magnetic field gradient spin-echo (PFG-SE) NMR spectroscopy. The important PFG-SE technique known as DOSY is presented and discussed. This is a noninvasive technique that can provide individual multicomponent translational diffusion coefficients with good precision in a few minutes, without the need for radioactive isotopic labelling.

Review: Cyclodextrin Inclusion Complexes Probed by NMR Techniques

“…However, the molecular diffusion does not allow G to keep totally homogeneous, decreasing the echoes amplitude in 2τ. The relation between the spin echoes and the diffusion coefficient D is given by equation 3.13 (Souza & Laverde, 2002). = exp − exp − Dτ (3.14)…”

“…Even though the application of this sequence and the improvement on the echo amplitude with the Stimulated Spin Echo (STE), also discussed by Hahn in 1950, the application of static magnetic field gradients (G) resulted in the necessity to use high values of G to measure small values of D, which led to a increase in the echo and, consequently, a decrease in the noise/signal ratio, causing a difficulty in the detection (Souza & Laverde, 2002).…”

“…The advantages of PFG application are (Souza & Laverde, 2002): a. Separation of the echo attenuation caused by the D effect due to T 2 , doing the experiment to a fix τ interval between the RF pulses and changing the area of the gradient pulse, b.…”

“…So, to try to minimize such effects, various pulse sequences were developed, each with advantages and disadvantages according to the characteristics of the sample components. The main examples of DOSY pulse sequences are: BPPSTE (Bipolar Pulser Pair STimulated Echo), BPPLED (Bipolar Pulse Pair Longitudinal Eddy currents Decay), GCSTE (Gradient Compensated STimulated Echo) e GCSTESL (Gradient Compensated Stimulated Echo Spin Lock) (Souza & Laverde, 2002).…”

Aplicação de técnicas cromatográficas e espectrométricas como ferramentas de auxílio na identificação de componentes de óleos voláteis

“…Por outro lado, a análise dos espectros de RMN obtidos de misturas complexas, torna-se limitada para identifi car todos os componentes, especialmente àqueles presentes em quantidades reduzidas. Do mesmo modo, a complexidade da composição dos óleos voláteis é um fator limitante para a utilização da técnica RMN-DOSY, usada para análise de misturas (Souza;Laverde, 2002 Navickiene et al, 2006;Nascimento et al, 2007). Ao contrário do observado para estudos fi toquímicos de extratos vegetais bioativos (Lago et al, 2004a;Moreira et al, 2007), poucos são os estudos que reportam qual(is) a(s) substância(s) responsável(is) pelas atividades em óleo voláteis.…”