LF-NMR relaxation
The spin-spin 1H LF NMR measurements were carried out
with a Maran bench‐top pulsed NMR analyzer (Resonance Instruments,
Witney, UK) with a permanent magnet and an 18 mm probe head operating at
23.4 MHz.
Before each measurement, the samples were stabilized at 40°C for 40 min
and then allowed to equilibrate inside the instrument for 5 min. The
spin–spin relaxation time constant (T2) was generated
using a Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence. The CPMG
sequence consists in applying a 90 degree radiofrequency pulse to the
sample, followed by many 180 degree pulses. Each time a 180 degree pulse
was applied, the signal decay of the magnetic field was removed and a
single data point was acquired (Carr and Purcell, 1954; Meiboom and
Gill, 1958).
For all the samples 32 scans were accumulated. The number of echoes
acquired was 16,384 with a recycle delay of 6 s and τ between 200
to 550 μs. Receiver gain (RG) and magnetic field were calibrated before
each measurement. The signal processing was based on the PDCO inverse
Laplace transform optimization algorithm
with a 2 = 0.5 (Berman et al., 2013b;
Campisi-Pinto et al., 2018)
The self-diffusion measurements were carried out with a 20 MHz minispec
bench-top pulsed NMR analyzer (Bruker Analytic GmbH, Germany), equipped
with a permanent magnet, and a 10-mm temperature controlled probe head
according to Meiri et al. (2015). Prior to each measurement, the samples
were stabilized at 40°C for 40 min and then allowed to equilibrate
inside the instrument for 5 min.
The self-diffusion coefficient, D, was determined by the pulsed-field
gradient spin echo (PFGSE) method (Stejskal and Tanner, 1965). The pulse
sequence was used with 16 scans, τ of 7.5 ms, and a recycle delay
of 6 s. Typical gradient parameters were Δ of 7.5 ms, δ of
0.5 ms, time between the 90° pulse to the first gradient pulse of 1 ms,
and G of 1.6 T/m. Each reported value of the self-diffusion
coefficient (D) is the average of a minimum of ten measurements.