Molecular Mobility in Seed Tissues Studied by Spin Label ESR

1

• Molecular Mobility in Seed Tissues Studied by
  Spin Label ESR

Marcus A. Hemminga
2
Research Team Wageningen
Julia Buitink Mireille Claessen Ivon van den
Dries Folkert Hoekstra P. Adrie de
Jager Sponsored by the Netherlands Technology
Foundation
3
ESR Spectroscopy

• Conventional ESR and ST-ESR
• Using spin probes
• Rotational mobility range from 10-11 to 104 s
• Note 104 s is about 2 hr!
• Further information about
• Molecular packing
• Hydrogen-bonded network in the matrix

4
Motional Ranges in ESR
10-6
10-9
10-12
?c (s)


ESR
novel integral method
10-6
10-3
?c (s)
100
ST-ESR
5
ST-ESR Spectra
TEMPOL in glycerol
Absolute spectral intensity plotted
3 mT
magnetic field (mT)
6
Sugar-Water Samples
Rotational correlation time (s)
150
200
250
300
350
Temperature (K)
7
Motion at Glass Transition
-1
10
-2
10
more mobility
concentrated glasses (10-30 wt water)
glucose 0 maltose
tr(Tg)
-3
10
-4
10
-5
10
180
200
220
240
260
280
Tg (K)
8
Conclusions

• Spin probe ESR is a strong tool to obtain
  information in glassy materials about molecular
  motion over an enormous motional range?
  Application to seed cytoplasm will yield similar
  motional results

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Motivation

• To maintain genetic resources, storage stability
  of germplasm has to be predicted under low water
  content and temperature conditions
• Characterisation of molecular mobility in seeds
  kinetics of seed ageing
• Polar spin probe 3-carboxy-proxyl (CP)
  incorporated into seed cytoplasm ST-ESR

10
Food Science ? Biology

• Solid Foods
• shelf life
• low water content
• amorphous state
• high viscosity state
• low molecular mobility
• Control Properties
• technology
• consumers (crispy chips!)

• Seeds
• longevity
• reduced water content
• avoid crystalline state
• high viscosity state
• low molecular mobility

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Polar ESR Spin Probe CP
O
N
COOH
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ST-ESR Spectra of CP
Pea seeds at 0.09 g water/g dry weight CP
outside cells broadened by ferricyanide
-40C
-20C
0C
20C
30C
L"
L
1 mT
13
Spin Probe Motion
1000
0.07 g/g dw
0.12 g/g dw
7.5
9.7
100
Rotational correlation time (µs)
CP in pea embryonic axes
25.5
Tg
Ea(kJ/mol)
26.0
10
2.4
2.8
3.2
3.6
4.0
4.4
4.8
1000/T (K)
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Conclusions

• Below Tg activation energies are lower ? motion
  changes at a slower rate
• Chemical processes will be slowed down

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WLF Equation

• For model glasses and polymers we have the
  Williams-Landel-Ferry Equation
• log(tR/tR at Tg) -C1(T - Tg)/(C2 (T - Tg)
• C1 and C2 are the so-called universal constants
• Tg is the glass transition

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WLF Plots
C1 3.4 and C2 150.0
C1 17.4 and C2 51.6
0.0
0
pea axes
pea axes
-4
-0.4
log(tR/tR at Tg)
-8
-0.8
glycerol
-12
0
20
40
60
80
0
20
40
60
80
T-Tg
T-Tg
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Conclusions

• Melting of the cytoplasmic glass does not lead to
  such a large increase in motion as found for
  other glass forming materials
• The complex composition of the intracellular
  glass may be responsible for the moderate
  increase in mobility upon melting the glass

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Motion and Ageing Rates
100
80
Pea seeds at 0.07 g water/g dry weight Ageing
rates derived from Vertucci et al., 1994.
Ageing rate (days-1)
60
Rotational correlation time (µs)
40
20
-40
-20
0
20
40
60
80
100
Temperature (C)
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Motion Ageing Rates
pea seeds at different temperatures
Ageing rate (days-1)
Rotational correlation time (µs)
20
Conclusions

• There is a linear relationship between rotational
  motion and ageing rate
• This relationship may be of relevance for seed
  storage in gene banks