Molecular Motion in Glassy Systems

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• Molecular Motion in Glassy Systems

Marcus A. Hemminga
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Research Team Wageningen
Cor van den Berg Marieke van den Bosch Julia
Buitink Ivon J. van den Dries Dagmar van
Dusschoten Peter-Leon Hagedoorn Marcus A.
Hemminga P. Adrie de Jager Pieter C.M.M. Magusin
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Goals

• obtain insight in molecular details of model food
  systems
• scale 0.1 to 10 nm
• translate results to the supra-molecular state
  (mesoscopic state) co-operative motions?
• scale 10 nm to 1 ?m
• link mesoscopic state to macroscopic state
• scale 1 ?m to 1 mm

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Problems

• steps from molecular level to mesoscopic level
  not yet known
• determination of molecular motion and structure
  extremely complicated
• start experiments at molecular level
• 13C NMR of labelled sugar molecules
• spin probe ESR

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Magnetic Resonance

• Magnetic Resonance Spectroscopy
• NMR (nuclear magnetic resonance)
• observation of protons (1H) and carbon (13C)
  nuclei of water-carbohydrate samples
• information about structure and mobility on
  molecular scale
• ESR (electron spin resonance)
• observation of spin probes in water-carbohydrate
  samples
• information about molecular mobility

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1. Spin Probe ESR

• foreign spin probes needed
• high sensitivity easy to carry out
• information about molecular mobility
• indirect information about hydrogen-bonded
  network
• information about molecular packing

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2. Solid State 1H NMR

• assignment of NMR signals to protons in the
  molecules is difficult
• provides information about ratio of
  mobile/immobile protons
• second moment analysis of line shape gives
  information about local mobilities
• results can be related to the hydrogen-bonded
  network

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3. Solid State 13C NMR

• specific labelled molecules needed for assignment
• detailed information about molecular dynamics

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Research Aims

• elucidation of molecular mechanisms related to
  the glassy state
• changes in molecular structure and dynamics of
  the host and embedded biological softeners

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Materials and Conditions

• carbohydrates
• synthetic oligosaccharides
• mixed samples
• water content
• temperature

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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

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ESR Spin Probe TEMPOL
O
N
OH
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Motional Ranges in ESR
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ST-ESR Spectra
TEMPOL in glycerol
3 mT
magnetic field (mT)
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Sugar-Water Samples
80 wt maltoheptaose 80 wt glucose 20 wt
maltoheptaose 20 wt glucose
Rotational correlation time (s)
150
200
250
300
350
Temperature (K)
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Freeze-Concentrated Mixtures
10-1
10-2
concentrated glasses (20 wt water) glucose
(t) maltose (l) maltoheptaose ()

more mobility
tr(Tg)
10-3
l
t
10-4
10-5
200
210
220
230
240
250
260
270
Tg (K)
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Conclusions

• limit of molecular motion is 104 s (about 2 hr)
• at Tg spin probe mobility decreases with
  increasing water content, suggesting a better
  packing of the sugar-water system or stronger
  hydrogen bonds

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Maltose
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1H NMR on Maltose-Water
80 wt maltose
broad line - matrix and
immobile water sharp line - mobile water
broad line - matrix sharp line - mobile water
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Assignment of Proton Fractions
T
A (
immobile)
B (mobile)
ltT
maltose
H
O
H
O
g
2
2
T
maltose
H
O
g
2
gtT
maltose
H
O maltose
g
2
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T2 of Mobile Water
-3
10
wt water
more mobility
5 7 10 20
-4
T2 (ms)
10
-5
10
200
220
240
260
280
300
320
340
Temperature (K)
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Second Moment of Matrix
more proton packing less local motion
wt water
5 7 20
M2 (s -2)
reduced packing
deuterated by exchange
Temperature (K)
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Conclusions

• Upon increasing the water content of sugar
  glasses the effects at Tg are
• distance between sugar molecules increases
  leading to a higher water mobility
• better overall packing of water and sugar
  molecules in hydrogen-bonded network
• this explains the lower spin probe mobility

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C1-13C Glucose
O
position of 13C
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2D Exchange 13C NMR
reorientation
probe initial orientation
probe final orientation
90
90
90
t1
tm
t2
correlation between orientations over tm
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2D Exchange 13C NMR Spectra
anhydrous C1-13C glucose at 0 C
tm 30 s
tm 1 s
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Computer Simulation
1 jump per 10 ms average jump angle 3o

• Motional Model
• motion is described by
• correlation time lttcgt
• jump angle

lttau_cgt
1 jump per 10 ms average jump angle 12o
Tp (ms)
Simulation type jump probability
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Conclusions

• NMR exchange experiments yield detailed
  information on re-orientation processes of melts
  near Tg
• above Tg data are well-described by a limited
  random jump model
• average jump angle 12º
• jump every 30 ms at Tg 9 K
• lttcgt 2 s

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Outlook

• relate molecular information to macroscopic
  information (i.e. DMTA)
• develop general rules along which more complex
  systems as foods can be handled
• new information about suitable storage conditions
  and processing techniques in food science