Molecular Level Characterization of InorganicBioorganic Interactions and Interfaces by Solid State NMR: Amino Acids on Silica Surface

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Molecular Level Characterization of Inorganic

Bioorganic Interactions and Interfaces by Solid State
NMR: Amino Acids on Silica Surface


Ira Ben Shir
, Shifi Kababya, Asher Schmidt


Schulich Faculty of Chemistry and Russell Berrie Nanotechnology Institute,

Technion
-

Israel Institute of Technology, Haifa, Israel


The molecular interface between bioorganics and inorganics plays a key role in diverse scientific
and technological research areas including nanoelectronics, biomimetics, biomineralization, and
medical applications such as drug delivery systems and implant coatings. Yet, the
physical/chemical basis of recognition of inorganic surfaces by biomolecules remains unclear. The
molecular level elucidation of specific interfacial interactions and the str
uctural and dynamical state
of the surface bound molecules is of prime scientific importance. Tailoring solid state NMR
techniques, we accomplish these goals for two amino acids
-

L
-
[1
-
13
C,
15
N]alanine and [1
-
13
C,
15
N]glycine, loaded on amorphous silica surf
ace, SBA
-
15 mesoporous silica with a high surface
area.


The surface interacting moiety of both alanine and glycine was identified as the

NH
3
+

functional
group by
15
N{
1
H}SLF NMR.
29
Si{
15
N} and
15
N{
29
Si}REDOR NMR revealed intermolecular
interactions betwee
n

NH
3
+

and 3
-
4 surface Si species, predominantly Q
3
, with similar internuclear
N∙∙∙Si distances of 4.0
-
4.2 Å. Distinct dynamic states of the adsorbed biomolecules were identified
by
15
N{
13
C}REDOR NMR, indicating both bound and free populations, depending
on hydration level
and temperature. In the bound populations the

NH
3
+

group is surface anchored while the free
carboxylate end undergoes librations, implying the carboxylate has small or no contributions to
surface binding. When surface water clusters gro
w bigger with increased hydration, the motional
amplitude of the carboxyl end amplifies, until onset of dissolution occurs.


Differently for glycine, when loaded from an unsaturated solution, surface
-
induced crystallization
occurred as a competitive proces
s to surface binding. Two crystalline polymorphs (α and β) of
glycine were identified by characteristic
13
C chemical shifts, which differs also from the chemical
shift of adsorbed glycine.
15
N{
13
C} and
15
N{
29
Si} REDOR NMR results substantiate their crystal
line
nature.


Our measurements provide the first
direct
, comprehensive, molecular
-
level identification of the
bioorganic
-
inorganic interface, showing binding functional groups, geometric constraints,
stoichiometry and dynamics, both for the adsorbed amino
acid and the silica surface.