20
H. Kaur et al.
Fig. 9 Self-assembled conformation of triptorelin decapeptide as a function of pH. a Electron
micrograph image of the small nanotubes at low pH (black arrow) coexisting with large nanotubes
(red arrow) formed at low and high pH respectively. b ATR-FTIR absorption spectra in amide
I region of small (green line, pH 5.5) and large (blue line, pH 8.5) nanotube structures. c ATRFTIR spectra depicting pH-dependent ionizing states of Histidine (H2) residue of the decapeptide.
Adapted with permission from [47]. Copyright, 2015, Macmillan Publishers Limited
Tomar et al. [26] carried out a pH-dependent ionizing study based on the l-Phe
self-assembly to identify the role of electrostatic interaction in the emergence of
varied aggregation states in the aqueous system with ATR-FTIR spectroscopic tool.
The ionization of l-Phe at 100 mM concentration was probed at pH values 1.5,
5.8, and 12.2 as depicted in Fig. 10. Pertaining to pH sensitivity, diamond crystal
assembly was utilized for spectral acquisition at extreme pH values. As noted from
the ATR-FTIR spectra in Fig. 10 panel a, l-Phe acquires a cationic state at pH 1.5 and
display vibrational modes of protonated amine groups at ~1560 and ~1643 cm
−1 and
a vibrational feature of a neutral form carboxylic acid (COOH) at ~1720 cm
−1 . At pH
value of 12.2 (Fig. 10, panel c), the anionic state of the amino acid displays neutral
amine bending mode β(NH 2 ) (~1355 cm
−1 ), and negatively charged carboxylate
groups as COO s
– and COO as
– modes positioned at ~1410 cm
−1 and ~1560 cm
−1 ,
respectively. The IR absorption spectra of l-Phe at a pH value of 5.8 shows the
zwitterionic form of the amino acid (Fig. 10, panel b). The ATR-FTIR spectra of
varying ionization states were compared with the SEM results observed for the
drop-casted samples which presented a distinct morphological verification for the
l-Phe sample at neutral pH (fibrillar state) and cationic and anionic pH values (flake
structure) (Fig. 10, panel d, e, f), correlating well with the ATR-FTIR results. It was
concluded that the inhibition of the fibrillar morphology of l-Phe beyond neutral pH
state arises due to inter-molecular electrostatic interactions that dominate over the
π–π stacking interactions among aromatic moieties.
From these studies, it could be summarized that the information about the chemical profile and interactive forces provided by the ATR-FTIR technique can directly
correlate with the morphological changes occurring during the self-assembly process.
H. Kaur et al.
Fig. 9 Self-assembled conformation of triptorelin decapeptide as a function of pH. a Electron
micrograph image of the small nanotubes at low pH (black arrow) coexisting with large nanotubes
(red arrow) formed at low and high pH respectively. b ATR-FTIR absorption spectra in amide
I region of small (green line, pH 5.5) and large (blue line, pH 8.5) nanotube structures. c ATRFTIR spectra depicting pH-dependent ionizing states of Histidine (H2) residue of the decapeptide.
Adapted with permission from [47]. Copyright, 2015, Macmillan Publishers Limited
Tomar et al. [26] carried out a pH-dependent ionizing study based on the l-Phe
self-assembly to identify the role of electrostatic interaction in the emergence of
varied aggregation states in the aqueous system with ATR-FTIR spectroscopic tool.
The ionization of l-Phe at 100 mM concentration was probed at pH values 1.5,
5.8, and 12.2 as depicted in Fig. 10. Pertaining to pH sensitivity, diamond crystal
assembly was utilized for spectral acquisition at extreme pH values. As noted from
the ATR-FTIR spectra in Fig. 10 panel a, l-Phe acquires a cationic state at pH 1.5 and
display vibrational modes of protonated amine groups at ~1560 and ~1643 cm
−1 and
a vibrational feature of a neutral form carboxylic acid (COOH) at ~1720 cm
−1 . At pH
value of 12.2 (Fig. 10, panel c), the anionic state of the amino acid displays neutral
amine bending mode β(NH 2 ) (~1355 cm
−1 ), and negatively charged carboxylate
groups as COO s
– and COO as
– modes positioned at ~1410 cm
−1 and ~1560 cm
−1 ,
respectively. The IR absorption spectra of l-Phe at a pH value of 5.8 shows the
zwitterionic form of the amino acid (Fig. 10, panel b). The ATR-FTIR spectra of
varying ionization states were compared with the SEM results observed for the
drop-casted samples which presented a distinct morphological verification for the
l-Phe sample at neutral pH (fibrillar state) and cationic and anionic pH values (flake
structure) (Fig. 10, panel d, e, f), correlating well with the ATR-FTIR results. It was
concluded that the inhibition of the fibrillar morphology of l-Phe beyond neutral pH
state arises due to inter-molecular electrostatic interactions that dominate over the
π–π stacking interactions among aromatic moieties.
From these studies, it could be summarized that the information about the chemical profile and interactive forces provided by the ATR-FTIR technique can directly
correlate with the morphological changes occurring during the self-assembly process.