Fundamentals of ATR-FTIR Spectroscopy and Its Role …
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4.1 Probing Biomolecular Self-Assembly Process
The existence of all bio-systems has relied primarily upon their evolution and
adaptation to the changing environment by forming intricately complex architectures through the self-assembly process. Biomolecules like, amino acids, peptides,
proteins, lipids, nucleic acids, etc. arrange and consolidate into higher-order assemblies involving cellular organelles, membranes, muscles, bones, and organs [35–37].
They may also accumulate in a disordered random fashion to form aggregated-like
structures within the biological system which are responsible for various diseases
for example, the formation of amyloid aggregates from amino acids, peptides, or
proteins, causing several degenerative disorders (like Parkinson’s disease) [38–40].
Inspired by nature’s ingenious process, self-assembly has been thoroughly employed
in the field of building energy devices, remediation technologies, designing model
biomaterials, nanofabrication, sensing, imaging, etc. [36, 37, 39, 41]. The formation of higher-dimensional structures due to the self-assembly process ensues from
the molecular-level interactions including coordination bonds, stacking interactions,
hydrogen bonding, electrostatic, and hydrophobic forces [35–37]. These molecular level interactions are predominantly governing the morphology of the selfassembly process. The surrounding conditions further influence their organizing
strength, which involves concentration, solution pH, temperature, ionic strength,
solvent composition, additives, etc. [36, 42].
In the recent past, a lot of research activities have been focused towards understanding the fundamentals of biomolecular self-assembly process and its relevance
for design and fabrication of various nanotechnological platforms [36, 37, 39, 43–
46]. Here, we are covering some of the selected studies focused in the direction
of amino acid and peptide self-assembly process using the ATR-FTIR spectroscopic
technique. Valery et al. [47] characterized the pH-dependent conformational changes
of the decapeptide self-assembly process. It is found that the switching of a decapeptide (Triptorelin) from a globular conformation at pH > 7.5 to an extended state below
pH 6.5. It is supported by the electron micrograph studies shown in panel a of Fig. 9.
Triptorelin decapeptide used for the study is pE 1 -H 2 -W 3 -S 4 -Y 5 -(D)W 6 -L 7 -R 8 -P 9 -
G 10 -NH 2 with three aromatic (Y 5 , W 3 , W 6 ) and three ionizing moieties (R 8 and Y 5
with pKa > 10, H 2 pKa 6.1). The studies were carried out in the pH range from 5 to 8.
The protonation state of histidine (-H 2 -) with the mentioned pH range was probed by
ATR-FTIR spectroscopy. From the spectra shown in panel b of Fig. 9, the absorption
peak at ~1097 cm
−1 depicted the positively charge imidazolium moiety of –H 2 – at
low pH of 5.5, while it deprotonates at higher pH of 8.5 with a peak absorbance at
~1106 cm
−1 . The ATR spectra in the amide I region in panel c of Fig. 9 shows the
formation of β-sheet structures through hydrogen bonding at lower pH state, which
gets weaker at high pH value. The observations from ATR-FTIR spectroscopy indicated that the conformational fate of the triptorelin decapeptide is primarily subjected
to a single –H 2 – protonation. This mechanistic study pin-pointed the viability of such
pH-dependent peptide as a molecular switch for designing responsive nanomaterials.
19
4.1 Probing Biomolecular Self-Assembly Process
The existence of all bio-systems has relied primarily upon their evolution and
adaptation to the changing environment by forming intricately complex architectures through the self-assembly process. Biomolecules like, amino acids, peptides,
proteins, lipids, nucleic acids, etc. arrange and consolidate into higher-order assemblies involving cellular organelles, membranes, muscles, bones, and organs [35–37].
They may also accumulate in a disordered random fashion to form aggregated-like
structures within the biological system which are responsible for various diseases
for example, the formation of amyloid aggregates from amino acids, peptides, or
proteins, causing several degenerative disorders (like Parkinson’s disease) [38–40].
Inspired by nature’s ingenious process, self-assembly has been thoroughly employed
in the field of building energy devices, remediation technologies, designing model
biomaterials, nanofabrication, sensing, imaging, etc. [36, 37, 39, 41]. The formation of higher-dimensional structures due to the self-assembly process ensues from
the molecular-level interactions including coordination bonds, stacking interactions,
hydrogen bonding, electrostatic, and hydrophobic forces [35–37]. These molecular level interactions are predominantly governing the morphology of the selfassembly process. The surrounding conditions further influence their organizing
strength, which involves concentration, solution pH, temperature, ionic strength,
solvent composition, additives, etc. [36, 42].
In the recent past, a lot of research activities have been focused towards understanding the fundamentals of biomolecular self-assembly process and its relevance
for design and fabrication of various nanotechnological platforms [36, 37, 39, 43–
46]. Here, we are covering some of the selected studies focused in the direction
of amino acid and peptide self-assembly process using the ATR-FTIR spectroscopic
technique. Valery et al. [47] characterized the pH-dependent conformational changes
of the decapeptide self-assembly process. It is found that the switching of a decapeptide (Triptorelin) from a globular conformation at pH > 7.5 to an extended state below
pH 6.5. It is supported by the electron micrograph studies shown in panel a of Fig. 9.
Triptorelin decapeptide used for the study is pE 1 -H 2 -W 3 -S 4 -Y 5 -(D)W 6 -L 7 -R 8 -P 9 -
G 10 -NH 2 with three aromatic (Y 5 , W 3 , W 6 ) and three ionizing moieties (R 8 and Y 5
with pKa > 10, H 2 pKa 6.1). The studies were carried out in the pH range from 5 to 8.
The protonation state of histidine (-H 2 -) with the mentioned pH range was probed by
ATR-FTIR spectroscopy. From the spectra shown in panel b of Fig. 9, the absorption
peak at ~1097 cm
−1 depicted the positively charge imidazolium moiety of –H 2 – at
low pH of 5.5, while it deprotonates at higher pH of 8.5 with a peak absorbance at
~1106 cm
−1 . The ATR spectra in the amide I region in panel c of Fig. 9 shows the
formation of β-sheet structures through hydrogen bonding at lower pH state, which
gets weaker at high pH value. The observations from ATR-FTIR spectroscopy indicated that the conformational fate of the triptorelin decapeptide is primarily subjected
to a single –H 2 – protonation. This mechanistic study pin-pointed the viability of such
pH-dependent peptide as a molecular switch for designing responsive nanomaterials.