Choline-amino acid ionic liquids: past and recent
achievements about the structure and properties of these really Bgreen^
Received: 13 March 2018 /Accepted: 8 April 2018 /Published online: 23 April 2018
International Union for Pure and Applied Biophysics (IUPAB) and Springer-Verlag GmbH Germany, part of Springer Nature 2018
The structure of choline-amino acid ionic liquids, atoxic task-specific solvents composed of materials originated from renewable
feedstocks, is reviewed in this letter. The varied and strong interactions that these liquids are capable of establishing are largely
dependent on their structure and confer them outstanding solvating properties with respect to a large number of different solutes.
Among the experimental methods capable of yielding structural insight, the energy-dispersive version of X-Ray diffraction, that
uses the Bremsstrahlung radiation of the X-Ray tube, is a technique very well suited to investigate these liquid systems. The
diffraction spectra of five choline-amino acid ionic liquids, recently measured, are reported and discussed; in particular, the
presence or absence of the medium-range order pre-peak is related to the presence of polar groups within the amino acid side
chain that destroys the hydrophobic interactions between aliphatic chains. In the final section, a recent example of choline-amino
acid ionic liquids as for ancient paper preservation and two other interesting results are discussed at the end.
Keywords Choline-amino acid ionic liquids
Since choline-amino acid ionic liquids (ChoAA-ILs) were in-
troduced in 2012 by Liu et al. (Liu et al. 2012), the interest in
these systems obtained from natural and renewable feedstocks
has increased significantly, as witnessed by the large number
of papers appeared in the last few years, and some new fields
of application have been opened. Indeed, though the use of
ChoAA-ILs in the field of biomass processing for which they
were originally developed—they possess the amazing proper-
ty of solubilizing lignin selectively from rice straw—has
steadily continued to grow (Hou et al. 2013b; Reddy 2015;
Wang et al. 2016;Toetal.2018), the powerful solubilization
properties of these liquids were extended to other systems, like
drugs (Alawi et al. 2015), to the extraction of noxious
substances in cigarette smoke (Zhang et al. 2016), and to
lignite and thermal coal pre-treatment (To et al. 2017). In
addition, these compounds have found use as lubricants (Mu
et al. 2015; Jiang et al. 2018; Zhang et al. 2018;Wuetal.
2018), as CO
capture agents (Zhang et al. 2013;
Bhattacharyya and Shah 2016; Saptal and Bhanage 2017),
and as antimicrobials (Foulet et al. 2016), in topical formula-
tions for drug delivery (De Almeida et al. 2017), and they
have shown to possess selective antiproliferative activity
against some breast cancer cell lines when complexed with
hemocyanins (Guncheva et al. 2015). Probably, the most strik-
ing feature of ChoAA-ILs lies in the fact that this variety of
properties goes along with their substantial harmlessness.
Indeed, they are considered not harmful for marine organism
(Baharuddin et al. 2016), as well as largely biodegradable
(Hou et al. 2013a; Yazdani et al. 2016; Foulet et al. 2016)
and atoxic (Zhang et al. 2018), though one study reported
toxicity with respect to some marine bacteria (Ventura et al.
2014). On the contrary, one of the greatest limits to their tech-
nological application is the very high viscosity of the liquid
phase (De Santis et al. 2015; Bhattacharyya and Shah 2016).
All these properties are deeply rooted in the various structural
motifs that ChoAA-ILs display.
This article is part of a Special Issue on BIonic Liquids and Biomolecules^
edited by Antonio Benedetto and Hans-Joachim Galla.
* Lorenzo Gontrani
Dipartimento di Chimica, Università di Roma BLa Sapienza^,
Piazzale Aldo Moro, 5, 00185 Rome, Italy
Biophysical Reviews (2018) 10:873–880