**1. Introduction**

Bioconjugated nanostructured materials resulting from the coupling of biomolecules with inorganic nanomaterials including nanotubes, nanowires, nanoparticles and nanosheets have attracted much attention during the last years as they exhibit unique features derived from combining synergistically the properties of the interacting components. These exclusive physico-chemical properties render these materials as suitable substrates with potential applications in diverse biological- [1,2] and material-related [3] areas such as biocatalysis,[4,5], drug delivery [6–8], biosensing [9–13] and medical diagnostics [14,15]. The functionalities resulting from these biohybrid materials are largely mediated by the biomolecule/inorganic surface interactions, which in turn are dictated by the structure-specific binding properties of the two partners. Accordingly, precise knowledge on the interactions between the biomolecule and the inorganic components is of fundamental relevance.

Among the different nanostructured materials, boron nitride nanotubes (BNNTs) have been proposed to be suitable candidates to be combined with biomolecules [16]. BNNTs are isosteres and structurally similar to carbon nanotubes (CNTs), in which alternating B and N atoms substitute for C atoms. However, these two type of nanotubes exhibit different physico-chemical properties. Whereas CNTs exhibit metallic or semiconducting behavior, which moreover is strongly dependent on the tube diameter, helicity and concentric layers, BNNTs are electrical insulators with a band gap of *ca*. 5.5 eV regardless of the tube geometry features [17]. Moreover, at variance with the non-polar C-C bonds in CNTs, the B-N bonds of BNNTs exhibit a certain polar character, the degree of which depends on the curvature of the nanotube. That is, the increase of the tube curvature induces the transformation of the sp2 hybrid character of the B and N atoms in large diameter BNNTs into a sp3 one in small diameter BNNTs. As recently shown by us [18], this has important consequences for the nature of interaction between functional molecules and the BNNTs walls; *i.e.*, polar molecules strongly chemisorb on small radius BNNTs, whereas interaction of non-polar molecules are energetically more favourable when physisorbed on large radius BNNTs. Furthermore, unlike CNTs, which present an inherent cytotoxicity [19], BNNTs have been found to be nontoxic [20] due to their high chemical and structural stability and high oxidation resistance, which alongside their uniformity and stability in dispersion in solution [21] make them suitable for biomedical applications.

Different experimental studies have focused on the interaction of peptides and proteins with BNNTs, showing a natural affinity between the two conjugates, which allows a direct immobilization of proteins on the BNNTs [22] as well as the isolation of individual BNNTs through a novel pathway based on peptide wrapping [23]. Moreover, biofunctionalized BNNTs via glycine interaction are good reactant substrates to obtain polysaccharide-coated BNNTs under mild conditions, in which the role of glycine is crucial during the interfacial process. The interactions of DNA and RNA with BNNTs have also been addressed and exploited to obtain nematic ordered ensembles of BNNT [24]. Other works have been devoted to assess the cytotoxicity of BNNTs when in contact with cells. Chen *et al*. [20] concluded that pristine BNNTs are inherently non-cytotoxic in view of the non-altered growth of human embryonic kidney (HEK) cells when cultured with BNNTs. Similar results were found by Ciofani and coworkers, in which coated-BNNTs presented a good cytocompatibility with human cells [25–28]. However, Goldberg and coworkers more recently found that BNNTs are actually cytotoxic for cells present in the lung alveoli and for HEK, in which the discrepancies with the other works were discussed and suggested to be due to the different morphology and size distribution of the BNNTs tested and the different assay techniques [29].

Theoretical works, mainly based on density functional theory (DFT) methods, have also studied the interaction of biomolecules with boron nitride nanostructures, most studies being limited to biomolecule building blocks, (amino acids and DNA and RNA nucleobases) due to the demanding computational cost of these calculations. Works on the gas-phase interaction of nucleobases using the local density approximation (LDA) and generalized gradient approximation (GGA) DFT levels of theory showed that this depends on the individual polarizations of the nucleobases [30–32]. The interaction of BNNTs with glycine (Gly) among other different amines has been studied in the gas-phase revealing an affinity of the BNNT with the NH2Gly group [33]. Study on the gas-phase interaction of the arginine (Arg), aspartic acid (Asp) and tryptophane (Trp) amino acids, with basic, acidic and aromatic side chain functionalities, respectively, at the LDA DFT level revealed that the binding is accompanied by charge transfer following the trend of Arg > Asp > Trp [34]. The binding of different biomolecules inside the cavity of BNNTs has also been studied at the LDA level [35].

The calculated weak interactions led the authors to suggest BNNTs to be suitable biological carriers due to the limited delivery kinetic barrier.

All these works focus on the intrinsic adsorption properties; *i.e.*, they are limited to the gas-phase and, accordingly, solvation effects were not accounted for. Moreover, each work addresses the interaction of biomolecules with a particular BNNT. Since it has been shown that the tubular radius can modulate the adsorption properties of BNNTs [18], which is also applicable for biomolecules, the nature of interaction can significantly be different depending on the radius of the BNNT. Moreover, for the particular works addressing the interaction of amino acids, no conformational exploration to find out the most stable amino acid/BNNT adduct was performed (*i.e.*, the initial amino acid conformation guesses were the most stable gas-phase structure), which is an important drawback due to the large conformational mobility of these molecules. In order to provide a more complete atomic-scale description of the interaction of amino acids with BNNTs, the present work reports a systematic periodic B3LYP-D2\* study, using a hybrid functional and including dispersion corrections, on the interaction of Gly with different zig-zag (*n*,0) single-walled BNNTs (*n* = 4, 6, 9 and 15) rendering nanotubes of different radius. Note that dispersive effects, not included in previous works, are expected to play a role in these systems. Moreover, with the aim to study in a more realistic way the interactions between biological systems and BNNTs, the very same Gly/BNNTs interaction study has been addressed considering a microsolvated environment modeled by the presence of seven water molecules. The effect of water has been analyzed from a structural and energetic point of view, with particular attention paid to the Gly/BNNT interface to determine whether the interaction is direct or bridged by the water molecules.

#### **2. Results and Discussion**
