In this study, chemical transformations of benzyl ester of О-(phenyl-2-acetamido-2,3-dideoxy-1-thio-β-d-glucopyranoside-3-yl)-d-lactoyl-l-alanyl-d-isoglutamine (SPhMDPOBn) on the fumed silica surface were examined, and the surface complex structure was characterized by temperature-programmed desorption mass spectrometry (TPD-MS), infrared spectroscopy (FTIR) and electrospray ion trap mass spectrometry (ES IT MS). Stages of pyrolysis of SPhMDPOBn in pristine state and on the silica surface have been determined. Probably, hydrogen-bonded complex forms between silanol surface groups and the C = O group of the acetamide moiety NH-(CH3)-C = O…H-O-Si≡. The thermal transformations of such hydrogen-bonded complex result in pyrolysis of SPhMDPOBn immobilized on the silica surface under TPD-MS conditions. The shifts ∆ν of amide I band (measured from 1,626 to 1,639 cm−l for SPhMDPOBn in pristine state) of 33 and 35 cm−l which occurred when SPhMDPOBn was immobilized on the silica surface may be caused by a weakening of the intramolecular hydrogen bonding of the SPhMDPOBn because the interaction with the silica surface as hydrogen bond with silanol groups is weaker than that in associates.
Muramyl dipeptideTemperature-programmed desorption mass spectrometry (TPD-MS)PyrolysisThioglycosidesElectrospray ion trap mass spectrometry (ESI IT MS)Fourier transform infrared spectroscopy (FTIR)
It has long been known that non-specific stimulation of the immune system can be brought about by exposure to bacteria or components extracted from bacterial cells . The minimum effective structure responsible for the immunoadjuvant activities of the bacterial cell wall was identified as a sugar-containing peptide of the peptidoglycan component [2, 3]. The smallest effective synthetic molecule was found to be an N-acetylmuramyl-l-alanyl-d-isoglutamine (MDP) [2, 3]. MDP was found to exert numerous immunomodulatory activities. However, the administration of MDP into different hosts was always associated with serious toxicity that hampered its use in man . Therefore, in an effort to generate MDP analogues with reduced toxicity and enhanced biological activities, several hundred derivatives were synthesized by chemical modification of the parent molecule [5–8].
Sulfur-containing compounds play an important role in living organisms in energy metabolism (energy production), blood clotting, and synthesis of collagen (the main protein of connective tissue in animals which is the major constituent of bones, fibrous tissues of the skin, hair, and nails) and also participate in enzyme formation. Thioglycosides are less investigated in contrast to O-glycosides. It is known that O-glycosidase is able to split O-glycosides, including of O-arylglycosides, in biological systems. Enzymes capable of cleaving the thioglycosidic bond are less common in nature and occur mainly in plants [9, 10]. While O-glycosidases are ubiquitous, plant myrosinase is the only known S-glycosidase . Thioglycosides possess significantly lower susceptibility to enzymatic hydrolysis than the corresponding oxygen glycosides . Also, thioglycosides have gained widespread use in carbohydrate chemistry as inhibitors of O-glycosidase and O-glycosyltransferase inhibitors . Nevertheless, unlike intensively investigated O-glycosides of MDP, S-glycosides have received relatively little attention. Currently, only three S-alkyl glycosides of MDP, namely, methyl and butyl β-glycosides and hexadecyl S-glycoside, have been obtained , although 1-thiomuramyl dipeptide itself was found to possess the adjuvant effect close to the action of muramyl dipeptide . For this reason, we synthesized the thioglycosides of MDP.
Fumed silica with controlled particle size, morphology and surface area, along with its chemical, thermal and easy functionalization properties, is suitable for application in adsorption, catalysis, chemical separation, drug delivery and biosensors [14–20]. Silica nanoparticle-MDP thioglycoside complexes' synthesis is a way of structure modification that results in enhanced bioavailability of MDP thioglycosides and prolonged action and simplifies delivery in biological systems . Parfenyuk et al.  have demonstrated the possibility of the application of silica nanoparticles for topical delivery of the immunomodulatory drug glucosaminylmuramyl dipeptide (GMDP; the chemically synthesized natural equivalent of peptidoglycan) to the peritoneal macrophages of women with endometriosis. Researchers have shown that the immunomodulatory effect of GMDP can be increased by its immobilization on silica nanoparticles.
The aim of this study was to examine chemical transformations of thiophenylglycoside of MDP with silica surface and to characterize the structure of the adsorbed films on silica by temperature-programmed desorption mass spectrometry (TPD-MS) and Fourier transform infrared spectroscopy (FTIR).
Powdery fumed silica (pilot plant at the Institute of the Surface Chemistry, Kalush, Ukraine; with a specific surface area of 270 m2/g) was used in this work. Fumed silica was previously heated on air for 2 h at 400°С to remove adsorbed organic substances.
Benzyl ester of О-(phenyl-2-acetamido-2,3-dideoxy-1-thio-β-D-glucopyranoside-3-yl)-D-lactoyl-L-alanyl-D-isoglutamine (SPhMDPOBn; Figure 1) was synthesized at the Department of Biological and Organic Chemistry of Taurida National V.I. Vernadsky University: SPhMDPOBn 1H-NMR (DMSO-d6) SAr: 7.11 to 7.24 (m, CHar); GlcNac: 4.75 (d, 1 H, J = 10 Hz), 1.79 (s, NAc), 7.98 (d, NHAc), 5.58 (d, C4-OH), 4.69 (bt, C6-OH); 1.25 (d, CH3CHCO); Ala: 1.25 (d, CH3), 7.11 to 7.24 (m, NH); Glu: 12.48 (bs, CO2R), 2.10 (t, γ-CH2), 1.74, 1.95 (m, β-CH2), 6.79, 7.24 (s, CONH2), 8.28 (d, NH) .
The details of the synthesis procedure of SPhMDPOBn have been previously reported .
Loading of MDP arylthioglycosides on the fumed silica surface
The sample of SPhMDPOBn with a concentration of 0.6 mmol/g on the silica surface was obtained by impregnation. It is known that the concentration of free silanol groups (isolated ≡ Si-OH groups), the main active sites, on the silica surface is equal to 0.6 mmol/g of silica . The weight of the MDP thioglycoside batch was such as to ensure a ratio of the concentration of modifier to that of silica surface silanol groups of 1:1. A 0.0121 g of SPhMDPOBn dissolved in 0.8 mL of 96% ethanol was added to 0.03 g of fumed silica in a Petri dish. The components were mixed and left on air at approximately 20°C till the solvent is evaporated (approximately 12 h). In the experiment, the air-dried sample was under investigation.
Instrument and procedures
Electrospray ionization ion trap mass spectrometry analysis
Mass spectra were obtained with the ion trap mass spectrometer Bruker HCT Plus (Bruker Daltonics, Bremen, Germany) equipped with an electrospray ionization source. Ionization was performed under electrospray conditions (flow rate 1.0 μL/min, spray voltage 4.8 kV, sheath gas 40 arb). All spectra were acquired at a capillary temperature of 25°C, and all ion guide voltages were tuned to maximize the abundance of the total ion current.
The analyte solutions (250 pmol/μL) were prepared in methanol. Methanol was of HPLC grade (Sigma, St. Louis, MO, USA).
Fourier transform infrared spectroscopy
FTIR spectra were recorded using a FT IR NEXUS spectrometer (Thermo Fisher Scientific Inc., Madison, WI, USA) at room temperature in the frequency range of 4,000 to 400 сm−1 in diffuse reflection mode at a resolution of 4 сm−1, a scan rate of 0.5 сm/s and number of scans of 150. In diffuse reflectance mode, the powdered samples were mixed with freshly calcined and milled KBr (1:100).
Method of temperature-programmed desorption mass spectrometry
TPD-MS experiments were performed in a MKh-7304A monopole mass spectrometer (Electron, Sumy, Ukraine) with electron impact ionization, adapted for thermodesorption measurements. A typical test comprised placing a 20-mg sample on the bottom of a molybdenum-quartz ampoule, evacuating to approximately 5 × 10−5 Pa at approximately 20°C and then heating at 0.15°C/s from room temperature to approximately 750°C. For all the samples, the sample vials were filled approximately 1/16 full, which helped limit interparticle diffusion effects [24–28]. Limiting the sample volume along with the high vacuum should further limit readsorption and diffusion resistance as described elsewhere [24–33]. The volatile pyrolysis products was passed through a high-vacuum valve (5.4 mm in diameter, a length of 20 cm and a volume of 12 mL) into the ionization chamber of the mass spectrometer where they were ionized and fragmented by electron impact. After mass separation in the mass analyzer, the ion current due to desorption and pyrolysis was amplified with a VEU-6 secondary-electron multiplier ("Gran" Federal State Unitary Enterprise, Vladikavkaz, Russia). The mass spectra and the P-T curves (where P is the pressure of volatile pyrolysis products, and T is the temperature of the samples) were recorded and analyzed using a computer-based data acquisition and processing setup. The mass spectra were recorded within 1 to 210 amu. During each TPD-MS experiment, approximately 240 mass spectra were recorded and averaged. During the thermodesorption experiment, the samples were heated slowly while keeping a high rate of evacuation of the volatile pyrolysis products. The diffusion effects can thus be neglected, and the intensity of the ion current can be considered proportional to the desorption rate.
Results and discussion
Electrospray ionization ion trap mass spectrometry analysis of О-(phenyl-2-acetamido-2,3-dideoxy-1-thio-β-d-glucopyranoside-3-yl)-d-lactoyl-l-alanyl-d-isoglutamine
The electrospray ionization ion trap mass spectrum (ESI IT MS) of SPhMDPOBn (Figure 2) shows a peak at m/z 697 which corresponds to the sodium adduct molecular ion [M + Na]+. The product ion at m/z 469 is most probably derived from m/z 402 fragment ion of SPhMDPOBn: [M-C10H11O2-C6H5S + 3Na+-2H+]+. The ion at m/z 247 was identified as [M + 3Na]3+/3.
TPD-MS analysis of О-(phenyl-2-acetamido-2,3-dideoxy-1-thio-β-d-glucopyranoside-3-yl)-d-lactoyl-l-alanyl-d-isoglutamine
As can be seen from the P-T curve (Figure 3), pyrolytic degradation of thiophenylglycoside of MDP in the pristine state proceeds in a relatively narrow temperature range from 150°С to 250°С in two main stages (Figure 4). The same two main stages are observed on the TPD-curves (Figure 5). Probably, these stages of pyrolysis result from the existence of SPhMDPOBn in α- and β-anomeric forms. Figure 4 illustrates a possible pyrolytic pattern and products.
At the first and the second stages (Figure 5), the elimination of the benzyl ester-protected carboxylic group of isoglutamine fragment takes place, which gives rise to a peak of the molecular ion of benzyl alcohol at m/z 108 (Figure 4). Fragmentation of benzyl alcohol via loss of the -OH group at m/z 17 leads to a common fragment seen for alkyl benzenes at m/z 91. Loss of CH2OH at m/z 31 from the molecular ion gives m/z 77 corresponding to the phenyl cation (Figure 4). Loss of aglycone and carbohydrate moiety occurs during the first and the second stages of pyrolysis. But it is observed that there are different ratios of peak intensities on the TPD-curve for molecular and fragment ions of corresponding products. Thus, the first stage proceeds via preferential removal of benzyl alcohol, while the second stage-by elimination of thiophenol. Aglycon is easily removed in the form of thiophenol under the pyrolysis of SPhMDPOBn. The intensity of a thiophenol molecular ion peak is high as the thiophenol molecular ion is stable. The thiophenol molecular ion is stabilized by the presence of π-electron systems, which are capable of accommodating a loss of one electron more easily. The fragmentation of thiophenol molecular ion under electron impact is shown in Figure 6. Isomerization of the molecular ion of thiophenol to thioketone can take place, as the fragmentation pattern used here provides a reasonable explanation of the observed release of -CS and fragment ions from the molecular ion (Figure 6).
Moreover, the characteristic peak at m/z 125 common to amino sugars is observed in the mass spectrum . Pyrolysis of SPhMDPOBn on the silica surface is more complex. As can be seen from the P-T curve (Figure 7), pyrolysis begins at a lower temperature and proceeds in a wider temperature range. At the same time, there are products such as thiophenol, benzyl alcohol and carbohydrate fragment with m/z 125, which were observed during the pyrolysis of SPhMDPOBn in the pristine state. However, the sequence of their stages and temperature range are changing. Thermal decomposition of SPhMDPOBn on the silica surface (Figures 7 and 8) also proceeds via the elimination of aglycon and carbohydrate moieties. The set of peaks in mass spectra of SPhMDPOBn adsorbed on the silica surface (Figure 8) is the same as that for the pyrolysis of pristine SPhMDPOBn (Figure 5).
Probably, a hydrogen-bonded complex forms between the silanol surface groups and the C = O group of the acetamide moiety: NH-(CH3)-C = O…H-O-Si≡. The thermal transformations of such hydrogen-bonded complex results in the pyrolysis of SPhMDPOBn immobilized on the silica surface under TPD-MS conditions.
The IR spectra of the silica sample are depicted in Figure 9. The band at 3,745 cm−1 is assigned to the stretching vibration of isolated silanol groups (≡Si-OH). The wide band in the 3,700- to 3,000-cm−1 interval corresponds to the overlapping of the O-H-stretching modes of adsorbed water and Si-OH stretchings [35, 36]. A small peak at approximately 1,628 cm−1 can be attributed to the proton-containing components σOH (silanol groups and the deformation vibrations of the O-H groups in physically adsorbed molecular water at the silica surface) [37–39]. Bands centered at 1,980 and 1,867 cm−1 represent overtones and combinations of intense Si-O fundamental modes (two component bands of Si-O-Si stretching modes) (Table 1).
Assignments of the main silica bands in the 700- to 4,000 cm−1region
The Si-O-Si and Si-O vibration bands appeared, respectively, at 1,083 and 809 cm−1 for the silica sample. The symmetric vibrations of the silicon atoms in a siloxane bond occur at approximately 809 cm−1 (νas-Si-O-Si). The largest peak observed in the silica spectrum is present at approximately 1,197 cm−1 and is dominated by antisymmetric motion of silicon atoms in siloxane bonds (νas-Si-O-Si).
The infrared spectra of SPhMDPOBn can be divided into several spectral regions. The IR spectra of SPhMDPOBn in the range 4,000 to 3,100 cm−1 are dominated by absorption arising from the symmetric and asymmetric N-H stretching modes. The IR spectrum of SPhMDPOBn adsorbed on the silica surface in the range 4,000 to 3,100 cm−1 shows a widened band near 3,313 cm−1 representing the N-H stretching mode, which is partially overlapped by the bands of the silica matrix (Figure 9). The maximum at 3,313 cm−1 is assigned to the N-H groups which were involved in hydrogen bonding interactions with the surface hydroxyl groups.
The bands in the IR spectra of SPhMDPOBn in the pristine state and adsorbed on the silica surface in the region 3,100 to 2,800 cm−1 are assigned as the symmetric and antisymmetric stretching vibrations of the С-Н bonds in a methylene group (in pristine state: νs = 2,850 cm−1 and νas = 2,925 cm−1; on the silica surface: νs = 2,850 cm−1 and νas = 2,931 cm−1).
The 1,800- to 1,700-cm−l region involves bands due to the C = O stretching modes of benzyl ester-protected carboxylic group of isoglutamine fragment. The bands at 1,724 cm−l in the spectrum of SPhMDPOBn in pristine state and at 1,728 cm−l on the silica surface referred to the ester C = O stretch mode.
The 1,700- to 1,500-cm−l region is dominated by the strong amide I and amide II bands. The conformational-sensitive amide I and amide II bands are the most intensive bands in the spectra of SPhMDPOBn in pristine and adsorbed states. Amide I band absorption originates from the C = O stretching vibration of the amide group, coupled to in-plane N-H bending and C-N stretching modes. The exact frequency of this vibration depends on the nature of the hydrogen bonding involving C = O and N-H groups, which encodes the secondary structure of a dipeptide. The amide I band is usually consists of a number of overlapping component bands representing helices, β-structures, β-turns and random structures. The amide I band of SPhMDPOBn in pristine state consists of two separate component bands at 1,626 and 1,639 сm−1 (Figure 9). The amide I band of SPhMDPOBn adsorbed on silica is composed of the following maxima: at 1,659 and 1,674 сm−1 (Figure 9, Table 2). The maximum in the spectrum at 1,624 cm−1 (Figure 9B, line 3) is assigned to proton-containing components σOH (silanol groups and the deformation vibrations of the O-H groups in physically adsorbed molecular water at the silica surface). So, amide I and amide II bands are not obscured by overlapping with absorption bands of physically adsorbed molecular water. The intensity of the infrared band at 3,745 cm−l assigned to the OH-stretching vibrations of isolated silanol groups on silica is decreased after immobilization of SPhMDPOBn. This is indicated on the hydrogen bonding of the SPhMDPOBn molecule with silanol groups. The amide I band at 1,626 and 1,639 сm−1 was shifted to 1,659 and 1,674 сm−1, respectively, for adsorbed-on-silica SPhMDPOBn molecules. That is, the amide I band is shifted to higher wavenumbers (Figure 9, Table 2). The shift of the amide I band of the adsorbed SPhMDPOBn by 33 and 35 cm−1, respectively, to higher wavenumbers may be caused by a weakening of the intramolecular hydrogen bonding of the SPhMDPOBn because of the interaction with the silica surface [41, 42]. This testifies that the binding to the silica surface occurs due to peptide fragment resulting in the change of its conformation under adsorption. The amide II band represents mainly N-H bending with the C-N stretching vibrations and is conformationally sensitive. The amide II of SPhMDPOBn in pristine state absorbs at 1,535 and 1,568 сm−1. The amide II of SPhMDPOBn on the silica surface has a complex structure and centered at 1,547 сm−1 (Figure 9, Table 2).
Absorption frequencies of amide I and amide II bands and N-H stretching modes of SPhMDPOBn
Аmide I ( ν(сm−1))
Аmide II ( ν(сm−1))
Pr, pristine state; Ad, adsorbed on the silica surface.
Earlier using 1H-NMR and nuclear Overhauser effect spectroscopy, it was shown that MDP consists of two type II adjacent β-turns forming an S-shaped structure [43, 44]. The first β-turn is formed by a C10 hydrogen bonding between the Ala NH and the acetamido C = O of N-acetylmuramic acid. The existence of the second β-turn is assumed by the presence of a free carboxamide group of isoglutamine [43, 44]. Correlation between amide frequency and protein secondary structure found in the literature is listed in Table 2. We can assume from the comparison correlation between amide frequency in FTIR spectra of SPhMDPOBn (Table 2) and protein secondary structure found in the literature (Table 3) that, probably, SPhMDPOBn in the pristine state adopt β-sheet conformation and in the adsorb state, combination of β-sheet and β-turn structures.
Assignments of amide bands to the secondary structure of peptides and proteins (literature data)
The spectral region 1,400 to 1,200 сm−1 is characterized by overlapping deformation vibrations of the C-H bond in methyl and methylene groups of peptide fragment, stretching vibrations of the С-О bond in carbonyl group and amide III vibrations (stretching vibrations of С-N bond and N-H bend in plane) and the Si-O-Si, Si-O and O-Si-O vibration bands of the silica matrix.
The stages of pyrolysis of aglycone, peptide fragment and carbohydrate residue of thiophenylglycoside of muramyl dipeptide in the pristine state and adsorbed on the silica surface have been determined. Decomposition of thiophenylglycoside of muramyl dipeptide in pristine state occurs within the narrow temperature range from 150°C to 250°C. The decomposition of thiophenylglycoside of muramyl dipeptide adsorbed on the silica surface undergoes certain reactions to produce pyrolysis products such as thiophenol, benzyl alcohol and carbohydrate fragment with m/z 125 in the temperature range from 50°C to 450°C. Probably, the hydrogen-bonded complex forms between silanol surface groups and the C = O group of the acetamide moiety NH-(CH3)-C = O…H-O-Si≡. The thermal transformations of such hydrogen-bonded complex result in the pyrolysis of SPhMDPOBn immobilized on the silica surface under TPD-MS conditions.
The intensity of the infrared band at 3,745 cm−l assigned to the OH stretching vibrations of isolated silanol groups on silica decreased after the immobilization of SPhMDPOBn. This indicated the hydrogen-bonding of SPhMDPOBn molecule with silanol groups.
The shifts ∆ν of the amide I band (measured from 1,626 to 1,639 cm−l for SPhMDPOBn in the pristine state) of 33 and 35 cm−l which occurred when SPhMDPOBn was immobilized on the silica surface may be caused by a weakening of the intramolecular hydrogen bonding of the SPhMDPOBn because the interaction with the silica surface as hydrogen bond with silanol groups is weaker than that in the associates.
LRA is a Ph.D. degree holder and a Junior Research Fellow. TVK is a Ph.D. degree holder, a Senior Researcher, Head of the Laboratory of the Kinetics and Mechanisms of Chemical Transformations on Solid Surfaces. BBP is a Junior Research Fellow. VNT is a Ph.D. degree holder and a Senior Laboratory Assistant. AEZ is a Dr. Sci. holder and a Professor of the Department of Organic and Biological Chemistry, the Faculty of Biology and Chemistry. VYC is Dr. Sci. holder and a Professor and the Head of the Department of Organic and Biological Chemistry, Faculty of Biology and Chemistry.
temperature-programmed desorption mass spectrometry.
This work was partially supported by the grant UKC2-7072-KV-12 from the U.S. Civilian Research & Development Foundation (CRDF Global) with funding from the United States Department of State and by the grant M/299-2013 from the State Agency of Ukraine for Science, Innovation and Information.
Chuiko Institute of Surface Chemistry, The National Academy of Sciences of Ukraine
Taurida National V.I. Vernadsky University
McDonald C, Inohara N, Nuñez G: Peptidoglycan signaling in innate immunity and inflammatory disease. J Biol Chem 2005, 280: 20177–20180. 10.1074/jbc.R500001200View Article
Ohkuni H, Norose Y, Ohta M, Hayama M, Kimura Y, Tsujimoto M, Kotani S, Shiba T, Kusumoto S, Yokogawa K, Kawata S: Adjuvant activities in production of reaginic antibody by bacterial cell wall peptidoglycan or synthetic N-acetylmuramyl dipeptides in mice. Infect Immun 1979, 24: 313–318.
Merser C, Sinaÿ P, Adam A: Total synthesis and adjuvant activity of bacterial peptidoglycan derivatives. Biochem Biophys Res Comm 1975, 66: 1316–1322. 10.1016/0006-291X(75)90503-3View Article
Lederer E: Natural and Synthetic Immunomodulators Derived from the Mycobacterial Cell Wall. Pythagora: Roma; 1988.
Dzierzbicka K, Wardowska A, Trzonkowski P: Recent developments in the synthesis and biological activity of muramylpeptides. Curr Med Chem 2011, 18: 2438–2451. 10.2174/092986711795843173View Article
Ishida H, Kigawa K, Kitagawa M, Kiso M, Hasegawa A, Azuma I: Synthesis and immunoadjuvant activity of 1-α-O, 1-β-S- and 6-O-(2-tetra-decylhexadecanoyl) derivatives of N-acetylmuramoyl-L-alanyl-D-isoglutamine methyl ester. Agric Biol Chem 1991, 55: 585–587. 10.1271/bbb1961.55.585View Article
Wang ZF, Xu J: Synthesis of muramyl dipeptide analogs by incorporation of 3,3,3-trifluoroalanine. Chin Chem Lett 2000, 11: 297–300.
Hasegawa A, Hioki Y, Kiso M, Okumura H, Azuma I: Synthesis of 1-thio-N-acetylmuramoyl-L-alanyl-D-isoglutamine derivatives, and their biological activities. J Carbohydr Chem 1982, 1: 317–323. 10.1080/07328308208085104View Article
Driguez H, Thiem J, Beau JM: Glycoscience: Synthesis of Substrate Analogs and Mimetics. Berlin: Springer; 1997.
Bourderioux A, Lefoix M, Gueyrard D, Tatibouet A, Cottaz S, Arzt S, Burmeister WP, Rollin P: The glucosinolate-myrosinase system. New insights into enzyme-substrate interactions by use of simplified inhibitors. Org Biomol Chem 2005, 3: 1872–1879. 10.1039/b502990bView Article
Shen H, Byers LD: Thioglycoside hydrolysis catalyzed by β-glucosidase. Biochem Biophys Res Comm 2007, 362: 717–720. 10.1016/j.bbrc.2007.08.043View Article
Barr BK, Holewinski RJ: 4-Methyl-7-thioumbelliferyl-β-D-cellobioside: a fluorescent, nonhydrolyzable substrate analogue for cellulases. Biochemistry 2002, 41: 4447–4452. 10.1021/bi015854qView Article
Rosenholm JM, Meinander A, Peuhu E, Niemi R, Eriksson JE, Sahlgren C, Lindén M: Targeting of porous hybrid silica nanoparticles to cancer cells. ACS Nano 2008, 3: 197–206.View Article
Trewyn BG, Slowing II, Giri S, Chen H-T, Lin VSY: Synthesis and functionalization of a mesoporous silica nanoparticle based on the sol–gel process and applications in controlled release. Acc Chem Res 2007, 40: 846–853. 10.1021/ar600032uView Article
Barbé C, Bartlett J, Kong L, Finnie K, Lin HQ, Larkin M, Calleja S, Bush A, Calleja G: Silica particles: a novel drug-delivery system. Adv Mater 2004, 16: 1959–1966. 10.1002/adma.200400771View Article
Slowing II, Trewyn BG, Giri S, Lin VSY: Mesoporous silica nanoparticles for drug delivery and biosensing applications. Adv Funct Mater 2007, 17: 1225–1236. 10.1002/adfm.200601191View Article
Mersal GAM, Khodari M, Bilitewski U: Optimisation of the composition of a screen-printed acrylate polymer enzyme layer with respect to an improved selectivity and stability of enzyme electrodes. Biosens Bioelectron 2004, 20: 305–314. 10.1016/j.bios.2004.01.023View Article
Wang J, Liu J: Fumed-silica containing carbon-paste dehydrogenase biosensors. Anal Chim Acta 1993, 284: 385–391. 10.1016/0003-2670(93)85324-DView Article
Chen H, Wang Y, Dong S, Wang E: Direct electrochemistry of cytochrome C at gold electrode modified with fumed silica. Electroanalysis 2005, 17: 1801–1805. 10.1002/elan.200503298View Article
Parfenyuk EV, Alyoshina NA, Antsiferova YS, Sotnikova NY: Silica Nanoparticles as Drug Delivery System for Immunomodulator GMDP. New York: Momentum; 2012.
Zemlyakov AE, Tsikalova VN, Azizova LR, Chirva VY, Mulik EL, Shkalev MV, Kalyuzhin OV, Kiselevsky MV: Synthesis and biological activity of aryl S-β-glycosides of 1-thio-N-acetylmuramyl-L-alanyl-D-isoglutamine. Russ J Bioorg Chem 2008, 34: 223–229. 10.1134/S106816200802012XView Article
Armistead CG, Tyler AJ, Hambleton FH, Mitchell SA, Hockey JA: Surface hydroxylation of silica. J Phys Chem 1969, 73: 3947–3953. 10.1021/j100845a065View Article
Delgado JA, Gómez JM: Estimation of adsorption parameters from temperature-programed-desorption thermograms: application to the adsorption of carbon dioxide onto Na − and H − mordenite. Langmuir 2005, 21: 9555–9561. 10.1021/la050966uView Article
Nicholl SI, Talley JW: Development of thermal programmed desorption mass spectrometry methods for environmental applications. Chemosphere 2006, 63: 132–141. 10.1016/j.chemosphere.2005.07.015View Article
Miller JB, Siddiqui HR, Gates SM, Russell JJN, Yates JJT, Tully JC, Cardillo MJ: Extraction of kinetic parameters in temperature programmed desorption: a comparison of methods. J Chem Phys 1987, 87: 6725–6732. 10.1063/1.453409View Article
Nicholl SI, Talley JW, Silliman S: Model verification of thermal programmed desorption-mass spectrometry for estimation of release energy values for polycyclic aromatic hydrocarbons on mineral sorbents. Environ Toxicol Chem 2004, 23: 2545–2550. 10.1897/03-356View Article
Kulyk K, Ishchenko V, Palyanytsya B, Khylya V, Borysenko M, Kulyk T: A TPD-MS study of the interaction of coumarins and their heterocyclic derivatives with a surface of fumed silica and nanosized oxides CeO2/SiO2, TiO2/SiO2, Al2O3/SiO2. J Mass Spectrom 2010, 45: 750–761. 10.1002/jms.1765View Article
Park J-H, Yang RT: Predicting adsorption isotherms of low-volatile compounds by temperature programmed desorption: iodine on carbon. Langmuir 2005, 21: 5055–5060. 10.1021/la046866qView Article
Rudziński W, Borowiecki T, Dominko A, Pańczyk T: A new quantitative interpretation of temperature-programmed desorption spectra from heterogeneous solid surfaces, based on statistical rate theory of interfacial transport: the effects of simultaneous readsorption. Langmuir 1999, 15: 6386–6394. 10.1021/la9800147View Article
Joly J-P, Perrard A: Determination of the heat of adsorption of ammonia on zeolites from temperature-programmed desorption experiments. Langmuir 2001, 17: 1538–1542. 10.1021/la001129pView Article
Kulik TV: Use of TPD–MS and linear free energy relationships for assessing the reactivity of aliphatic carboxylic acids on a silica surface. J Phys Chem C 2011, 116: 570–580.View Article
Kulik TV, Azizova LR, Palyanytsya BB, Zemlyakov AE, Tsikalova VN: Mass spectrometric investigation of synthetic glycoside of muramyl dipeptide immobilized on fumed silica surface. Mater Sci Eng, B 2010, 169: 114–118. 10.1016/j.mseb.2009.12.003View Article
Kulyk TV, Palyanytsya BB, Borodavka TV, Borysenko MV: Supramolecular structures of chitosan on the surface of fumed silica. In Nanomaterials and Supramolecular Structures. Edited by: Shpak AP, Gorbyk PP. Dordrecht: Springer; 2010:259–268.
Della Volpe C, Dirè S, Pagani E: A comparative analysis of surface structure and surface tension of hybrid silica films. J Non-Cryst Solids 1997, 209: 51–60. 10.1016/S0022-3093(96)00547-9View Article
Griffith GW: Quantitation of silanol in silicones by FTIR spectroscopy. Indust Eng Chem Prod Res Dev 1984, 23: 590–593.View Article
Sárkány J: Effects of water and ion-exchanged counterion on the FTIR spectra of ZSM-5. I. NaH-ZSM-5: H-bonding with OH groups of zeolite and formation of H+(H2O) n . Appl Catal A 1999, 188: 369–379. 10.1016/S0926-860X(99)00257-4View Article
Little LH: Infrared Spectra of Adsorbed Species. New York: Academic; 1966.
Iler RK: The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica. New York: Wiley; 1979.
Kiselev AV, Lygin VI: Infrared Spectra of Surface Compounds. New York: Wiley; 1975.
Hasegawa M, Low MJD: Infrared study of adsorption in situ at the liquid–solid interface: III. Adsorption of stearic acid on silica and on alumina, and of decanoic acid on magnesia. J Colloid and Interf Sci 1969, 30: 378–386. 10.1016/0021-9797(69)90405-6View Article
Parfitt GD, Rochester CH: (Eds): Adsorption from Solution at the Solid/Liquid Interface. London: Academic; 1983.
Fermandjian S, Perly B, Level M, Lefrancier P: A comparative 1H-N.M.R. study of MurNAc- L -Ala- D -iGln (MDP) and its analogue murabutide: evidence for a structure involving two successive β-turns in MDP. Carbohydr Res 1987, 162: 23–32. 10.1016/0008-6215(87)80197-0View Article
Sizun P, Perly B, Level M, Lefrancier P, Fermandjian S: Solution conformations of the immunomodulator muramyl peptides. Tetrahedron 1988, 44: 991–997. 10.1016/S0040-4020(01)86129-9View Article
Adochitei A, Drochioiu G: Rapid characterization of peptide secondary structure by FT-IR spectroscopy. Rev Roum Chem 2011, 56: 783–791.
Hering JA: FTIR spectroscopy for analysis of protein secondary structure. In Biological and Biomedical Infrared Spectroscopy. Edited by: Barth A, Haris PI. Amsterdam: IOS; 2009:129–167.
Wellman SE, Hamodrakas SJ, Kamitsos EI, Case ST: Secondary structure of synthetic peptides derived from the repeating unit of a giant secretory protein from Chironomus tentans . Biochim Biophys Acta Protein Struct Mol Enzymol 1992, 1121: 279–285. 10.1016/0167-4838(92)90157-9View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.