General synthetic methods for the grafting of peptide chains onto polyoxometalate clusters by the use of general activated precursors have been developed. Using a solution-phase approach, pre-synthesized peptides can be grafted to a metal oxide cluster to produce hybrids of unprecedented scale (up to 30 residues). An adapted solid-phase method allows the incorporation of these clusters, which may be regarded as novel hybrid unnatural amino acids, during the peptide synthesis itself. These methods may open the way for the automated synthesis of peptides and perhaps even proteins that contain "inorganic" amino acids.
Here we report a suite of approaches for the isolation of asymmetrically grafted organic-inorganic hybrid Mn-Anderson polyoxometalate compounds (TBA) 3 [MnMo 6 O 18 ((OCH 2) 3 CNHR 1)((OCH 2) 3 CNHR 2)] (where TBA ¼ tetrabutylammonium). Both a "pre-functionalization" route (for compound 1-R 1 ¼-COC 14 H 9 , R 2 ¼-H) using two different TRIS-based ligands ((HOCH 2) 3 CNHR), and a "post-functionalization" of the preformed TRIS Mn-Anderson compound (R 1 ¼ R 2 ¼-H) were demonstrated. Compounds 2 (R 1 ¼-COC 15 H 31 , R 2 ¼-CO(CH 2) 2 COOH) and 3 (R 1 ¼-COC 15 H 31 , R 2 ¼-H) are some of the first reported examples of asymmetric Mn-Anderson compounds to have been synthesized by the latter route. The reliable and broadly applicable chromatographic method used to isolate these compounds relies on the difference in affinity of compounds' organic moieties for reverse phase (RP) media; the target asymmetric cluster will have an intermediate affinity, between that of the two symmetric by-products. For instances where this is not the case, we have prepared and isolated a "universal" asymmetric Mn-Anderson precursor 4 (R 1 ¼-C(O)OC 14 H 11 , R 2 ¼-H), which can be used as a precursor to synthesize practically any asymmetric Mn-Anderson system. The use of 4 as an "universal" precursor was successfully demonstrated in the synthesis and isolation of compound 5 (R 1 ¼-COC 2 H 5 , R 2 ¼-H), which would not be accessible by a simple 'one pot' approach. In addition to removing a significant barrier to the exploitation of asymmetric Mn-Anderson clusters as new functional materials, the methods presented here should be applicable to a range of other hybrid organic-inorganic clusters.
Ion‐mobility mass spectrometry has been explored as a new technique in the analysis of polyoxometalates and was utilized for the differentiation between photoswitchable isomers of organic–inorganic hybrid compounds (see picture).
Abstract:Incorporating the building blocks of nature (e.g., peptides and DNA) into inorganic polyoxometalate (POM) clusters is a promising approach to improve the compatibilities of POMs in biological fields. To extend their biological applications, it is necessary to understand the importance of different non-covalent interactions during self-organization. A series of Anderson POM-peptide hybrids have been used as a simple model to demonstrate the role of different interactions in POM-peptide (biomolecules) systems. Regardless of peptide chain length, these hybrids follow similar solution behaviors,
A comprehensive study of the isomer-independent synthesis of TRIS ((HOCH(2))(3)CNH(2)) Mn-Anderson compounds from Na(2)MoO(4)·2H(2)O, via the corresponding octamolybdate species, is presented. Three octamolybdate salts of [Mo(8)O(26)](4-) in the β-isomer form, with tetramethylammonium (TMA), tetraethylammonium (TEA) and tetrapropylammonium (TPA) as the counter cation, were synthesised from the sodium molybdate starting material. Fine white powdery products for the three compounds were obtained, which were fully characterised by elemental analysis, TGA, solution and solid state Raman, IR and ESI-MS, revealing a set ratio of Na and organic cations for each of the three compounds; (TMA)(2)Na(2)[Mo(8)O(26)] (1), (TEA)(3)Na(1)[Mo(8)O(26)] (2) and (TPA)(2)Na(2)[Mo(8)O(26)] (3), and the analyses also confirmed that the three compounds all consisted of the octamolybdate in the β-isomeric form. ESI-MS analyses of 1, 2 and 3 show similar fragmentation for these β-isomers compared to the previously reported study for the α-isomer ((TBA)(4)[α-Mo(8)O(26)]) (A) in the synthesis of ((TBA)(3)[MnMo(6)O(18)((OCH(2))(3)CNH(2))(2)]) (B), and compounds 1, 2 and 3 were successfully used to synthesise equivalent TRIS Mn-Anderson compounds: (TMA)(3)[MnMo(6)O(18)((OCH(2))(3)CNH(2))(2)] (4), (TEA)(3)[MnMo(6)O(18)((OCH(2))(3)CNH(2))(2)] (5) and (TPA)(2)Na(1)[MnMo(6)O(18)((OCH(2))(3)CNH(2))(2)] (6), as well as Na(3)[MnMo(6)O(18)((OCH(2))(3)CNH(2))(2)] (7). This is the first example where symmetric organically-grafted Mn-Anderson compounds have been synthesised in DMF from anything but the {Mo(8)O(26)} α-isomer.
General synthetic methods for the grafting of peptide chains onto polyoxometalate clusters by the use of general activated precursors have been developed. Using a solution‐phase approach, pre‐synthesized peptides can be grafted to a metal oxide cluster to produce hybrids of unprecedented scale (up to 30 residues). An adapted solid‐phase method allows the incorporation of these clusters, which may be regarded as novel hybrid unnatural amino acids, during the peptide synthesis itself. These methods may open the way for the automated synthesis of peptides and perhaps even proteins that contain “inorganic” amino acids.
We report here the straightforward synthesis and characterisation of a series Anderson-type hybrid polyoxometalates in high yield, functionalised with carboxylic acid following the reaction of anhydride precursors with the starting hybrid cluster ([n-N(C 4 H 9) 4 ] 3 [MnMo 6 O 18 ((OCH 2) 3 CNH 2) 2 ]). Seven new structures have been obtained, five of which have acid-terminated ligands. Six of these structures have been isolated with a yield higher than 80% with high purity. This reaction is limited by the bulkiness of the anhydride used; this effect can be employed to selectively synthesise one isomer out of three other possibilities. The acid groups and aromatic platforms attached to the clusters can act as building tools to bridge several length scales and engineer molecular packing within the crystal structure. The presence of acids should also change the hydrophilicity of the clusters, and therefore the way they interact with hydrophilic surfaces. We also show a potential relationship between the acid group interaction in the packing diagram and the cluster's tendency to interact with a hydrophilic surface. In addition to reporting a derived synthetic path to new acid-terminated Mn-Anderson-type hybrids, we describe here a new way to program self-assembly motifs of these compounds in the crystal structure and at interfaces.
Herein a library of hybrid Mn-Anderson polyoxometalates anions are presented: 1, [(MnMo6 O18 )((OCH2 )3 -C-(CH2 )7 CHCH2 )2 ](3-) ; compound 2, [(MnMo6 O18 )((OCH2 )3 C-NHCH2 C16 H9 )2 ](3-) ; compound 3, [(MnMo6 O18 )((OCH2 )3 C-(CH2 )7 CHCH2 )1 ((OCH2 )3 C-NHCH2 C16 H9 )1 ](3-) ; compound 4, [(MnMo6 O18 )((OCH2 )3 C-NHC(O)CH2 CHCH2 )2 ](3-) and compounds 5-9, [(MnMo6 O18 )((OCH2 )3 C-NHC(O)(CH2 )x CH3 )2 ]), where x = 4, 10, 12, 14, and 18 respectively. The compounds resulting from the cation exchange of the anions 1-9 to give TBA (a) and DMDOA (b) salts, and additionally for compounds 1, 2 and 3, tetraphenylphosphonium (PPh4 ) (c) salts, are explored at the air/water interface using scanning force microscopy, showing a range of architectures including hexagonal structures, nanofibers and other supramolecular forms. Additionally the solid-state structures for compounds 1c, 2c, 4a, 6a, 9a, are presented for the first time and these investigations demonstrate the delicate interplay between the structure of the covalently derivatised hybrid organo-clusters as well as the ion-exchange cation types.
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