Mechanism of the formation of a dehydrated ion by an unusual loss of oxygen at the 4'-carbonyl group of abscisic acid methyl ester in electron ionization mass spectrometry

Methyl ester of abscisic acid (ABA), a plant hormone, gives a dehydrated ion at m/z 260 in electron ionization mass spectrometry (EI-MS). This dehydrated ion had been considered to be derived only from the elimination of the tertiary hydroxyl group at C-1'. We found that 34% of the dehydrated ion was formed by elimination of the oxygen atom at the 4'-carbonyl group, and the remaining 66% by elimination of the 1'-hydroxyl group. This unusual elimination of the carbonyl oxygen was shown with [4'-(18)O]ABA methyl ester. Involvement of the 4'-carbonyl oxygen in dehydration was observed in methyl ester of phaseic acid (PA), a natural metabolite of ABA, but not in 1'-deoxy-ABA methyl ester or isophorone. This suggested that the 1'-hydroxyl group was necessary for the elimination of the 4'-carbonyl oxygen. ABA methyl esters labeled with stable isotopes showed that hydrogen atoms at the 1'-hydroxyl group and at C-4 or -5 or -3' or - 5' or -7' were eliminated with the 4'-carbonyl oxygen. These results allow us to propose a formation mechanism of the dehydrated ion derived from the elimination of 4'-carbonyl oxygen and hydrogen atoms at C-4 and 1'-oxygen in ABA methyl ester as follows: first, ionization at the 1'-hydroxyl group occurs to give an ion radical, and the proton at the 1'-oxygen migrates to the 4'-carbonyl oxygen after the bond fission between C-1'-C-6'; second, migration of the proton at C-4 to the 1'-oxygen is followed by migration of the protons at C-5 and C-7' to C-4 and C-5, respectively; finally, the proton at the 1'-oxygen migrates to the 4'-hydroxyl group, and H(2)O at C-4' is eliminated to give the dehydrated ion. Our findings point out that a dehydrated ion is not always derived from the elimination of a hydroxyl group.     

Protonation Sites in Gaseous Pyrrole and Imidazole: A Neutralization–Reionization and ab initio Study

Mild gas-phase acids C₄H₉⁺ and NH₄⁺ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization–reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6–311G(2d, p) as 866, 845 and 786 kJ mol^-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2-and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck–Condon effects which deposit on average 183 kJ mol^-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.     

RCM as a tool to freeze conformation of monosaccharides: synthesis of a β-mannopyranoside mimic adopting a conformation close to the biologically relevant B2,5 boat

The synthesis of a β-d-mannopyranoside analog, fully identical to the naturally occurring d-mannopyranose in terms of hydroxyl pattern, and displaying a skew-boat conformation close to a B_2,5 boat strongly believed to be adopted by the oxycarbenium transition state during glycosidic bond cleavage of β-mannane by family 26 β-mannanase, is described. The conformationally locked analog has been obtained by tethering the C-2 and C-5 carbon atoms of the sugar ring with a three carbon bridge using RCM methodology. Conformation of the mannose analog has been confirmed by NMR and molecular modelling.     

Efficient Regioselective Synthesis of Mono-2-O-Sulfonyl-cyclodextrins by the Combination of Sulfonyl Imidazole and Molecular Sieves

Abstract

Cyclodextrins are cyclic oligosaccharides consisting of six or more α-1,4-linked D-glucopyranose units, which possess primary hydroxyl groups at the C-6 positions and secondary hydroxyl groups at the C-2 and C-3 positions. Because cyclodextrins have a hydrophobic and optically active interior, they have been utilized as transporters of hydrophobic molecules and small molecular mimics of enzymes. The chemical modification of cyclodextrins has been investigated in order to improve these characteristics. Sulfonations of the primary or secondary hydroxyl groups of cyclodextrin have been applied for further functionalization of cyclodextrin, and several methods for regioselective sulfonations have been developed. Among these strategies, selective monotosylation of the C-6 hydroxyl group is done relatively easily by reaction of α or β-cyclodextrin and p-toluenesulfonyl chloride in pyridine^1,2 or in alkaline aqueous solution.^3,4 However, sulfonation of the secondary hydroxyl groups is more difficult and new sulfonation methods must be developed to provide precursors for cyclodextrin analogues such as amino and sulfide analogues. Several strategies for the sulfonation of one C-2 hydroxyl group have been reported. However, because reaction conditions can require specific sulfonation reagent,⁵ alkaline condition,^3-7 strict anhydrous conditions,^8,9 or use of protected C-6 hydroxyl groups,^10,11 the methodology is not convenient to employ.     

Cyclic bipyridine glycosides from the marine-derived actinomycete Actinoalloteichus cyanogriseus WH1-2216-6

Cyanogrisides A-D (1-4), four new glycosidic derivatives of bipyridine featuring a novel cyclic glycoside generated by vicinal hydroxyl groups of an aglycone with both the anomeric center and the adjacent carbonyl of a keto sugar, were isolated from the marine-derived actinomycete Actinoalloteichus cyanogriseus WH1-2216-6. The structures of 1-4 were elucidated by spectroscopic analysis, X-ray single crystal diffraction, CD spectra, and chemical methods.     

Proton affinity ladder for uridine and analogs: influence of the hydroxyl group on the sugar ring conformation

A ladder of relative proton affinities (PA) for a series of modified uridines (e.g. araU, ddU, 5BrU, 5BrdU and 5IU) was established from competitive dissociations of proton-bound heterodimers using Cooks and co-workers' kinetic method. The studied heterodimers are constituted of a modified nucleoside and either an amino acid or a nucleoside with known PA value. These non-covalent heterodimers were prepared under electrospray conditions to be selected and dissociated into the ion-trap analyzer. These results allowed our PA ladder of uridine and deoxyuridine analogs substituted at the C-5 position in the uracil ring to be extended. From this scale, it was showed that the substitution of hydrogen atom at the C-2' position in the sugar ring by a hydroxyl group involves a decrease of about 7 kJ mol(-1). The experimental values for U, 5MeU, dU, 5MedU, ddU and araU are consistent with those obtained by DFT calculations (B3P86/6-31+G//B3LYP/6-31G(.)). Several neutral and protonated conformations of these compounds were considered, in particular the ring conformation of furanose and the orientation of the base with respect to the sugar ring. These calculated results showed the influence of sugar substituent on the conformation of the neutral form of theses nucleosides. However, the most stable protonated structure is the same for all the studied nucleosides except for araU, where the position of the anti 2'-OH group imposes a specific conformation.     

Elimination of water from the molecular ion of cycloheptanol

Under electron impact cycloheptanol decomposes by four fragmentation paths: (1) α-cleavage with subsequent losses of C¹-C⁵ fragments, (2) elimination of water, (3) loss of the hydrogen atom from C-1 and (4) loss of the hydroxyl group. The mechanism of water elimination was investigated by means of deuterium labelling. 1,4-Elimination of water predominates in cycloheptanol, with the stereospecific cis-1,3-elimination also being operative. The loss of water is preceded by extensive exchange of the hydroxyl hydrogen with those of the ring. This is attributed to a very facile transannular interaction of the hydroxyl group with the C-3 to C-6 positions that are made accessible due to conformational properties of the 7-membered ring. A kinetic model is proposed, describing migrations of the ring hydrogen atoms.     

New phenylboronic esters derived from inositol [1]

We have prepared two new tetracyclic phenylboronic esters 4 and 5 derived from myo-inositol and from 1,2-O-isopropylidene-myo-inositol, respectively. The structures of these compounds were established from NMR and IR spectra, elemental analyses, and an X-ray diffraction study in the case of 4 . Compound 4 is a tetracyclic derivative of the less stable conformer of inositol (five axial hydroxy groups and one equatorial) with two dioxaboroline rings at opposite faces of the six-membered ring, one formed between the boron atom and the axial hydroxyl groups at C-3 and C-5 and the other between the boron atom and the hydroxyl groups at C-4 and C-6, and a dioxaborolidine ring bridging C-1 and C-2 at axial and equatorial positions. A similar structure was found for 5 with the difference that bridging C-1 and C-2 there is a dioxolane ring. The boron atoms are planar with their attached atoms, stabilized by retrocoordination between the boron and oxygen and carbon atoms, respectively. The two phenyl rings that are in the same face of the molecule are essentially parallel, with a dihedral angle between planes of 28.26 ± 0.79°.     

Cu催化羟基自由基氧化断裂纤维二糖分子糖苷键反应及其在纤维素低温解聚中的应用

纤维素是葡萄糖通过β-1,4-糖苷键链接而成的高聚物,在木质纤维素中含量最高,结构稳定,较难水解.糖苷键的解聚主要有三种方式:酶水解、酸水解以及碱降解.酶解的优点是反应条件温和、副产物少,但存在成本高、活性低等缺点,限制了其大规模的工业化生产.碱水解纤维素的同时伴随着葡萄糖的peeling-off反应得到异变糖酸,需要消耗大量的碱,并且强碱也存在腐蚀性强和回收难等问题.酸水解是目前工业上常用的纤维素水解方法,在保持较高葡萄糖选择性的同时,通过对反应条件的控制(提高反应温度和酸浓度)来提高纤维素的水解效率,但是硫酸对设备的腐蚀性强,也难以回收,不符合绿色化学的发展要求.固体酸是近年来研究较多的纤维素水解催化剂.固体酸虽然腐蚀性弱、易回收,但是其活性低,水热稳定性较差,目前还不具备大规模生产的条件.本文发展了一种羟基自由基活化断裂糖苷键的方法,利用羟基自由基的高活性在低温下实现糖苷键的选择性断裂,同时羟基自由基与糖苷键作用后转化为无毒无害的水和氧气,将不会对环境造成污染.我们首先以纤维二糖作为纤维素的模型分子,通过羟基自由基能够优先与糖苷键反应得到葡萄糖和葡萄糖酸的实验证实所提出的方法的可行性.实验表明,来自H2O2的·OH自由基能够在铜基催化剂作用下选择性氧化断裂其糖苷键,生成葡萄糖和葡萄糖酸.比如:采用均相CuSO4体系,纤维二糖转化率约为20%时,葡萄糖和葡萄糖酸的选择性分别为28.5%和32.3%.采用多相CuO/SiO2(4 wt%CuO)体系,纤维二糖转化率约为20%时,葡萄糖和葡萄糖酸的选择性约分别为23.3%和25.7%,并且该催化剂具有良好的循环使用性能.与·OH类似,CuSO4催化过硫酸钾生成的·SO4-自由基也能够有效转化纤维二糖,在纤维二糖转化率为20%时,葡萄糖和葡萄糖酸的选择性分别为36.6%和39.9%.利用这种·OH和·SO4-自由基氧化的方法,也能够在较低温度下(333 K)解聚纤维素中的糖苷键.我们发展了H2O2浸渍预处理纤维浸渍预处理纤维素的方法,通过部分破坏纤维素糖苷键,提高了纤维素的水解活性.比如:处理后的纤维素在413 K条件下反应12 h,纤维素转化率和葡萄糖选择性分别达到约36.1%和42.5%.XRD结果表明,处理后的纤维素的晶体结构未发生明显的变化.FT-IR表征结果显示处理后的纤维素表面生成了大量的羧酸基团.     

Formation of intermolecular hydrogen bonds in light-sensitive crystals of C-2′-(o-oxyphenyl)vinyl-N-p-methylphenyl nitrone,C-2′-(2″-oxy-5″-bromophenyl)vinyl-N-p-methylphenyl nitrone,C-2′-(2″-oxy-5″-bromophenyl)-vinyl-N-phenyl nitrone,and C-2′-(o-oxyphenyl)vinyl-n-methyl nitrone

This paper studies the crystal structure of new substituted light-sensitive azomethine N-oxides (nitrones): C-2′-(o-oxyphenyl)vinyl-N-p-methylphenyl nitrone (1), C-2′-(2″-oxy-5″-bromophenyl)vinyl-N-p-methylphenyl nitrone (2), C-2′-(2″-oxy-5″-bromophenyl)-vinyl-N-phenyl nitrone (3), and C-2′-(o-oxyphenyl)vinyl-N-methyl nitrone (4). In contrast to the compounds studied earlier [1, 2], C-2′-(β-oxy-α-naphthyl)vinyl-N-p-methylphenyl nitrone (5), C-2′-(β-oxy-α-naphthyl)vinyl-N-phenyl nitrone (6), C-2′-(o-oxyphenyl) vinyl-N-phenyl nitrone (7), and C-2′-(o-oxyphenyl)vinyl-N-p-bromophenyl nitrone (8), the nitrones studies in this work have anti-rather than syn-orientations of the nitrone and hydroxyl groups. Due to this spatial arrangement of the proton-donating hydroxyl and proton-accepting nitrone groups, molecules in crystals 1–4 are bonded by intermolecular hydrogen bonds (IHB) to form chains but not centrosymmetric dimeric associates (CDA). Two types of chain arrangements were revealed: “head-to-tail” and “head-to-tail, tail-to-head”. It is shown that the introduction of an alkyl substituent instead of an aryl one at the nitrogen atom of the nitrone group in 4 leads to a change in the geometry of the IHB in the H-associate. It is proven that the hydroxyl proton can undergo an intermolecular O→O transfer in the chain of hydrogen bonds in crystals 1–4, which can give rise to photochemical transformations in these crystals. Institute of Chemical Physics in Chernogolovka, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 349–362, March–April, 1996. Translated by L. Smolina     

A kinetic and thermodynamic study of the glycosidic bond cleavage in deoxyuridine

Density functional theory was used to study the thermodynamics and kinetics for the glycosidic bond cleavage in deoxyuridine. Two reaction pathways were characterized for the unimolecular decomposition in vacuo. However, these processes are associated with large reaction barriers and highly endothermic reaction energies, which is in agreement with experiments that suggest a (water) nucleophile is required for the nonenzymatic glycosidic bond cleavage. Two (S(N)1 and S(N)2) reaction pathways were characterized for direct hydrolysis of the glycosidic bond by a single water molecule; however, both pathways also involve very large barriers. Activation of the water nucleophile via partial proton abstraction steadily decreases the barrier and leads to a more exothermic reaction energy as the proton affinity of the molecule interacting with water increases. Indeed, our data suggests that the barrier heights and reaction energies range from that for hydrolysis by water to that for hydrolysis by the hydroxyl anion, which represents the extreme of (full) water activation (deprotonation). Hydrogen bonds between small molecules (hydrogen fluoride, water, or ammonia) and the nucleobase were found to further decrease the barrier and overall reaction energy but not to the extent that the same hydrogen-bonding interactions increase the acidity of the nucleobase. Our results suggest that the nature of the nucleophile plays a more important role in reducing the barrier to glycosidic bond cleavage than the nature of the small molecule bound, and models with more than one hydrogen fluoride molecule interacting with the nucleobase provide further support for this conclusion. Our results lead to a greater fundamental understanding of the effects of the nucleophile, activation of the nucleophile, and interactions with the nucleobase for this important biological reaction.     

Production and fragmentation of negative ions from neutral N-linked carbohydrates ionized by matrix-assisted laser desorption/ionization

Although negative ion fragmentation mass spectra of neutral N-linked carbohydrates (those attached to Asn in glycoproteins) provide much more structural information than spectra recorded in positive ion mode, neutral carbohydrates are reluctant to form negative ions by matrix-assisted laser desorption/ionization (MALDI) unless ionized from specific matrices such as nor-harmane or adducted with anions such as chloride. This paper reports the results of experiments to optimize negative ion formation from adducts of N-linked glycans with respect to ion abundance and fragment ion production. The best results were obtained with 2,4,6-trihydroxyacetophenone (THAP) as the matrix with added ammonium nitrate as the salt providing the anion. This approach is demonstrated to be applicable for a wide range of N-linked glycan structures. Phosphate adducts, analogous to those that are usually encountered in electrospray spectra from N-glycans released by protein N-glycosidase F, were produced by addition of ammonium phosphate to the matrix but in relatively low yield allowing competitive ionization of endogenous anionic compounds leading to complex spectra. Fragmentation of the nitrate adducts, which were formed in higher yield, generally paralleled that seen by collision-induced dissociation following ionization by electrospray, with the first stage of the dissociation being the elimination of the nitrate with a proton from one of the hydroxyl groups of the sugar. The spectra of the resulting [M-H](-) species displayed very specific fragment ions, mainly cross-ring and C-type glycosidic cleavage products, that revealed more structural (linkage and branching) information of the compounds than the mainly glycosidic cleavage products that dominated the positive ion spectra.     

Communication: Double Bond Migration of a 1,5-Anhydro-3-C-p-Tolylsulfonyl-D-Hex-2-Enitol Derivative and the Corresponding 5a-Carba-Dl-Sulfonyl Sugar

Although the rate of proton abstraction (kinetic acidity) frequently plays an essential role in determination of reaction pathways and is of theoretical interest,¹ it is still controversial whether an oxygen atom activates or deactivates the abstraction of an α-hydrogen atom of an ether. For example, it is well known that oxidative elimination of a seleno group gives an allyl ether as the major product, indicating the oxygen atom deactivates the kinetic acidity.² Abstraction of the equatorial hydrogen atom at C-2 of 6-methyl-1,3-oxathiane-3,3-dioxide 1 is slower than that at C-4.³ On the other hand, the bridgehead hydrogen atom (H_b) adjacent to the oxygen atom of piperazinedione (2) is abstracted more readily than that of the alternative one (H_a).⁴     

The impact of protonation and deprotonation of 3-methyl-2'-deoxyadenosine on N-glycosidic bond cleavage

The enzyme-substrate contacts that are believed to be involved in depurination by proton transfer have been modelled by protonation and deprotonation of 3-methyl-2'-deoxyadenosine (3-MDA) using quantum mechanical calculations in the gas-phase and solution media. The change in the charge distribution on the sugar ring and nucleobase that is introduced by the protonation and deprotonation strongly affects the N-glycosidic bond length. The unimolecular cleavage and hydrolysis of the N-glycosidic bond, involving D(N)*A(N) and A(N)D(N) pathways, have been considered at several levels of theory. The trend in the energy barriers is A(N)D(N) > cleavage > D(N)*A(N). All probable proton transfer reactions resulting from enzyme-substrate contacts do not facilitate the N-glycosidic bond cleavage of 3-MDA. The deprotonation of 3-MDA that may result from the interaction between H6 and enzyme do not facilitate bond cleavage. The protonation at N7 induces more positive charge on the sugar ring and further facilitates the depurination relative to the protonation at N1. The changes in the charges calculated on the ribose and nucleobase are in good relationship with the C1'-C2', C1'-O4', and N-glycosidic bond lengths along the cleavage. The change in energy barrier ΔE of glycosidic bond cleavage from the gas-phase to solution media strongly depends on the charge of the species.     

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC(6)H(4)CH[double bond, length as m-dash]CHCO(+)/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH[double bond, length as m-dash]CH(2)]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the C(α)-CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC(6)H(5), respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.     

Structural investigation of the antibiotic sporaviridin: 11—Molecular secondary ion mass spectral studies on the constituent pentasaccharides viridopentaoses

Molecular secondary ion mass spectra of three pentasaccharides, viridopentaoses A, B and C, using various matrices, are discussed. The appearance of the molecular ion species is dependent upon the relative proton affinities between the sample and the organic matrix material. However, the presence of sodium ion, rather than proton, also greatly influences the appearance of the molecular ion species. The glycosidic linkages are mainly cleaved between the glycosidic oxygen atom and the anomeric carbon atom to give informative sugar sequence ions. These fragmentations have been confirmed by the linked scanning technique (B/E).     

Chemo-enzymatic synthesis of conformationally constrained oligosaccharides

N-Acetyllactosamine derivative 4, which has a methylene amide tether between C-6 and C-2', was enzymatically glycosylated using rat liver alpha-2,6-sialyltransferase (ST6GalI) or recombinant human fucosyltransferase V (FucT-V) to give conformationally constrained trisaccharides 5 and 6, respectively. The methylene amide linker of 4 was installed by a two-step procedure, which involved acylation of a C-6 amino function of a LacNAc derivative with chloroacetic anhydride followed by macrocyclization by nucleophilic displacement of the chloride by a C-2' hydroxyl. The conformational properties of 4 were determined by a combination of NOE and trans-glycosidic heteronuclear coupling constant measurements and molecular mechanics simulations and these studies established that the glycosidic linkage of 4 is conformationally constrained and resides in only one of the several energy minima accessible to LacNAc. The apparent kinetic parameters of transfer to LacNAc and conformationally constrained saccharides 3 and 4 indicates that fucosyltransferase V recognize LacNAc in its A-conformer whereas alpha-2,6-sialyltransferase recognizes the B-conformer of LacNAc.     

Promiscuity of carbonic anhydrase II. Unexpected ester hydrolysis of carbohydrate-based sulfamate inhibitors

Carbonic anhydrases (CAs) are enzymes whose endogenous reaction is the reversible hydration of CO(2) to give HCO(3)(-) and a proton. CA are also known to exhibit weak and promiscuous esterase activity toward activated esters. Here, we report a series of findings obtained with a set of CA inhibitors that showed quite unexpectedly that the compounds were both inhibitors of CO(2) hydration and substrates for the esterase activity of CA. The compounds comprised a monosaccharide core with the C-6 primary hydroxyl group derivatized as a sulfamate (for CA recognition). The remaining four sugar hydroxyl groups were acylated. Using protein X-ray crystallography, the crystal structures of human CA II in complex with four of the sulfamate inhibitors were obtained. As expected, the four structures displayed the canonical CA protein-sulfamate interactions. Unexpectedly, a free hydroxyl group was observed at the anomeric center (C-1) rather than the parent C-1 acyl group. In addition, this hydroxyl group is observed axial to the carbohydrate ring while in the parent structure it is equatorial. A mechanism is proposed that accounts for this inversion of stereochemistry. For three of the inhibitors, the acyl groups at C-2 or at C-2 and C-3 were also absent with hydroxyl groups observed in their place and retention of stereochemistry. With the use of electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry (ESI-FTICR-MS), we observed directly the sequential loss of all four acyl groups from one of the carbohydrate-based sulfamates. For this compound, the inhibitor and substrate binding mode were further analyzed using free energy calculations. These calculations suggested that the parent compound binds almost exclusively as a substrate. To conclude, we have demonstrated that acylated carbohydrate-based sulfamates are simultaneously inhibitor and substrate of human CA II. Our results suggest that, initially, the substrate binding mode dominates, but following hydrolysis, the ligand can also bind as a pure inhibitor thereby competing with the substrate binding mode.     

13C-NMR spectral studies on the distribution of substituents in some cellulose derivatives

¹³C-NMR spectra of trityl cellulose (Tr-Cell), tosyl cellulose (Ts-Cell), cellulose S-methyl xanthate (Cell-M-Xan), and cellulose formate (CF) in dimethylsulfoxide-d₆ were analyzed at 50.4 MHz. It was found that the distribution of substituents in the anhydroglucose units of these cellulose derivatives can be estimated from their ring carbon spectra. The results showed that (i) in Tr-Cell having degree of substitution (DS) lower than 1, the hydroxyl groups at C-6 carbon position are selectively tritylated, (ii) in the case of Ts-Cell, the difference in the relative DS value among three different types of hydroxyl groups is not large, although the relative reactivities of hydroxyl groups toward tosylation decrease in the order C-6 > C-2 > C-3, (iii) in Cell-M-Xan, the hydroxyl groups at C-3 carbon position are mainly substituted, and (iv) the ease of formylation is C-6 > C-2 > C-3. The 100.8 MHz ¹³C-NMR spectra of O-methyl cellulose (MC) revealed that the reactivity order in commercial MC prepared from alkali cellulose is C-6 ? C-2 > C-3. Concerning MC, its water solubility was also discussed in terms of the distribution of substituents along the cellulose chain.     

Relation of Certain Infrared Bands to Starch Crystallinity

Starch is a homoglycan composed of but a single type of sugar unit. Nature has chosen the starch granule as an almost universal from for packaging and sturing carbohydrate in green plants. In granule form, starch is quasi-crystalline, water-insoluble, and dense. In structure of amylose, a hydrogen bond exists between the hydroxyl group at C-2 of one α-D-glucopyranosyl unit and the C-3 hydroxyl group of the adjacent ct-D-glucopyranosyl unit with the C-3 hydroxyl group donating the hydrogen atom in the hydrogen bond. The starch chains within the amorphous region are presumable available for reaction. With extensive chemical derivatization of starch in which the granule crystal structure is maintained essentially inact.