The relationship between intramolecular hydrogen bonds and certain physical properties of regioselectively substituted cellulose derivatives

This article tries to provide some direct evidence about the relationship between the intramolecular hydrogen bonds in cellulose and their corresponding effect on physical properties. The formation of intramolecular hydrogen bonds has been proved to contribute directly to certain physical properties of cellulose, such as its solubility in solvents having different polarities, the relative reactivities of the hydroxyls in a repeating unit and its crystallinity, using a 6-O-methylcellulose (6MC) film that was known¹ to have intramolecular hydrogen bonds. The excellent solubility of 6MC when compared with other cellulose derivatives indicated a lack of interchain hydrogen bonds. A comparison of the relative reactivities between the C-2 and C-3 position hydroxyls in 6MC also indicates that intramolecular hydrogen bonds once formed in 6MC films are possibly maintained even after dissolution in solvents. In addition, the poor crystallinity exhibited by 6MC supports the idea that crystallization in cellulosics may be dependent more upon preferencial interchain hydrogen bonding at the C-6 position hydroxyls than upon a uniform structure such as that found in 6MC, where every structural unit is completely and regioselectively substituted, distinguishing it from other synthetic polymers such as polyolefins and polyesters. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 717–723, 1997     

Regioselectively Modified Cellulose and Chitosan Derivatives for Mono- and Multilayer Surface Coatings of Hemocompatible Biomaterials

In this study different synthetic strategies were developed and applied to introduce solely or in combination heparin/heparansulfate-like functional groups such as N-sulfo, O-sulfo, N-acetyl, and N-carboxymethyl groups into chitosan and cellulose with highest possible regioselectivity and completeness and defined distribution along the polymer chain. Completely substituted 6-amino-6-deoxycellulose and related derivatives were prepared from tosylcellulose (DS 2.02; C₆ 1.0) by nucleophilic substitution with azido groups only in the 6-position at 50 °C with subsequent reduction to amino groups and completely removing tosyl groups in the 2,3-position. 2,6-Di-O-sulfocellulose was prepared using the reactivity difference between C-2, C-6 and C-3 of cellulose. The reactivity difference between amino groups and hydroxyl groups was used to prepare various N-substituted derivatives. Partially 2,6-di-O-sulfated cellulose was obtained from trimethylsilylcellulose by the insertion of sulfurtrioxide into the Si–O ether linkage. Partially 3-O-sulfocellulose was synthesized by protecting C-2 and C-6 with trifluoroacetyl groups. A copper–chitosan complex was used to synthesize 6-O-sulfochitosan with a DS of 1.0 at C-6 and various partially 6-O-desulfonated products are possible. Using the phthalimido group to increase the solubility of chitosan in DMF, the regioselectivity of 3-O-sulfo groups was improved by regioselective 6-O-desulfonation of nearly complete 3,6-O-disulfochitosan. The platelet adhesion properties of immobilized regioselectively modified water-soluble derivatives on membranes have been tested in vitro. Some regioselectively modified chitosan and cellulose derivatives are potential candidates for the surface coatings of biomaterials if the regioselective reactions are somewhat further optimized.     

The assignment of IR absorption bands due to free hydroxyl groups in cellulose

Interpretation of the IR hydroxyl absorption bands in cellulose has been limited to the inter- and intramolecularly hydrogen-bonded hydroxyl groups in the crystalline form. This paper attempts to assign IR frequencies due to ‘free‘ or non-hydrogen bonded hydroxyl groups by using a curve fitting method. The almost completely methylated cellulose derivatives of tritylcellulose (previously used in related studies) exhibited small IR bands due to hydroxyl groups. The IR bands were assumed to appear under stereohindered conditions and thus resulted in a mixture of bands which included the contribution of free hydroxyl groups. The curve fitting method deconvoluted the IR bands into three bands in the OH stretching region: they were interpreted in terms of free or hydrogen bonded hydroxyl groups. The assignments were confirmed by comparison of an almost completely methylated derivative with partially methylated derivatives having different degrees of substitution. In addition, intramolecular hydrogen bonds involving OH at the C-3, C-2 and C-6 positions were shown to be easily formed, even between extremely small numbers of unsubstituted hydroxyl groups present, and thus cause perturbation of the specific deconvoluted band. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Availability and state of order of hydroxyl groups on the surfaces of microstructural units of crystalline cotton cellulose

The relative accessibilities of the hydroxyl groups of the D-glucopyranosyl units of hydrocellulose have been studied by means of the reaction of N,N-diethylaziridinium chloride, which produces 2-(diethylamino)ethyl cellulose. The deviation in the distribution of substituents among the 2-O-, 3-O-, and 6-O-positions of the D-glucopyranosyl residues in a hydrocellulose from that in a disordered cellulose in which the three types of hydroxyl groups are equally accessible is the basis for estimating the selective accessibilities of the hydroxyl groups in the crystalline cellulose. A particular hydrocellulose, lying within the range of leveling-off degree of polymerization, was studied in detail; this hydrocellulose, designated EHC (“Exemplar Hydrocellulose”), was formed from fibrous cotton by hydrolysis for 0.67 hr in 2.5N hydrochloric acid at reflux. EHC exhibited higher selective accessibility (larger deviation from equal accessibility) of the hydroxyl groups at C-2, C-3, and C-6, than samples of hydrocellulose formed in shorter or longer periods of hydrolysis. This selective accessibility is discussed in terms of intra- and intermolecular hydrogen bonding on the surfaces of crystalline microstructural units in EHC.     

The role of hydroxyl groups in interchain interactions in cellulose Iα and Iβ

Complex networks of hydrogen bonds within the cellulose I_α and I_β contribute greatly to cellulose's anisotropic physical properties such as material stiffness. The interchain hydrogen bonding interactions through hydroxyl groups are isolated in each of the three lattice planes of the adjacent chains within the unit cell of two allomorphs of natural cellulose. In our density function theory study with dispersion corrected Perdew–Burke–Ernzerhof (PBE‐D2) functional, these hydroxyl groups participate in strong hydrogen bonding interactions (?24.8 and ?24.8 kcal/mol for cellulose I_α and I_β, respectively) in the side‐to‐side lattice plane. Unexpectedly, the hydroxyl groups also participate significantly in hydrogen bonding interactions (?11.0 and ?12.4 kcal/mol for cellulose I_α and I_β, respectively) in one of the diagonal lattice planes in both cellulose I_α and I_β. Both PM7 and PBE‐D2 method predict that the overall interaction is asymmetric and stronger in the right diagonal lattice plane. While hydrogen bonding interactions are strongest in side‐to‐side lattice plane as expected, the role of hydrogen bonding interactions for keeping the sheet together is more significant than previously thought.     

Organic hybrid of chlorinated polyethylene and hindered phenol. IV. Modification on dynamic mechanical properties by chlorinated paraffin

Binary systems of chlorinated polyethylene (CPE) and chlorinated paraffin (CP) or 3,9‐bis[1,1‐dimethyl‐2{β‐(3‐tert‐butyl‐4‐hydroxy‐5‐methylphenyl) propionyloxy}ethyl]‐2,4,8,10‐tetraoxaspiro[5,5]‐undecane (AO‐80) and their ternary systems were investigated by dynamic mechanical analysis, thermal analysis, and infrared spectrum analysis. Adding CP into CPE/AO‐80, in which one novel relaxation appears above the glass‐transition temperature of CPE, can increase not only the peak height but also the minimum value between two peaks. The tan δ value in the middle of the two peaks for CPE/CP/CPE was found to be proportional to the slope (d ln E′/dT) of the E′ curve at an identical temperature. The addition of CP caused changes in many of the hydrogen bonds: a decrease in hydrogen bonds between the hydroxyl groups of AO‐80, a reinforcement of hydrogen bonds between the hydroxyl groups of AO‐80 and α‐hydrogens of CPE, and the formation of other hydrogen bonds between the carbonyl groups of AO‐80 and α‐hydrogens of CPE. Those changes are useful to improve the temperature dependence of tan δ and to enhance the stability of the dynamic mechanical properties. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 23–31, 2001     

Solid-state NMR studies of methyl celluloses. Part 1: regioselectively substituted celluloses as standards for establishing an NMR data basis

Three methyl celluloses with completely uniform substitution pattern, 2-O-methyl cellulose (1), 3-O-methyl cellulose (2) and 6-O-methyl cellulose (3), were prepared according to the cationic ring opening polymerization approaches starting from substituted 1,2,4-orthopivalate derivatives of d-glucose. These samples allowed for the first time to sort out the methyl substitution effects on solid-state NMR chemical shifts and relaxation. Dipolar dephasing experiments allowed the detection and assignment (¹H, ¹³C) of the methyl groups. In 1 and 2, these resonances overlapped with those of C-6, whereas in 3, the methyl signal experienced a low-field shift into the region of C-2,3,5. ¹³C T₁ experiments were used to verify different relaxation behavior of the carbon sites, particularly the short relaxation time of at the carbon substitution site next to the methyl groups. This effect was used to unambiguously identify the ¹³C chemical shifts of the carbons carrying the methoxyl substituent, although they overlap with all resonances in the C-2,3,5 region. The data obtained for the standard samples with uniform substitution will now be used as the basis for determining methylation patterns and substitution degree in commercial methyl celluloses.     

Nucleotides,Part LXIII,New 2-(4-Nitrophenyl)ethyl(Npe)- and 2-(4-Nitrophenyl)ethoxycarbonyl(Npeoc)-Protected 2′-Deoxyribonucleosides and Their 3′-Phosphoramidites – Versatile Building Blocks for Oligonucleotide Synthesis

A series of new base-protected and 5′-O-(4-monomethoxytrityl)- or 5′-O-(4,4′-dimethoxytrityl)-substituted 3′-(2-cyanoethyl diisopropylphosphoramidites) and 3′-[2-(4-nitrophenyl)ethyl diisopropylphosphoramidites] 52 – 66 and 67 – 82 , respectively, are prepared as potential building blocks for oligonucleotide synthesis (see Scheme). Thus, 3′,5′-di-O-acyl- and N ²,3′-O,5′-O-triacyl-2′-deoxyguanosines can easily be converted into the corresponding O⁶-alkyl derivatives 6 , 8 , 10 , 12 , 14 , and 16 by a Mitsunobu reaction using the appropriate alcohol. Mild hydrolysis removes the acyl groups from the sugar moiety (→ 9 , 11 , 13 , 15 , and 19 (via 18 ), resp.) which can then be tritylated (→ 38 – 42 ) and phosphitylated (→ 57 – 61 ) in the usual manner. N ²-[2-(4-nitrophenyl)ethoxycarbonyl]-substituted and N ²-[2-(4-nitrophenyl)ethoxycarbonyl]-O⁶-[2-(4-nitrophenyl)ethyl]-substituted 2′-deoxyguanosines 5 and 7 , respectively, are synthesized as new starting materials for tritylation (→ 28 , 35 , and 37 ) and phosphitylation (→ 54 , 56 , 70 , and 78 ). Various O⁴-alkylthymidines (see 20 – 24 ) are also converted to their 5′-O-dimethoxytrityl derivatives (see 43 – 47) and the corresponding phosphoramidites (see 62 – 66 and 79 – 82 ).     

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 1 1 ). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate ( 3 ); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride ( 7 ); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride ( 9 ), hexa (6)-O-methyl-α-cyclodextrin ( 13 ). The direct substitution is shown to be even more efficient for β-cyclodextrin ( 16 ), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate ( 17 ), while the indirect strategy fails. The compounds are characterized by extensive use of ¹³C- and ¹H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.     

Phosphorylated cellulose propionate derivatives as thermoplastic flame resistant/retardant materials: influence of regioselective phosphorylation on their thermal degradation behaviour

Cellulose ester derivatives having phosphoryl side-chains were synthesized by phosphorylation of two types of cellulose propionate (CP); the difference between the two CPs was whether the primary hydroxyl group at C6 had been fully propionylated or not. Dimethyl phosphate, dimethyl thiophosphate, diethyl phosphate, or diethyl thiophosphate was introduced into the residual hydroxyl positions of the CPs. Chemical composition of the respective derivatives was characterized by elemental analysis and a combined use of saponification and HPLC quantification of the released propionic acid. Their thermal properties were investigated by DSC and TGA, and an intermediate residue of the pyrolysis was also examined by FT-IR spectroscopy. From the thermal degradation measurements using TGA, the C6-O phosphorylation was found to noticeably prevent the CP derivatives from weight loss in the pyrolysis process under dynamic air, i.e., providing them with a flame-resistance functionality, whereas the C2-O and C3-O phosphorylation did not give rise to such an appreciable resistance effect. A discussion was focused on the difference in pyrolysis mechanism between the phosphorylated CPs. However, most samples of the CP derivatives showed a clear T _g considerably lower than the onset temperature of the thermal degradation. Thus we suggest that it is possible to design thermoplastic flame resistant/retardant materials based on cellulose, by controlling the substitution distribution of the phosphoryl and propionyl groups introduced.     

Higher Sucrose Analogs: Homologation of a Glucose Unit of Sucrose by Two Carbon Atoms

Abstract

The primary hydroxyl groups (at C-6 and C-6′) in 2,3,4,3′4′-penta-O-benzyl-l′-O-methoxymethyl sucrose (2) can be reactively differentiated with tert-butyldiphenylsilyl chloride. Reaction of 2 with TBDPSCl afforded only one monosilylated product protected at C-6′ (6). The regioisomeric monoprotected sucrose 8 was prepared by selective deprotection of the double silylated derivative 7. Compound 6 was converted into 2,3,4,3′,4′-penta-O-benzyl-6-carbomethoxymethylidene-1′-O-methoxymethylsucrose 10 in three steps. Osmylation of the double bond in 10 afforded stereoisomeric homologated sucroses: 11a [6(S),7(R)] and 11b [6(R),7(S)] in the ratio 3:2. A large downfield shift of the H-1 (up to 0.5 ppm) was observed for 6′-silylated derivatives.     

Structural characterization of ethyl cellulose and its use for preparing polymers with viologen moieties

The characteristics of ethyl cellulose (EC) were studied by chemical reaction and the spectra method. EC was first tosylated with tosyl chloride, and then the tosylated EC (Ts-EC) reacted with sodium iodide (NaI). By comparing the NMR spectra of EC and iodine-substituted EC (EC-I), a free hydroxyl group was proved to exist at the C-6 of the D-glucose unit of cellulose. Viologen moieties were introduced into the EC chain by the reaction of Ts-EC with 4,4′-bipyridine and n-propyl bromide. The existence of viologen was confirmed by ¹H-NMR, IR, and ESR. © 1993 John Wiley & Sons, Inc.     

Directing-protecting groups for carbohydrates. Design, conformational study, synthesis and application to regioselective functionalization

A novel concept of regioselective transformation of secondary hydroxyl groups in carbohydrates is presented. First, the relative reactivity of the free hydroxyl groups of onoprotected d-glucose derivatives was assessed using acetylation as a model reaction. As a result, acylation of these polyols gave a mixture of monosubstituted products in which the 3-O functionalized derivatives predominated. Novel hydrogen bond acceptor protecting groups were next designed to modulate the 4-OH and 3-OH reactivity in the hope to mediate higher regioselective transformations. A molecular modeling study later validated by spectroscopic analysis predicted additional intramolecular hydrogen bonds between the hydroxyl groups and pyridyl-containing protecting groups. Taking advantage of this induced hydrogen bond network, we achieved regioselective acetylation of the hydroxyl group at position 3 without protecting any secondary hydroxyl groups of the carbohydrate moiety. This designed protecting/directing group increased the nucleophilicity and the steric hindrance of position 3. As a result, optimization of the reaction conditions enabled the monoacetylation (not affected by steric hindrance) of 6-O-protected glucopyranosides at position 3 and selective silylation (affected by steric hindrance) of position 2 in high isolated yields and regioselectivities. This result certainly opens doors to the regioselective open glycosylation of carbohydrates.     

Shape of the proton potential in an intramolecular hydrogen-bonded system. Part II

The crystals of 5,5′-dibromo-3-diethylaminomethyl-2,2′-biphenol N-oxide were studied by X-ray and FT-IR spectroscopy. Within this molecule two short OHO intramolecular hydrogen bonds are formed. The NO?H⁺?O⁻ bond between the OH and the N-oxide groups is very strong, of 2.419(7) Å between the oxygen atoms. The proton potential of this hydrogen bond is flat, broad and has probably no barrier—consequently it could not be located from X-ray diffraction data. The other hydrogen bond formed between two hydroxyl groups appears asymmetrical from FT-IR spectra, and shows also relatively limited proton polarizability. The molecular conformation is non-planar, due to strong overcrowding effect between the oxygen atoms involved in the hydrogen bonds.     

Order of Reactivity of OH/NH Groups of Glucosamine Hydrochloride and N-Acetyl Glucosamine Toward Benzylation Using NaH/BnBr in DMF

The order of reactivity of OH and NH groups of glucosamine hydrochloride (GlcNH₂^.HCl) and N-acetyl glucosamine (GlcNAc) toward benzylation with NaH/BnBr in DMF was investigated. For GlcNH₂^.HCl, benzyl groups were introduced in the order of N-Bn > N-Bn₂ > 1-O-Bn > 6-O-Bn > 4-O-Bn > 3-O-Bn; for GlcNAc, benzyl groups were introduced in the order of 1-O-Bn > 6-O-Bn > 4-O-Bn > 3-O-Bn > N-Bn. A range of partially benzylated 2-N,N′-dibenzyl glucopyranosides and GlcNAc derivatives were obtained in a single step.     

The Synthesis of Four Dideoxygenated Analogues of β-Maltosyl-(1→4)-Trehalose

Abstract

Four derivatives of β-maltosyl-(1→4)-trehalose were prepared, each with two deoxy functions in one of the constitutive disaccharide building blocks. 2,3-Di-O-acetyl-4,6-dideoxy-4,6-diiodo-α-D-galactopyranosyl- (1→4) ?1,2,3,6-tetra-O-acetyl-D-glucopyranose (3) was employed as a precursor for the 4?,6?-dideoxygenated tetrasaccharide 9: coupling of 3 with 2,3,6-tri-O-benzyl-α-D-glucopyranosyl 2,3,6-tri-O-benzylidene-α-D-glucopyranoside (4) furnished the tetrasaccharide 5 which was deiodinated and deprotected to yield the target tetrasaccharide 9. Secondly, the dideoxygenated maltose derivative 3-deoxy-4,6-O-isopropylidene-2-O-pivaloyl-β-D-glucopyranosyl- (1→4) ?1,6-anhydro-3-deoxy-2-O-pivaloyl-β-D-glucopyranose (10) was ring-opened to the anomeric acetate 11. A [2+2] block synthesis with 4 in TMS triflate mediated glycosylation gave a tetrasaccharide which was deprotected to the 3″,3?-dideoxygenated analogue of β-maltosyl-(1→4)-trehalose. For the third tetrasaccharide, 2,3,2″,3′-tetra-O-benzyl-α,α-trehalose was iodinated at the primary positions and deiodinated in the presence of palladium-on-carbon, then this acceptor was selectively glycosylated with hepta-O-acetyl-maltosyl bromide (20). Removal of protective groups furnished the maltosyl trehalose tetrasaccharide deoxygenated at positions C-6 and C-6′. to prepare a 3,3′-dideoxygenated trehalose, the free hydroxyl groups of 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranosyl 2-O-benzyl-4,6-O-(R)-benzylidene-α-D-glucopyranoside (25) were reduced by Barton-McCombie deoxygenation. One of the benzylidene groups was opened reductively with sodium cyanoborohydride. The resulting free hydroxyl group at the 4′-position was glycosylated in a Koenigs-Knorr reaction with 20 to yield the 3,3′-dideoxygenated tetrasaccharide 32, the fourth target oligosaccharide, after deprotection.     

PREPARATION AND STRUCTURE OFFIVE DERIVATIVES OFβ-(1→3)-D-GLUCAN ISOLATED FROM PORIA COCOS SCLEROTIUM*

A new solvent of cellulose (1.5 mol/L NaOH/0.5 mol/L urea aqueous solution) was used as one of the homogeneous reaction media of polysaccharides for methylation, hydroxyethylation and hydroxypropylation. A water insoluble β-(1→3)-D-glucan, sample PCS3-Ⅱ, isolated from fresh sclerotium of Poria cocos was sulfated in dimethyl sulfoxide (Me2SO), carboxymethylated in NaOH, isopropanol solution, as well as methylated, hydroxyethylated and hydroxypropylated in the new solvent system, respectively, to obtain five water-soluble derivatives coded as S-PCS3-Ⅱ, C-PCS3-Ⅱ, M-PCS3-Ⅱ, HE-PCS3-Ⅱand HP-PCS3-Ⅱ. Their chemical structure and distribution of substitution were characterized by infrared spectroscopy (IR), elementary analysis (EA), ^1H-NMR, ^13C-NMR, 2D-COSY, 2D-TOCSY and 2D-^1H-detected ^1H ^13C HMQC spectra. The results reveal that the relative reactivity of hydroxyl groups of the β(1→3)-D-glucan is in the order C-6 ＞ C-4 ＞ C-2 on the whole. The substitution of the samples S-PCS3-Ⅱ, C-PCS3-Ⅱ and M-PCS3-Ⅱ occurred mainly at C-6 position and secondly at C-4 and C-2 positions, and that of HE-PCS3-Ⅱ occurred at C-6 and C-4 positions and of HP-PCS3-Ⅱ almost completely occurred at C-6 position. The degrees of substitution (DS) obtained from ^13C-NMR range from 0.23 to 1.27. The water solubility of the derivatives is in the order S-PCS3-Ⅱ ＞ C-PCS3-Ⅱ ＞ M-PCS3-Ⅱ ＞ HE-PCS3-Ⅱ ＞ HP-PCS3-Ⅱ. This work provides a novel and nonpolluting process for the methylation, hydroxyethylation and hydroxypropylation of β-(1→3)-D-glucan.     

A DFT-based study of the hydrogen-bonding interactions between epicatechin and methanol/water

Tea polyphenols are essential components that give tea its medicinal properties. Methanol and water are frequently used as solvents in the extraction of polyphenols. Hydrogen-bonding interactions are significant in the extraction reaction. Density functional theory (DFT) techniques were used to conduct a theoretical investigation on the hydrogen-bonding interactions between methanol or water and epicatechin, an abundant polyphenol found in tea. After first analyzing the epicatechin monomer's molecular geometry and charge characteristics, nine stable epicatechin (EC) H₂O/CH₂OH complex geometries were discovered. The presence of hydrogen bonding in these improved structures has been proven. The calculated hydrogen bond structures are very stable, among which the hydrogen bond bonded with a hydroxyl group has higher stability. The nine complex structures’ hydrogen bonds were thought to represent closed-shell-type interactions. The interaction energy with 30O-31H on the epicatechin benzene ring is the strongest in the hydrogen bond structure. While the other hydrogen bonds were weak in strength and mostly had an electrostatic nature, the hydrogen bonds between the oxygen atoms in H₂O or CH₂OH and the hydrogen atoms of the hydroxyl groups in epicatechin were of moderate strength and had a covalent character. Comparing the changes in the hydrogen bond structure vibration peak, the main change in concentration peak is the hydrogen bond vibration peak in the complex. Improved the study on the hydrogen bond properties of CH₂OH and H₂O of EC.     

Metal complexes of a phenol-tailed porphyrin with different hydrogen bonds

Hydrogen bonds are very common and important interactions in biological systems, they are used to control the microenvironment around metal centers. It is a challenge to develop appropriate models for studying hydrogen bonds. We have synthesized two metal complexes of the phenol-tailed porphyrin, [Zn(HL)] and [Fe(HL)(C₆H₄(OH)(O))]. X-ray crystallography reveals that the porphyrin functions as a dianion HL^2? and the phenol OH is involved in hydrogen bonds in both structures. In [Zn(HL)], an intramolecular hydrogen bond is formed between the carbonyl oxygen and OH. In [Fe(HL)(C₆H₄(OH)(O))], the unligated O(5) of the ligand is involved in two hydrogen bonds, as a hydrogen bond donor and a hydrogen bond acceptor. The overall electronic effect on the ligand could be very small, with negligible impact on the structure and the spin state of iron(III). The structural differences caused by the hydrogen bonds are also discussed.