Pets & Animals

Ab initio and density functional study of the conformational space of 1C4 ALFA -L-fucose

Description
Ab initio and density functional study of the conformational space of 1C4 ALFA -L-fucose
Categories
Published
of 13
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  — —    Ab Initio   and Density Functional Study of the Conformational Space of   1 C 4  - L -Fucose ´ ´ ´ GABOR I. CSONKA *  and KRISZTINA ELIAS Department of Inorganic Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary IMRE G. CSIZMADIA Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, CanadaReceived 9 February 1996; accepted 26 April 1996 ABSTRACT The conformational space of   1 C   - L -fucose was searched by the MM2 * -SUMM 4 molecular mechanics conformationalsearch technique .  The molecular geometriesof the first 17 structures of lowest energy were analyzed at the HF  3  21G, Ž . Ž . 6  31G  d  , and generalized gradient approximation GGA DFT levels of theory .   1997 by John Wiley & Sons, Inc . Introduction he orientations of carbonhydrate hydroxyls T affect the reactivity of sugars in glycosylationreactions . 1,2 The reliable prediction of the orienta-tions of these hydroxyl groups would greatly helpinvestigators and would lead to a better under-standing of the molecular recognition of sugar Ž molecules e . g . , in cell adhesion, metastasis, fertil- . ization, or embryonic development , and carbohy-drate-receptor binding . 3, 5 The numerous inter- andintramolecular OH  O interactions in sugars andpolysaccarides make these analyses extremelycomplicated .  According to multidimensional con- * Author to whom all correspondence should be addressed . formational analysis, the threefold rotation of   n hydroxyl groups, in principle, can generate 3 n dif-ferent conformers .  The    and     anomers and Ž aldopyranosyl ring puckering two nonequivalent,chairlike  4 C and  1 C , 6 skew and skew-boat forms 71 4 . should also be considered leads to additionalstructures .  This complexity makes the sugars andoligosaccharides excellent information encodersthat may be decoded by a given receptor duringthe biomolecular binding process .  This process isentropically unfavorable due to the decrease of rotational and translational entropy . 8  10 To esti-mate the magnitude of this entropy decrease, theentropy of the unbinded carbohydrates would beknown . The conformation of carbohydrates in solutioncan be established by the combination of NMR ( )Journal of Computational Chemistry, Vol. 18, No. 3, 330  342 1996  1997 by John Wiley & Sons CCC 0192-8651 /97 /030330-13  CONFORMATIONAL SPACE spectroscopy and molecular mechanics and dy-namics techniques . 11 However, there are seriousproblems with the force-field calculations that arisefrom the insufficient quality parameter sets forsaccharides . 12 The comprehensive, high-quality  abinitio  calculations may help to point out the prob-lems with the force fields . The  L -fucose group is an important building block of polysaccharides . 13 It has a sufficientlysimple structure; consequently, its hydroxyl inter-actions can be studied at a rather high level of theory .  The conformational space for  1 C   - L - 4 fucose is shown in Figure 1 .  Note that only thecarbon atoms are numbered .  These same numberswill be used to denote the oxygen atoms attachedto those carbons throughout this article .  Followingconventional notation, the idealized dihedral an-gles are designated by g  ,  t , and  g   for  gauche Ž . Ž . clockwise 60   ,  anti  180   , and  gauche  counter- Ž . clockwise   60   , respectively, for the C—C—O—H torsions .  It should be noted that the  anti position is denoted by the letter ‘‘t’’ and not by theletter ‘‘a,’’ because the latter is reserved for thenotation of the axial positions of hydroxyls insugars . For aldopyranohexoses, the threefold rotationalof the five hydroxyl groups and the C—C bond of the hydroxymethyl group, in principle, can gener- FIGURE 1.  Schematic representation of 81 possibleminimum energy rotamers of   1 C   - L -fucose. The 4 ( )idealized C  x  +1   C  x   O  H torsions are denoted by( ) ( )g+, t, and g  for  gauche  clockwise 60   ,  anti   180   ,( )and  gauche  counterclockwise   60   , respectively,where  x  =1, 2, 3, 4. ate 729 different conformers 14,15 for a given anomerand chair form .  Our previous study showed thatthe OH  O interaction between one of the ringhydroxyl groups and the hydroxymethyl groupdiffers considerably from the three interactions be-tween the four ring hydroxyl groups . 15 Moreover,the hydroxymethyl group rotates more freely thanthe ring hydroxyl groups . 15 In   - L -fucose, the lackof a hydroxymethyl group considerably reducesthe possible number of minima on the potential Ž . energy hypersurface PEH  .  The remaining fourhydroxyl groups provide 81 possible minima on Ž . the PEH cf  .  Fig .  1  . A proton NMR investigation of 23 monosac-charides 16 showed that intramolecular OH  O in-teractions influence the chemical shifts in sugaranions and explains the differences between thep K   values of the anomeric hydroxyl groups .  Also,NMR signals of the equatorial hydroxyls are influ-enced by the equatorial or axial positions of theirneighbors .  A semiquantitative description of thetorsions of the H—C—O—H bonds was also at-tempted using the vicinal and long-range couplingconstants . 16 The smaller magnitude of the vicinalcoupling constants of the axial hydroxyls was at-tributed to an assumed gauche H—C—O—H di- Ž . hedral angle   44   .  The larger vicinal couplingconstants of the equatorial hydroxyls were at-tributed to partially  anti  H—C—O—H dihedralangles .  For   - D , L -fucose, the H—C1—O—H andH—C4—O—H dihedral angles were assumed to be  44   and  45  , respectively . 16 In the present article, the conformational spaceof   1 C   - L -fucose is searched by the MM2 * -SUMM 4 method . 17 We analyzed the first 17 low-energystructures at the MM2, 18 AM1 19 , PM3 20 , HF  3  21G, Ž . 6  31G  d  , and generalized gradient approximation Ž . GGA DFT levels of theory .  We are looking for thelowest level of the theory that provides chemicallyuseful accuracy .  We will compare the results of thepresent study to the results obtained by high-levelmethods for 1,2-ethanediol . 21 The predicted dihe-dral angles can be directly compared to the protonNMR results . 16 Computational Methods The search for stable rotamers in the conforma-tional space of   1 C   - L -fucose was carried out 4 using the MacroModel 4 . 5 program package . 22 TheMM2 *  force field available in MacroModel has been used .  It differs from the srcinal MM2 force JOURNAL OF COMPUTATIONAL CHEMISTRY   331  ´ ´ CSONKA, ELIAS, AND CSIZMADIA field 18 only in that it employs the point-chargeCoulomb potential to describe the electronic elec-trostatic interactions . Theconformationalspacewassearched using a particularly efficient systematicunbounded multiple minimum search technique Ž . 17 SUMM that is available in MacroModel .  Theresulting 415 rotamers were minimized to yield 17unique rotamers within an energy window of 40kJ  mol above the global minimum .  Geometry op-timizations were carried out with a truncated Ž .  23 Newton conjugate gradient TNCG techniquewith the maximal number of iterations set to 200and using a convergence criterion of 0 . 01 for thegradient norm .  The global minimum was found 39times . The 17 minima obtained by the MM2 * -SUMMsearch were further optimized by AM1, PM3, HF, Ž . and generalized gradient approximation GGA Ž . and hybrid density functional DFT methods us-ing the Berny algorithm combined with redundantinternal coordinates built into the GAUSSIAN 94program . 24 We employed the following combinations of theGGA  DFT functionals: 1.  The  BP  or Becke  Perdew method, in whichBecke’s exchange functional 25 is combinedwith Perdew’s correlation functional . 26 2.  The  B3P  hybrid method .  B3P is a linear com- bination of various exchange and correla-tional functionals in the form:    Ž .       A    E  HF    1   A    E  S   B    E  B x x x     E  VWN   C    E  P86 c c       where  E  HF ,  E  S , and   E  B are the HF, x x x Slater, and Becke exchange functionals; and     E  VWN and   E  86 are the Vosko, Wilk, c c and Nussair 27 and Perdew 26 correlation   functionals, respectively .  Note that   E  B is x a gradient correction to the S  WVN or   LSDA for exchange, and   E  P86 is a gradi- c ent correction for correlation . The constants  A ,  B , and  C  are those deter-mined by Becke by fitting heats of formations Ž .  28  A  0 . 2,  B  0 . 72,  C  0 . 81  .  Note that Ž . Becke used the Perdew  Wang PW91 func-tional instead of P86 . 28 3.  The  B3LYP  hybrid method .  It is a logicalextension of Becke’s three-parameter concept Ž using different correlational functionals e . g . , . LYP in the form:    Ž .       A    E  HF    1   A    E  S   B    E  B x x x Ž .       1  C    E  VWN   C    E  LYP c c The constants  A ,  B , and  C  are selected to beequal to those determined by Becke for theB3P method . 26 GAUSSIAN 94 24 uses numerical quadrature toevaluate the DFT integrals .  The quadrature schemeis defined by the number of points in the radialand angular directions .  The geometries were opti-mized with a fine-pruned grid having 75 radialshells and 302 angular points per shell that re-sulted in about 7000 points per atom . The HF and GGA  DFT calculations were car- 29 Ž . Ž . ried out using 3  21G, 6  31G  d  , 6  31  G  d ,  p  , Ž . 30 and 6  311  G  d ,  p  basis sets . Energetics RELATIVE STABILITIES Ž . The MM2 * , HF, GGA  DFT total energies  E  , Ž . energy differences   E  , and corrected HF  Ž . Ž . 6  31F  d  zero-point energy differences   ZPE of the stable rotamers of   1 C   - L -fucose that are pro- 4 vided by the MM2 * -SUMM search within a 40-kJ  mol energy window are summarized in TableI .  The orientations of the four hydroxyl groups arenotated by t, g  , or g   corresponding to thenotation in Figure 1  .  This provides a convenientline notation for the orientation of four hydroxylgroups . 14,15 It should be noted that, due to theinteractions, the C—C—OH dihedral angles are Ž usually far from their idealized values e . g . , 180  . for t, 60   g   .  The results listed in Table I showqualitatively that the orientations of the hydroxylgroups are not independent of each other .  In themost stable conformation, the number of possibleOH  O interactions is maximal, which leads tothe formation of an intramolecular chain of hy-droxyl groups .  The formation of these chains leadsto counterclockwise or clockwise patterns as Ž . viewed from above the pyranose ring cf  .  Fig .  1  . The C1—O1—H group may interact with the C2—O2—H group in the clockwise direction .  The C2—O2—H group may interact with the C1—O1—Hgroup in the counterclockwise direction with theC3—O3—H group in the clockwise direction .  TheC3—O3—H group is in an exactly analogous situ-ation, whereas C4—O4—H may interact with theC3—O3—H group in the counterclockwise direc- VOL. 18, NO. 3 332  CONFORMATIONAL SPACE tion or with the endocyclic oxygen in the clockwisedirection .  The example for the counterclockwise Ž . Ž . Ž arrangement is the t a g  t t a conformation first . row in Table I  .  The example for the clockwise Ž . Ž . arrangement is the g   a g  g  g   a confor- Ž . mation second row in Table I  .  While conforma-tions 1, 2, and 3 shown in Table I have simpleunidirected chains of OH interactions, conforma-tions 5 and 6 have two lone pairs of OH oxygens Ž . in the middle O2 or O3 interacting with two Ž . hydrogen atoms at both sides non-unidirected  . This concentration of interactions results in some-what less stable rotamers of   1 C   - L -fucose . 4 The above-mentioned interactions dramaticallyreduce the number of possible rotamers and clearlyshow strong coupling between the hydroxylgroups .  In this way, the rotational entropy of thehydroxyl groups is decreased .  Similar undirectedpatterns were found earlier by French et al . 31 usingmolecular modeling techniques, by Cramer andTruhlar using AM1 and PM3 methods, 14 and byPolavarapu et al . 32 at the HF  4-31G level of theoryfor  D -glucose . ERROR SOURCES Previous articles report that the PM3 methodprovides improved description of the intermolecu-lar hydrogen bonding . 33,34 However, for theintramolecular OH    O interaction in 1,2-ethanediol, the AM1 and PM3 methods fail tosupply good quality results . 35 Our results supportthe failure of AM1 and PM3 methods for therotamers of   1 C   - L -fucose .  The AM1 and PM3 4 methods are unable to provide the correct minimaand energetic order .  For example, rotamers 3, 4, 8,9, and 13  17 in Table I are missing from the AM1and PM3 conformational space and some new   Ž . Ž . Ž . Ž . Ž . minima e . g . , g   a g  t t a , g   a t t g  a , or t a Ž .  g  t g  a appear .  We checked these rotamers bythe HF  3  21G method and they were not pre-dicted to be stationary .  The AM1 and PM3 meth-ods predict rotamer 10 to be the most stable andthe energetic order of the remaining conformers iscompletely intermingled relative to higher levelmethods .  Similar discrepancies were experiencedearlier for the hydroxyl rotamers of the   - D -glu-cose 14 and for the relative stability of alternativechair forms of     - D -glucose . 36 The MM2 *  rotamers 13, 15, and 16, in Table I,which are not predicted to be stationary by the HFand GGA  DFT methods, converge smoothly toone of the other 14 rotamers, which are predicted Ž . to be minima cf  .  Table I  .  Earlier results for 1,2-ethanediol 20,37 show that the so-called tGt confor-mation of 1,2-ethanediol, which corresponds to the Ž . t g   a conformation of hydroxyl groups 3 and 4 in 1 C   - L -fucose, is predicted to be in high-energy 4 state .  In this specific conformation the two hy-droxyl groups are in the  anti  position and a lonepair  lone pair repulsive interaction occurs .  Thisconformationis necessarily unstable and the higherlevel methods clearly support this view .  The MM2 * method fails to reproduce this effect correctly . Our recent study of 1,2-ethanediol 21 also showedthat the MM2 *  force field does not provide quanti-tative agreement with the higher level calculations   Ž .  e . g . , MP2, CCSD t for the OH  O interactions . Ž . The HF  3  21G, 6  31G  d  , and GGA  DFT resultsin Table I show clearly the differences between theenergy ordering .  The HF  3  21G method provides Ž . larger relative energies    E  , whereas the Ž . HF  6  31G  d  method provides smaller   E  valuesfor the various conformers .  For these basis sets, the basis set dependence of the   E  values at the HF Ž . level is considerable   30 % . The GGA  DFT methods introduce some elec-tron correlation effects .  The BP and B3P   E  values Ž are frequently between the HF values:   E  HF  3  . Ž Ž .. Ž Ž .. 21G    E  BP  6  31G  d    E  HF  6  31G  d Ž . cf  .  conformations 3, 4, 5, 6, 7, 9, and 12 in Table I  . However, there are several conformations, namely2, 8, 10, 11, 14, and 17, for which the BP methodprovides considerable stabilization .  The common Ž . feature in these conformations is the g  a hy-droxyl at the C4 atom .  Further analysis shows that Ž . this g  a hydroxyl group interacts with the ringoxygen; thus, the inclusion of the electron correla-tion is necessary to better recover this effect .  Moredetails will be given in the following section .  Forall the other conformations, the HF, BP, B3P, and Ž . B3LYP  6  31G  d  results agree well with eachother .  For a more convenient overview, Figure 2shows the energy differences in graphic form .  The Ž . BP  6  31G  d  points shown are connected by a Ž . line .  The extra stability caused by the g   a posi-tion of the fourth hydroxyl group is easily seenfrom the Figure for the conformations, 2, 8, 10, 11,14, and 17 . The results in Table I show considerable agree-ment for the relative energies calculated with vari-ous DFT methods .  The largest difference is 0 . 6kcal  mol, and differences below 0 . 1 kcal  mol are Ž . not rare cf  .  conformers 3, 4, and 9 in Table I  .  Theenergetic effects of the basis set extension from JOURNAL OF COMPUTATIONAL CHEMISTRY   333  ´ ´ CSONKA, ELIAS, AND CSIZMADIA      T     A     B     L     E     I .      (     )     (     )     (     )     L     i    n    e     N    o     t    a     t     i    o    n    s ,     M     M     2     * ,     H     F    a    n     d     G     G     A  –     D     F     T     T    o     t    a     l     E    n    e    r    g     i    e    s ,    r    e     l    a     t     i    v    e    e    n    r    g     i    e    s         ,    a    n     d     R    e     l    a     t     i    v    e     Z    e    r    o   -     P    o     i    n     t     E    n    e    r    g     i    e    s             Z     P     E     f    o    r     S     t    a     b     l    e     R    o     t    a    m    e    r    s       1     o     f     C         -        L    -     F    u    c    o    s    e     F    o    u    n     d     b    y     M     M     2     *  –     S     U     M     M     S    e    a    r    c     h .       4      T    o    r    s     i    o    n  a      (     )     (     )     (     )     (     )    a    n    g     l    e    s     M     M     2     *     H     F     /     3          2     1     G     H     F     /     6          3     1     G        d      B     P     /     6          3     1     G        d      B     3     P     /     6          3     1     G        d      B     3     L     Y     P     /     6          3     1     G        d      b    c     d    c     d    c    c     d    c     d    c     d    c      (     )     (     )     N    o .     1    a     2     3     4    a        E               E       E               E       E               E             Z     P     E        E               E       E               E       E               E      1     t    g          t     t          9     7 .     0     7     0 .     0     0          6     0     5 .     1     2     7     0     9     0 .     0     0          6     0     8 .     4     8     4     5     0     0 .     0     0     0 .     0     0          6     1     1 .     9     4     8     1     9     0 .     0     0          6     1     3 .     5     3     6     4     0     0 .     0     0          6     1     1 .     9     4     5     7     5     0 .     0     0     2    g    +    g    +    g    +    g    +          9     1 .     7     8     1 .     2     6          6     0     5 .     1     2     3     7     5     2 .     1     0          6     0     8 .     4     8     1     4     4     1 .     9     2     0 .     0     0          6     1     1 .     9     4     8     0     6     0 .     0     8          6     1     3 .     5     3     5     5     9     0 .     5     1          6     1     1 .     9     4     7     5     7     0 .     6     9     3    g    +    g    +    g    +    g    +          8     9 .     9     0     1 .     7     1          6     0     5 .     1     2     1     3     9     3 .     5     8          6     0     8 .     4     8     0     1     7     2 .     7     1          0 .     1     6          6     1     1 .     9     4     4     2     7     2 .     4     7          6     1     3 .     5     3     2     4     7     2 .     4     7          6     1     1 .     9     4     2     8     3     2 .     5     2     4     t     t     t     t          8     9 .     3     0     1 .     8     6          6     0     5 .     1     1     9     7     3     4 .     6     2          6     0     8 .     4     7     9     0     0     3 .     4     5          0 .     3     1          6     1     1 .     9     4     1     6     8     4 .     0     9          6     1     3 .     5     2     9     9     8     4 .     0     3          6     1     1 .     9     4     0     4     4     4 .     0     2     5    g    +    g    +    g          t          8     8 .     5     0     2 .     0     5          6     0     5 .     1     1     8     8     2     5 .     1     9          6     0     8 .     4     7     8     5     2     3 .     7     5          6     1     1 .     9     4     2     1     8     3 .     7     7     6    g    +     t     t     t          8     8 .     3     0     2 .     1     0          6     0     5 .     1     1     9     0     5     5 .     0     4          6     0     8 .     4     7     8     7     7     3 .     5     9          6     1     1 .     9     4     2     4     4     3 .     6     1     7     t    g    +    g          t          8     6 .     8     8     2 .     4     4          6     0     5 .     1     1     7     0     0     6 .     3     3          6     0     8 .     4     7     6     9     1     4 .     7     6          6     1     1 .     9     4     0     1     0     5 .     0     8     8     t    g    +    g    +    g    +          8     6 .     0     1     2 .     6     5          6     0     5 .     1     1     9     1     8     4 .     9     6          6     0     8 .     4     7     8     4     8     3 .     7     8          6     1     1 .     9     4     4     7     0     2 .     1     9          6     1     3 .     5     3     2     1     1     2 .     6     9     9     t    g    +    g    +    g              8     5 .     8     7     2 .     6     8          6     0     5 .     1     1     7     6     3     5 .     9     4          6     0     8 .     4     7     7     2     8     4 .     5     3          6     1     1 .     9     4     0     9     2     4 .     5     7          6     1     3 .     5     2     9     0     6     4 .     6     1     1     0     t    g         g    +    g    +          8     1 .     7     0     3 .     6     8          6     0     5 .     1     1     9     4     9     4 .     7     7          6     0     8 .     4     7     8     2     4     3 .     9     3          6     1     1 .     9     4     4     3     8     2 .     4     0     1     1     t    g          t    g    +          8     1 .     2     0     3 .     8     0          6     0     5 .     1     1     7     5     7     5 .     9     7          6     0     8 .     4     7     7     0     3     4 .     6     9          6     1     1 .     9     4     2     8     7     3 .     3     4     1     2     t    g         g    +    g             8     0 .     8     9     3 .     8     7          6     0     5 .     1     1     7     5     4     5 .     9     9          6     0     8 .     4     7     7     2     7     4 .     5     4          6     1     1 .     9     4     0     6     3     4 .     7     5     e      1     3     t    g          t    g             8     0 .     2     7     4 .     0     2           1     1     4    g    +     t     t    g    +          7     4 .     8     9     5 .     3     1          6     0     5 .     1     1     1     6     5     9 .     6     9          6     0     8 .     4     7     2     3     2     7 .     6     4          6     1     1 .     9     3     9     0     1     5 .     7     6     e      1     5    g    +     t     t    g             7     3 .     2     3     5 .     7     0           6  e      1     6     t     t     t    g             7     3 .     2     3     5 .     7     0           4     1     7     t     t     t    g    +          7     2 .     2     6     5 .     9     3          6     0     5 .     1     0     8     9     2     1     1 .     4     0          6     0     8 .     4     7     1     2     5     8 .     3     1          6     1     1 .     9     3     6     0     7     7 .     6     1     a      T     h    e     h    y     d    r    o    x    y     l    g    r    o    u    p    s    a    r    e    n    u    m     b    e    r    e     d    a    c    c    o    r     d     i    n    g     t    o     F     i    g    u    r    e     1    a    n     d     t     h    e     t ,    g    + ,    a    n     d    g         s    y    m     b    o     l    s    a    r    e    u    s    e     d     t    o    n    o     t    a     t    e     t     h    e     i     d    e    a     l     i    z    e     d    p    o    s     i     t     i    o    n    o     f     t     h    e     h    y     d    r    o    x    y     l     h    y     d    r    o    g    e    n .     T     h    e    a    x     i    a     l    p    o    s     i     t     i    o    n     (     )      b    c     a     i    s    s     h    o    w     f    o    r     t     h    e     f     i    r    s     t    a    n     d     f    o    u    r     t     h     h    y     d    r    o    x    y     l    g    r    o    u    p    s .     V    e    r     t     i    c    a     l     b    a    r    s     d    e    n    o     t    e     t     h    e     b    r    e    a     k    o     f     t     h    e    c     h    a     i    n .     K     i     l    o     j    o    u     l    e    s    p    e    r    m    o     l    e .     K     i     l    o    c    a     l    o    r     i    e    s    p    e    r    m    o     l    e ,    z    e    r    o  -    p    o     i    n     t    v     i     b    r    a     t     i    o    n    a     l    e    n    e    r    g     i    e    s    w    e    r    e     (     )      d    e     c    a     l    c    u     l    a     t    e     d     f    r    o    m     h    a    r    m    o    n     i    c    v     i     b    r    a     t     i    o    n    a     l     f    r    e    q    u    e    n    c     i    e    s     d    e     t    e    r    m     i    n    e     d    a     t     H     F     /     6          3     1     G     d     l    e    v    e     l    a    n     d    s    c    a     l    e     d     b    y    a    c    o    m    m    o    n     l    y    u    s    e     d     f    a    c     t    o    r    o     f     0 .     8     9 .     H    a    r     t    r    e    e .     U    n    s    u    c    c    e    s    s     f    u     l    a     t     t    e    m    p     t    s     t    o     f     i    n     d     t     h    e    g     i    v    e    n    c    o    n     f    o    r    m    e    r .     T     h    e    n    u    m     b    e    r    a     f     t    e    r     t     h    e    a    r    r    o    w    s     h    o    w    s     t     h    e     f     i    n    a     l    c    o    n     f    o    r    m    a     t     i    o    n    r    e    a    c     h    e     d    a     f     t    e    r     t     h    e    g    e    o    m    e     t    r    y    o    p     t     i    m     i    z    a     t     i    o    n . VOL. 18, NO. 3 334
Search
Similar documents
View more...
Tags
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks