J. Phys. Chem. 1987, 91, 4466-4470
4466
monk approximation, there is a good agreement with the Raffenetti calculation for the ClH-NH, complex. The S C F C I calculation of Brciz et al.' provides a value of 2606 cm-I for the uCIHtransition close to the value 2550 cm-' of Raffenetti. The anharmonicity effects, however, are quite important because they decrease this transition by an additional 357 cm-l. Another comparison is instructive. In the harmonic approximation, the SCF calculation gives a value of 2888 cm-' for the uCIHtransition of the ClH-NH, complex. The comparison with the value 2249 cm-' of the S C F C I calculation which includes the anharmonicity effects in the vibrational study shows that both electronic correlation effects and anharmonicity effects are responsible for a decrease of nearly 640 cm-' for this transition. At the level of experimental study the infrared spectra of ClH-NH, complex has been recorded for the first time by Ault and Pimentel12 in a nitrogen matrix at low temperature. More recently several author^'^-'^ have shown that the assignment of the infrared spectra is strongly dependent on the nature of the matrix. For the uCIH transition, the present result, which disregards the environmental effects cannot be compared to the experimental . . ~ stretch, however, seems to be less data. The v ~ ~symmetric sensitive to the matrix effects and the present values of 162 and 201 cm-' calculated a t the SCF and S C F C I levels may be compared to the values 166 and 130 cm-' recorded in Ar and N2 matrices, r e ~ p e c t i v e l y . ~A~ ~similar '~ analysis stands for both C1H-NH2CH3 and BrH-NH3 complexes. For BrH-NH,, the uBrHWNsymmetric stretch has been recently assigned at 110 cm-I by subtraction from an N H 3 rocking mode.16 The corresponding
+
+
+
(12) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1972, 77, 1649. (13) Schriver, L.: Schriver, A,; Perchard, J. P. J . Am. Chem. SOC.1983, 105, 3843. (14) Barnes, A. J.; Beech, T. R.; Mielke, Z. J . Chem. Soc., Faraday Trans. 2 1984. 80. 455. (15) Barnes, A. J.; Kuzniarski, J. N. S.; Mielke, Z. J. Chem. SOC.,Faraday Trans. 2 1984, 80, 465. (16) Barnes, A. J.; Wright, M. P.J . Chem. SOC.,Faraday Trans. 2 1986, 82, 165.
value, 144 cm-I, calculated in the present work confirms this assignment. The mechanical anharmonicity of this symmetric stretch is not very large because the first calculated overtone occurs at 297 cm-'. The present S C F calculation presumably overestimates this overtone mode by 30-50 cm-I. As a consequence, it is not obvious that the band located at 302 cm-I may readily be assigned to this overtone which may be a rocking or a bending mode. The final assignment of the uC1H-N symmetric stretching mode of methylamine-hydrogen chloride complex has not yet been made. The fundamental and the first overtone transitions have been calculated at 144 and 302 cm-I, respectively. Taking into account the overestimation due to the S C F calculation, this symmetric stretching mode is expected to lie in the range of 110-120 cm-'. On the other hand, the 220-cm-' band possibly corresponds to this overtone. Conclusion
It has been first c o n f i i e d that the differences observed between the S C F and S C F C I levels of calculation for the ClH-NH, complex are substantial and lead to a strong reduction of the vXH transition value. The second relevant point is the inadequacy of the harmonic approximation in describing the vibrational properties of such medium-strength hydrogen-bonded systems well. In Table IV, the differences calculated between the uXH transitions taking into account the whole set of coefficients of the potential energy and the um transitions in the harmonic approximation demonstrate that only a variational method leads to a correct description of coupled anharmonic vibrational modes. In most of the cases treated here, the perturbation method, even at the second order, is certainly not convergent.
+
Acknowledgment. Y.B. and C.M. are indebted to the "Centre de calcul d'Orsay" for allowing them to make use of facilities on N A S 9080 and IBM 3090/200. Registry No. HC1, 7647-01-0; HBr, 10035-10-6; NH3, 7664-41-7; NHZCH,, 74-89-5; Dz,7782-39-0.
Calculated Circular Dichroism of the n-r* Transition In N-Acetylglucosamines Aaron H. Coben and Eugene S. Stevens* Department of Chemistry, University Center at Binghamton. State University of New York. Binghamton. New York 13901 (Received: January 16, 1987; In Final Form: May 4 , 1987)
The rotational strength of the amide n-r* transition of 2-acetamido-2-deoxy-~-glucopyranoses is calculated as a function of CI hydroxyl, C3 hydroxyl, and acetamido group orientations. The results explain features of the circular dichroism (CD) solvent dependence observed with a-GlcNAc-OMe and 0-GlcNAc-OMe, including a strong anomeric effect. The results are also relevant to the CD of glycosaminoglycans.
Introduction
Beychok and Kabat first reported carbohydrate Cotton effects near 220 nm and assigned them to the n-r* transition of acetamido gr0ups.l That assignment has been supported in subsequent work.2,3 In an early theoretical treatment of the circular dichroism (CD)4 of acetamido sugars it was shown that a major source of optical activity was coupling of the n-r* transition with the strong acetamido r-r* transition near 190 nm.j
There are now three reasons for reexamining and extending that early theoretical study. First, the solution properties of polysaccharides in general, and glycosaminoglycans in particular, have attracted increased attention as appreciation for their biological significance has Circular dichroism has proved a sensitive probe of polysaccharide conformation8 and the n-r* C D of glycosaminoglycans has been noted to display substantial ~ a r i a b i l i t y , ~but - ' ~the significance of that variability will remain ( 5 ) Yeh, C.-Y.; Bush, C. A. J. Phys. Chem. 1974, 78, 1829-1833.
(1) Beychok, S.; Kabat, E. A. Biochemistry 1965, 4 , 2565-2574. (2) Stone, A. L. Biopolymers 1971, 10, 739-751. (3) Coduti, P. L.; Gordon, E. C.; Bush, C. A. Anal. Biochem. 1977, 78,
9-20. (4) Abbreviations used are a-GlcNAc, 2-acetamido-2-deoxy-a-~-glucopyranose; 8-GlcNAc, 2-acetamido-2-deoxy-~-~-glucopyranose; a-GlcNAcOMe, Omethyl-2-acetamidc-2-deoxy-a-~glucopyraide; p - G l c N A d M e , O-methyl-2-acetamido2-deoxy-~-~-glucopyranoside: CD, circular dichroism.
0022-3654/87/2091-4466$01 .50/0
(6) Solution Properties of Polysaccharides; Brant, D. A,, Ed.: ACS Symposium Series 150; American Chemical Society: Washington, DC, 1981. (7) Molecular Biophysics of the Extracellular Matrix; Arnott, S., Rees, D. A,, Morris, E. R.,Eds.; Humana: Clifton, NJ, 1984. (8) Morris, E. R.,Frangou, S. A. In Techniques in Carbohydrate Metabolism; Elsevier: London, 1981; Vol. B308, pp 109-160. (9) Cowman, M. K.; Bush, C. A.; Balazs, E. A. Biopolymers 1983, 22, 1319-1 334.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4467
n-a* C D of N-Acetylglucosamines
unclear without some understanding of the fundamental origin of the CD and its dependence on conformational features. Second, the original model calculations implied that, for 8-anomers (e.g., 0-GlcNAc), the C D depends strongly on the orientation of the C3 hydroxyl group but not at all on the Cl hydroxyl group, which is surprising, considering the relatively symmetrical placement of those equatorial groups with respect to the C2 acetamido group. Third, a recent extensive study by Buffington et al." of the C D solvent dependence of a-GlcNAc-OMe and 0-GlcNAc-OMe provides a wealth of new experimental data which can be examined in light of an extended theoretical model.
H O\,i H
Calculational Methods
Ab initio calculations on molecules as large and flexible as acetamido sugars are still not practical in spite of the significant progress in applying nonempirical methods to calculating the C D of somewhat smaller molecules.12 In choosing a semiempirical model of C D calculations one has first to consider whether any one of three fundamental mechanisms, Le., "static field", B-m, and p-p,l3-I5 is likely to be dominant, or alternatively one must evaluate the contributions from all three. The amide n-r* transition has been studied extensively from this point of view, largely with reference to peptide and polypeptide CDI6l8 but also for the case of acetamido sugars.19 The conclusion has consistently been that the static field mechanism is dominant, apparently on account of the very small oscillator strength of the n-r* transition and the availability of a strong electric-dipole-allowed transition (.-a*) nearby in energy with which to couple. We therefore restrict our attention to this mechanism in the present model. The static field mechanism for amide optical activity has often been described in detail. Specifically, the rotational strength, &, of the n-a* transition at 210 nm arising from static field mixing with the strong a-a* transition near 190 nm (transition ob), is given Roa
= -[Voa,ob/(Eob
- Eoa)] Im pob'mao
O\
\
H Figure 1. Schematic drawing of the &anomer of 2-acetamido-2-deoxyo-glucopyranose.
a+o.lol Y
(1) -0.20 0 0 80
where m and ~1 denote magnetic and electric dipole transition moments and Eoband Eo, are transition energies. We use the experimentally measured value of pdm and a CNDO/S calculation of ma: to give Im pob'mao = 2.0 D pB. In Schellman's method16J8of calculating Voa,&,it is represented as the electrostatic interaction of that part of the n-a* electric quadrupole transition moment in a plane perpendicular to the amide C O bond, with the permanent partial charges, q,, of all atoms other than those of the amide group.5 For a coordinate system with the origin at the amide oxygen atom, x in the direction of thz OC bond, and y in the O C N plane such that t h e y coordinate of the nitrogen atom is positive, Voa,ob becomes, in atomic units
where f is the Slater exponent for oxygen (2.275), y i and zi are the coordinates of the atom having partial charge qi, r, is the distance of partial charge q,, from the amide oxygen atom, and the summation is over partial charges. The result is where Ro, is in D
P "
H
pB.5
(10) Cziner, D. G.; Stevens, E. S.;Moms, E. R.;Rees, D.A. J. Am. Chem. SOC.1986, 108, 3790-3795. (1 1) Buffington, L.; Hamilton, C.; Knowles, J.; Lodewick, P.; McAllister. L.; Neidhart, D., submitted for publication. (12) Bohan, S.; Bouman, T. D. J. Am. Chem. Soc. 1986,108,3261-3266. (13) Kirkwood, J. G. J . Chem. Phys. 1937, 5, 479-491. (14) Kirkwood, J. G. J. Chem. Phys. 1939, 7, 139. (15) Schellman, J. A. Accts. Chem. Res. 1968, I , 144-151. (16) Schellman, J. A.; Oriel, P. J . Chem. Phys. 1962, 37, 2114-2124. (17) Wpody, R. W.; Tinoco, I. J. Chem. Phys. 1967, 46, 4927-4945. (18) Stigter, D.; Schellman, J. A. J. Chem. Phys. 1969, 52, 3397-3403. (19) Yeh, C. Y.Ph.D. Thesis, Illinois Institute of Technology, 1973. (20) Peterson, D. L.;Simpson, W. T. J . Am. SOC.1955, 77, 3929-3930; 1957 79, 2375-2382.
C,-OH
120 .
180 1
240 5 300
L
ROTATION (DEG)
Figure 2. 'Calculated n-r* rotational strength of 2-acetamido-2-deoxya-D-glUCOpyranOSeas a function of rotation of the C3 hydroxyl group (calculated at 10' increments), for several orientations of the acetamido group. Dashed portions of the curve indicate sterically unfavored conformations.
Atomic coordinates for a-GlcNAc were taken from the X-ray study of Johnson2' with the exception that the C(3)OH bond angle was increased from the reported value of 79O to a value of 1loo. Coordinates for 0-GlcNAc were calculated from those of the a-anomer. Evaluation of eq 2 and 3 was then carried out as a function of rotation of the Cl and C3 hydroxyl groups about the corresponding OC bond, and as a function of acetamido group rotation about the NC(2) bond (Figure 1). The reference states for these rotations, corresponding to 0' rotations, are those in which O(1)H eclipses C ( l ) H , O(3)H eclipses C(3)H, and the amide C O group eclipses C(2)H; Le., in which the amide NH group is approximately trans to C(2)H. Positive values of rotation for the C(1)OH group correspond to clockwise rotation of the group as viewed from the oxygen atom toward C(1). Positive values of rotation for the C(3)OH group correspond to counterclockwise rotation of the group as viewed from the oxygen atom toward C(3). Positive values of rotation for the acetamido group refer to clockwise rotation of the group as viewed from the amide nitrogen toward C(2). Including acetamido group rotation leads to a description of substituent effects on n-a* C D which is qualitatively different from that of previous work. N M R data22-25indicate that the ~
~~~
(21) Johnson, L. N. Acta Crystallogr. 1966, 21, 885-891. Cartesian coordinates are generated from the unit cell dimensions (a, b, c, 8) by x = a - c(sin e), y = b, z = c(cos e), where 8 = fl - 90. (22) Cerezo, A. S . Chem. Ind. (London) 1971, 96-97. (23) Hirano, S. Agric. Biol. Chem. 1972, 36, 1071-1073.
4468
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
Cohen and Stevens o,,
0
, ,
/
1
v
120.
0 i-
a I
?
0 12
0
I
0
120
240
0
Figure 3. Contours of the calculated n-r* rotational strength of 2acetamido-2-deoxy-&~-glucopyranoseas a function of rotation of the C, and C3 hydroxyl groups for an acetamido group rotation of -20'. Contours were generated by linear interpolation from points calculated at 30' increments. Dashed lines indicate sterically unfavored conformations.
C,-OH
ROTATION (DEG)
Figure 4. Contours of the calculated n-n* rotational strength of 2acetamido-2-deoxy-@-~-glucopyranose as a function of rotation of the C , and C3 hydroxyl groups for an acetamido group rotation of 0'. Contours were generated by linear interpolation from points calculated at 30' increments.
acetamido group is oriented with the NH group approximately trans to C(2)H. The acetamido group was therefore rotated only within the range of *20°. Some of the considered rotations of the C3 hydroxyl group bring it into close contact with the amide oxygen atom. We used contact parametersz6 of 2.7 A for oxygen-oxygen pairs, and 2.3 A for oxygen-hydrogen pairs, to denote sterically unfavored conformations.
Results In a-GlcNAc the hydrogen atom of the axial C, hydroxyl group makes only a small contribution to the calculated n-r* rotational strength. Figure 2 shows results as a function of C3 hydroxyl group rotation, for several orientations of the acetamido group in the range *20°. Sterically unfavored conformations are indicated by the broken line. Figures 3-5 show results for &GlcNAc. When the acetamido group is rotated -10' or - 2 O O (i.e., toward C3), the calculated rotational strength is only slightly dependent on CI hydroxyl group orientation (Figure 3). Conversely, when the acetamido group is rotated +loo or +20° (Le., toward Cl), the results are only slightly dependent on C3 hydroxyl group orientation (Figure 5 ) . When the acetamido group is oriented with the amide CO bond eclipsing the C(2)H bond (Le., Oo rotation), the orientations of the C1 and C3 hydroxyl groups are both important determinants of rotational strength (Figure 4), as expected for the relatively high symmetry of the @-anomer in the region of the amide chromophore. Discussion In the earlier work5 the acetamido group was fixed in the orientation observed in the crystal geometry of a-GlcNAc, Le., (24) Schamper, T. J. Carbohydr. Res. 1974, 36, 233-237. (25) Bush, C. A.; Duben, A.; Ralapati, S.Biochemistry 1980,19,501-504. (26) Ramachandran, G. N.; Ramakrishnan, C.; Sasisekharan, V. J. Mol. Biol. 1963, 7, 95-99.
\
I
I
.
120
240
C,-OH ROTATION (DEG)
C,-OH ROTATION (DEG)
Figure 5. Contours of the calculated n-r* rotational strength of 2acetamido-2-deoxy-@~glucopyranoseas a function of rotation of the C, and C, hydroxyl groups for an acetamido group rotation of +20°. Contours were generated by linear interpolation from points calculated at 30' increments.
rotated -23O relative to an orientation in which the amide CO bond eclipses the C(2)H bond.21 The earlier results therefore represent a subset of our present results (see the curve labeled -2OO in Figure 2 for a-GlcNAc and Figure 3 for @-GlcNAc). For those cases our calculated rotational strengths are larger than those calculated previously, by approximately a factor of 2, on account of the larger C(3)OH bond angle we used (see above). A major motivation for the present work was to provide guidance in interpreting the variable n-r* C D observed in glyc o s a m i n o g l y c a n ~ .X-ray ~ ~ ~ ~diffraction studies indicate that in the solid-state structures of glycosaminoglycans the acetamido group is found within a wide range of orientation^,^'-^^ the extreme values of the range being -30' (by our convention) in compact hyaluronate structure^^*^^* and +30° in the 2-fold helical allomorph of dermatan sulfate.34 The present results show that the n-r* CD of acetamido sugars is expected to depend on acetamido orientation for both a-and p-anomers. Application of these results to specific glycosaminoglycans is left to future case-by-case analyses; application to the CD of keratan sulfate has been com~ l e t e d .The ~ ~ present extension of the earlier model thus provides an interpretive tool for relating glycosaminoglycan CD to the orientation of substituent acetamido groups. Such an application does not depend on the present model being quantitatively accurate. Calculated rotational strengths in the present formalism depend on the wave functions used to parametrize eq 1,l6-I8 and the simple zero-order functions used here cannot be expected to be highly accurate. The qualitative nature of the dependence of C D on orientation of the acetamido group and orientation of the C1and C3hydroxyl groups, on the other hand, are less dependent on parameterization. A second major result of considering explicitly the dependence of C D on acetamido group orientation is that the CD of the p-anomer has been found to be dependent mainly on the C3 hydroxyl group orientation only if the acetamido group is rotated toward C3, the orientation previously e ~ a m i n e d .If, ~ on the other hand, the acetamido group is rotated toward C,, the CD of the ~~
(27) Arnott, S.; Guss, J. M.; Hukins, D. W. L.; Dea, I. C. M., Rees, D. A. J. Mol. Biol. 1974, 88, 175-184. (28) Guss, J. M.; Hukins,D. W. L.; Smith, P. J. C.; Winter, W. T.; Amott, S.: Moorhouse. R.: Rees. D. A. J. Mol. Biol. 1975. 95. 359-384. (29) Winter, W: T.; Arnott, S.; Isaac, D. H.;Atkins, E. D. T. J. Mol. Biol. 1978, 125, 1-19. (30) Cad, J. J.; Winter, W. T.; Amott, S.J . Mol. Biol. 1978, 125, 21-42. (31) Mitra, A. K.; Arnott, S.; Sheehan, J. K. J. Mol. Biol. 1983, 169, '
813-827. .- . -..
(32) Mitra, A. K.; Raghunathan, S.;Sheehan, J. K.; Arnott, S. J . Mol. Biol. 1983, 169, 829-859. (33) Arnott, S.; Mitra, A. K.; Raghunathan, S. J . Mol. Biol. 1983, 169,
-
861-877 - - - . -.
(34) Mitra, A. K.; Amott, S.;Atkins, E. D. T.; Isaac, D. H. J. Mol. Biol.
1983, 169, 873-901.
(35) Millane, R. P.; Mitra, A. K.: Arnott, S. J. Mol. Biol. 1983, 169, 903-920. (36) Stevens, E. S.; Lin, B. Biochim. Biophys. Acta 1987, 924, 99-103. (37) Djerassi, C. Optical Rotatory Dispersion; McGraw-Hill: New York, 1960.
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4469
n-r* C D of N-Acetylglucosamines TABLE I:; , ]e[ for the n-r* CD Band of a-ClcNAc-OMe and 6-ClcNAc-OMe in Various Solvents"
2+o.10/ c)
Y
solvent
1,1,1,3,3,3-hexafluoro-2-p1 .opanol trifluoroethanol heptafluorobutanol acetonitrile dioxane water methanol ethanol ethylene glycol 3-methyl-2-butanol 2-propanol 2-methyl-2-propanol
+7.5 +3.6 +3.2 -0.52 -1.8 -3.7 -4.7 -4.9 -5.5 -5.5 -5.6 -6.8
+0.38 -0.85 -0.57 +1.1 +2.5 -3.4 -2.1 -2.4 -2.4 +1.6 +0.75 +1.3
OThe relationship between rotational strength, R , in units of D fiB, and [6],,,, in deg cm2 dmol-I, for a Gaussian band of width AA cenis given by e[,] = 0.75 X lo4 (Amx R/AX).37 The tered at A,, and AA with solvent, experimental data" show a variation in both A, (210 nm) but for our purposes, one can consider typical values of A,, = 1.3 X 10SR; Le., 1300 deg cm2 and AA (12 nm), for which [e],, dmol-' for each 0.01 D fig.
@-anomerdepends mainly on the C, hydroxyl group orientation (Figure 5). If the acetamido CO bond is cis to the C(2)H bond, the C D is determined approximately equally by both C1and C3 hydroxyl group orientations (Figure 4), as expected from the relatively high symmetry of the acetamido group environment in the @-anomer. We can also apply our results qualitatively to rationalize some of the features of the solvent dependence of a-GlcNAc-OMe and j3-GlcNAc-OMe" (Table I), with the proviso that quantitative results are not expected with the present model. Methyl pyranosides are not subject to mutarotation and experimental data for them can be interpreted in a more straightforward manner than for pyranoses, where the data usually refer to an equilibrium mixture of anomers. Our calculated results still have relevance to methyl pyranosides because the static field contribution from a hydroxyl hydrogen atom is not much different from the combined static field contribution from a methyl carbon atom and its three substituent hydrogen atoms. The most striking feature of the data in Table I is the much values observed for the a-anomer than for larger range of ,e[,] the @-anomer. Also, the a-anomer CD is more positive than the @-anomerC D in fluorinated alcohols, whereas the opposite is true in all other solvents. We apply the results of our calculations to account for these general features. @-GlcNAc-OMe. The conformational preference of the C1 methoxy group in pyranosides has received considerable attention. Of the three staggered methoxy group rotamers, the 60' rotamer is unfavored on account of steric interference with the acetamido group. The exo-anomeric effect would lead to the 180' and 300' rotamers both being stable in nonpolar solvent^,^^*^^ but the 180' rotamer is apparently sterically hindered, at least in methyl-2deoxy-@-~-glucopyranoside,~~~' so that the 300' methoxy rotamer is likely to predominate overwhelminglyin the present case. Figure 6 displays the calculated rotational strength of @-GlcNAcfor the 300' C , rotamer, as a function of C3 hydroxyl group rotation and acetamido group rotation. The range of [6],,, observed for the @-anomer in various solvents must therefore be accounted for in terms of a solvent dependence in the distribution of rotamers of the C3 hydroxyl group and, perhaps, also a solvent dependence (38) Lemieux, R. U.; Pavia, A. A.; Martin, J. C.; Watanabe, K.A. Can. J. Chem. 1969,47,4427-4439. (39) Painter, T. J. J. Chem. SOC.,Perkin Trans. 2, 1976, 215-228. (40) Lemieux, R. U. Ann. N.Y.Acad. Sci. 1973, 222, 915-934. (41) Lemieux, R. U.; Koto, S.; Voisin, D. In Anomeric Effect, origin and Conrequences;ACS Symposium Series 87; Szarek, W. A,, Horton, D., Eds.; American Chemical Society: Washington, DC, 1979; pp 17-29.
: I-
-0.15
-200
-
-0.20
o
60
120 i a o 240 300 C,-OH ROTATION (DEG)
Figure 6. Calculated n-r* rotational strength of 2-acetamido-2-deoxy@-D-glUCOpyranOSeas a function of rotation of the Cp hydroxyl group (calculated at 30' increments), for a Cl hydroxyl group rotation of +300° and acetamido group rotations of -20°, Oo, and +20°. Dashed portions of the curve indicate sterically unfavored conformations. TABLE 11: Correlation of Dielectric ConstanP of Pure Solvent with a[eI,,,.,,b solvent A le1 e 2-methyl-2-propanol 2-propanol dioxane ethylene glycol methanol ethanol acetonitrile water
-8.1 -6.35 -4.3 -3.1 -2.6 -2.5 -1.62 -0.3
11 18 2.2 39
33 24 39 79
"Data from CRC Handbook of Chemistry and Physics, 67th ed., 1986-1987. A[6],,, = [6],,,(a-anomer) - [e],,,(@-anomer).
in acetamido group orientation. The very small C D bands observed in the @-anomer in fluorinated alcohols must reflect a high degree of symmetry in the local environment of the planar acetamido group, such as would be provided by "mirror image" methoxy rotamers at C, and C3 (Le., 300' for both). The positive C D bands observed in acetonitrile and dioxane, and in the larger fluorinated alcohols, are also consistent with the preference for the 300' rotamer at C3 (Figure
6). In water and the smaller alcohols, the @-anomer displays a moderately large negative C D band. The environment of the acetamido chromophore must be distinctly more asymmetric in these solvents, and the present model indicates that two possibilities must be considered: (1) an acetamido group rotation toward C, (e.g., +20°) or (2) a C3 hydroxyl group rotation toward the acetamido group (e.g., 120'). The present model further indicates that C D is not likely to be able to distinguish between these two alternatives. a-GlcNAc-OMe. The C D of the a-anomer is more positive than that of the @-anomerin fluorinated alcohols. This is indeed the CD anomeric difference expected from the model when other conformational preferences are not changed, since the methoxy oxygen contribution to CD is negative and is farther removed from the chromophore in the axial position (compare Figures 2 and 6 at 300' C3 hydroxyl rotation). In all other solvents the a-anomer displays more negative CD than the @-anomer. This observation can be rationalized with the present model only in terms of a stabilization of the 120' C3 hydroxyl group rotamer in the a-anomer. Such an interpretation implies that the 120' rotamer of the C3 hydroxyl group is more stable in solvents of low polarity (e.g., 2-methyl-2-propanol,2-propanol) where the negative incremental
4470
J. Phys. Chem. 1987, 91, 4410-4476
change is the greatest. Indeed, in that orientation the C3hydroxyl group is directed toward the acetamido group (rather than away from it), and the intramolecular interactions stabilizing such an orientation would become more significant as the solvent polarity is lowered. Evidence supporting such a hypothesis, independent of the present theoretical model, is presented in Table 11, which displays a strong correlation between the incremental negative change observed in [O],, and the solvent dielectric constant. The proposal that the conformational preferences of substituent groups in these compounds depend on solvent is unexceptional. Determining the precise nature of those preferences, however, is not straightforward. The present work shows that CD, within limits, will reflect differences in the orientation of substituent groups in acetamido sugars. The calculations reported here, although necessarily approximate, direct attention to specific conformational features which are amenable to study by independent means such as NMR.
Conclusions
The theoretical model presented here displays the dependence of acetamido sugar n-7r* CD on specific conformational features. It provides an interpretive tool for relating the n-r* C D of glycosaminoglycans to the orientation of constituent acetamido groups. It also allows a qualitative rationalization of several features of the observed solvent dependence of the C D of aGlcNAc-OMe and 8-GlcNAc-OMe, including (1) the stronger solvent dependence of the a-anomer, (2) the positive C D of the a-anomer in strong hydrogen-bond-donor solvents, (3) the negative C D of the a-anomer in other solvents, and (4) the particularly weak CD of the &anomer in fluorinated alcohols. Acknowledgment. We thank Professor Lynn Buffington for making her experimental CD data available in advance of publication and for helpful discussions. This work was partially supported by NIH Grant GM24862 and N S F Grant CHE8509520.
Theoretical Studies of Transltlon-Metal Hydrides. 4. Comparison of the Transition-Metal Dihydride Ions CrH,' and MoH,' J. Bruce Schilling, W. A. Goddard HI,* and J. L. Beauchamp Arthur Amos Noyes Laboratory of Chemical Physics,+ California Institute of Technology, Pasadena, California 91 125 (Received: February 13, 1987)
The electronic and geometricstructure of two transition-metaldihydride cations, CrH2+and MoH2+,has been studied theoretically by using generalized valence bond and configuration interaction methods. MoH2+is found to have two equally favorable geometries: Re = 1.705 A, 0, = 6 4 . 6 O ; Re = 1.722 %I, Be = 112.3'. These lead to bond energies of &(HMO+-H) of 35.1 and 34.7 kcal/mol, respectively, compared with D,(Mo+-H) = 33.8 kcal/mol. CrH2+leads to an open geometry with Re = 1.635 8, and 0, = 107.5'. The bond energy is D,(HCr+-H) = 19.4 kcal/mol compared with D,(Cr+-H) = 26.9 kcal/mol.
I. Introduction The gas-phase activation of hydrocarbons by transition-metal positive ions is an active area of research.'-5 The proposed first step for most of these reactions after initial association is M+ + R-H R-M+ - H products (1) where the initial reaction step involves the insertion of the metal ion into a C-H bond. The initial intermediate thus involves two species u bonded to the metal ion. There is a growing amount of data, both e ~ p e r i m e n t a and l ~ ~theoretical,*-I0 ~~~~~ dealing with the bond dissociation energies for single species bound to transition-metal ions. Little is known, however, about the geometries of these metal insertion products or the strengths of the second bonds formed to the metals,"J2 both of which are important for an understanding of these metal ion reactions. As a step toward understanding species formed on metal insertion into u bonds and to compare first and second row metal ions, we investigated the two species CrH2+ and MoH2+. The similarity of the Cr+ and Mo' electronic states allows comparison of bonding differences due to orbital size differences of the two metals. The ground-state symmetries, geometries, and bond strengths should be useful in helping to explain differences in the reactivities of the two metal ions. Extension can also be made to predict the geometries and bonding in other transition-metal systems of the first and second transition series.
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11. Calculational Details
A . Basis Sets. The basis sets for the present study are indentical with those in previous work on metal hydride cations.8 +ContributionNo. 7552.
For Cr', the all-electron basis involved an optimized valence double [contraction (13s,10p,5d/5s,4p,2d).I3 For Mo+, the Ni core was replaced with an ab initio effective core potential14 so that Mo+ (1) (a) Halle, L. F.; Armentrout, P. B.; Beauchamp, J. L Orgunometullics 1982, 1, 963. (b) Hanratty, M. A.; Beauchamp, J. L.; Lillies, A. J.; Bowers, M. T. J. Am. Chem. SOC.1985, 107, 1788. (2) Tolbert, M. A.; Beauchamp, J. L. J. Am. Chem. SOC.1984,106,8117. (3) Aristov, N.; Armentrout, P. B. J. Am. Chem. Soc. 1986, 108, 1806. (4) (a) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. SOC.1983, 105,5197. (b) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. SOC.1983, 105, 7492. ( 5 ) Larsen, B. S.; Ridge, D. P. J . Am. Chem. SOC.1984, 106, 1912. (6) (a) Armentrout, P. B.; Halle, L. F.; Beauchamp, J. L. J . Am. Chem. SOC.1981, Z03,6501. (b) Stevens, A. E.; Beauchamp, J. L. Chem. Phys. Lett. 1981, 78, 291. (c) Mandich, M. L.; Halle, L. F.; Beauchamp, J. L. J. Am. Chem. SOC.1984, 106,4403. (d) Georgiadis, R.; Armentrout, P. B. J . Am. Chem. Soc. 1986, 108, 21 19. (e) Hettich, R. L.; Freiser, B. S. J. Am. Chem. SOC.1986, 108, 2537. (7) Elkind, J. L.; Armentrout, P. B. Znorg. Chem. 1986, 25, 1080. (8) (a) Schilling, J. B.; Goddard, W. A,, 111; Beauchamp, J. L. J. Am. Chem. SOC.1986, 108, 582. (b) Schilling, J. B.; Goddard, W. A,, 111; Beauchamp, J. L., submitted for publication. (c) Schiling, J. B.; Goddard, W. A., 111; Beauchamp, J. L. submitted for publication. (9) Alvarado-Swaisgwd, A. E.; Allison, J.; Harrison, J. F. J. Phys. Chem. 1985,89, 2517. (IO) (a) Carter, E. A.; Goddard, W. A., 111. J . Phys. Chem. 1984, 88, 1485. (b) Carter, E. A. Goddard, W. A., 111. J . Am. Chem. SOC.1986,108, 2180. (c) Mavridis, A,; Alvarado-Swaisgood, A. E.; Harrison, J. F. J . Phys. Chem. 1986, 90, 2584. (d) Harrison, J. F. J . Phys. Chem. 1986, 90, 3313. (1 1) Halle, L. F.; Crowe, W. E.; Beauchamp, J. L. Organometallics 1984, 3, 1694. (12) Alvarado-Swaisgwd, A. E.; Harrison, J. F. J . Phys. Chem. 1985, 89, 5198. (13) (a) Rap@, A. K.; Goddard, W. A., 111, to be published. (b) Rappe, A. K.; Smedley, T. A,; Goddard, W. A., 111. J. Phys. Chem. 1981,85, 2607. (14) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
0022-3654/87/2091-4470$01.50/0 0 1987 American Chemical Society
