J . Phys. Chem. 1989, 93, 7640-7644
7640
1 I
and water (and possibly between waters) below 90% R H are stronger than the average hydrogen-bond strengths of liquid water.
15% HOD
91XRH
.
I
2800
.
I
1400
,
. .I
. . I 1000
em"
Figure 10. Uncoupled oscillator Raman difference spectra (%RH - 0% RH) of HOD (7% D20in HzO) in BSA films at 91,56, and 4% RH in the OD stretching region. The upper spectrum is that of HOD in liquid
H20 at ambient temperature. vapor containing 15% HOD. At 4% R H we observe a single broad (2300-2650 cm-I) OD band centered at approximately 2500 cm-', whose intensity increases with hydration. Above 55% R H this band shifts to higher cm-l. Also shown in this figure is the 2525-cm-l OD stretching band of HOD in H 2 0solution. These results indicate that the hydrogen bonding between the protein
Conclusions The stepwise hydration of a protein film has been followed by using Raman spectroscopy and volume and refractive index measurements using integrated optics. These measurements have shown that as one hydrates a dry BSA film there is an immediate alteration of the secondary structure and hydraton of ionic groups. Frequeny shifts of the amide I band, intensity changes in the amide 111 region, and the skeletal C-C stretching band at 940 cm-' confirm an increase in the a-helical content with increasing water concentration. The hydration of carboxylate groups is observed at hydration levels as low as 2% RH. Half of these sites are hydrated by 20% R H with an average of 5 water molecules per site. Indirect evidence suggests that above 70% R H other sites are being hydrated. The concept of monolayer coverage is valid provided that one restricts the coverage specific sites. Measurement of the uncoupled OD oscillator spectra show that the water bound to the protein, at levels of hydration up to 90% RH, appears to be more strongly hydrogen bonded than water in liquid water. The hysteresis observed on dehydration of BSA films can be attributed to a higher a-helical content. The estimated helical content of the dehydrated film changed from 34% to 40% at the end of one cycle.
Interaction of COP and Water Investigated by a Combination of ab Initio and SOLDRI-WIMP Techniques James R. Damewood, Jr.,* Robert A. Kumpf, and Wolfgang C. F. Miihlbauer Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716 (Received: March 9, 1989)
The interaction of C02 and water is investigated by a combination of ab initio and SOLDRJ-MM2 techniques. Ab initio results at levels of theory as high as MP2/6-31 l+G**//6-31G** are found to be in reasonable agreement with experiment. The R,B potential energy surfaces generated for C02.H20by SOLDRI-MM2 and ab initio methods at the 6-31G** level are found to be in very good agreement with a root mean square deviation in the binding region of only 0.8 kcal/mol.
Introduction Carbon dioxide (CO,), being both the starting material for plant carbohydrate synthesis and the end organic product of oxidative degradation,' is ubiquitous in our environment. In biological systems, COz is often found in aqueous surroundings, and its condensation with water to form carbonic acid is of sufficient importance that the metalloenzyme, carbonic anhydrase, catalyzes this reaction with spectacular efficiency., In this work, we investigate the potential energy surface for the 1:l interaction of C 0 2 with water. These calculations provide information on the (1) Lehninger, A. L. Biochemistry, 2nd ed. Worth Publishers: New York, 1976. (2) For studies concerned with the condensation of CO, and H 2 0relevant
to modeling carbonic anhydrase, see: (a) JBnsson, B.; KarlstrBm, G.; WennerstriSm, H. Chem. Phys. Lett. 1976, 41, 317. (b) JBnsson, B.; Karlstrom, G.; Wennerstrijm, H.; Forsen, S.; Rom, B.; Almof, J. J . Am. Chem. Soc. 1977,99,4628. (c) Jonsson, B.; Karlstrom, G.; Wennerstrom, H. J. Am. Chem. SOC.1978, 100, 1658. (d) George, P.; Bock, C. W.; Trachtman, M. J . Comput. Chem. 1982,3,283. (e) Sokalski, W. A. In?.J. Quantum Chem. 1981, 20, 231. ( f ) Jean, Y.; Volatron, F. Chem. Phys. 1982, 65, 107. ( 9 ) Nguyen, M. T.; Ha, T.-K. J . Am. Chem. SOC.1984, 106, 599. (h) Liang, J.-Y.; Lipscomb, W. N. J . Am. Chem. SOC.1986, 208, 5051. (i) Sokalski, W. A. J . Mol. Struct. 1986, 138, 77. (j) Nguyen, M. T.; Hegarty, A. F.;Ha, T.-K. J . Mol. Struct. 1987, 150, 319. (k) Merz, K. M., Jr.; Hoffmann, R.; Dewar, M. J. S . J . Am. Chem. SOC.1989, I l l , 5636.
details of the hydration potential (COz + H 2 0 P COZ.H20) of C 0 2 and thus assist in generating a more accurate picture of how C 0 2 appears to other reactants or enzymes (such as carbonic anhydrase) in an aqueous environment. We investigate the hydration of COz through a combination of ab initio molecular orbital techniques3 and our recently developed SOLDRI-MM2 approach: Work in our laborat~ries~**~ has revealed the need for nonprejudicial searches of intermolecular potential energy surfaces for molecular hydration in order to accurately determine important features of the potential energy surface. The SOLDRI-MM2 approach was developed to provide an economical way to search, in a nonprejudicial fashion, the potential energy surface for intermolecular interaction. While our initial intent was to develop a rigorous search technique that would provide qualitative information on the shape of the inter(3) For an introduction to ab initio methods, see: Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (4) Miihlbauer, W. C. F.; Damewood, J. R., Jr. J . Phys. Chem. 1988,92, 3693. (5) (a) Damewood, J. R., Jr.; Kumpf, R. A. J. Phys. Chem. 1988,92,3449. (b) Kumpf, R. A.; Damewood, J. R., Jr. J. Phys. Chem. 1989,93,4478. (c) Kumpf, R. A.; Damewood, J. R., Jr. J . Chem. SOC.,Chem. Commun. 1988, 621.
0022-3654/89/2093-7640$01 SO10 0 1989 American Chemical Society
Ab Initio and SOLDRI-MM2 Study of C02 and Water TABLE I: Calculated Absolute Energies" for C02, la-f, compd 6-31G** MP2' 6-311+G** CO2 -187.634 176 -188.113 122 -187.691 280 la -263.662 995 -264.340 736 -263.749 069 lb -263.662 738 -264.340 709 -263.748 606 IC -263.660 505 -264.338419 -263.747 201 Id -263.659 896 -264.337 719 -263.746446 le -263.661 750 -264.339 820 -263.747 266 If -263.661 396 -264.339 159 -263.747 394 water -76.023 615 -76.221 787 -76.053 295
and Water MP2C -188.203629 -264.483 194 -264.482 802 -264.480896 -264.480 378 -264.481 606 -264.481 625 -76.274 117
" In hartrees. Maller-Plesset correction to 6-31G** energies. Maller-Plesset correction to 6-31 1+Gt*//6-31G** energies. molecular potential (location of minima, etc.), our studies on the interaction of formaldehyde and water4 revealed extremely encouraging quantitative results. Indeed, formaldehyde-water potential energy surfaces calculated with ab initio, 6-31G** methods and modified MM2 techniques agree very well, with root mean square deviations of 1.3 kcal/mol in the binding region (binding energy
