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Ab initio studies on proton transfer involving Schiff base and related nitrogen compounds
Journal of Molecular Structure, 212 (1989) 53-60 Elsevier Science Publishers B.V., Amsterdam - Printed
53 in The Netherlands
AB INITIO STUDIES ON PROTON TRANSFER INVOLVING SCHIFF BASE AND RELATED NITROGEN COMPOUNDS
N. SREERAMA
and SARASWATHI
VISHVBSHWARA
Molecular Biophysics Unit, Indian Institute of Science, Bangalore - 560 012 (India) (Received 6 December
1988)
ABSTRACT Proton transfer across cationic hydrogen bonds involving Schiff base, ammonia and related compounds has been studied at the 4-31G level. Proton transfer characteristics are correlated to the proton affinities of the species involved. Hydrogen bond strengths of these hydrogen bonds are correlated to the difference in the proton affinity of the donor and the acceptor. Influence of a neighbouring hydrogen bond on the proton transfer from Schiff base to ammonia and Schiff base to water is also discussed.
INTRODUCTION
Proton transfer through hydrogen bonds is an important mechanism by which certain chemical and biological processes are carried out [l-3]. The proton potential curve (proton transfer profile) between a donor and an acceptor depends on several factors such as the proton affinities of the donor/acceptor and the nature of the ionic or polar environment around the system. Scheiner et al. have carried out ab initio calculations on various models of hydrogen bonded systems involving Schiff base, ammonia and water in order to investigate the effect of hydrogen bond geometry and external ions on proton potential curves [ 4-61. Bacteria-rhodopsin (bR) is an interesting membrane protein, which acts as a light driven proton pump [ 7,8]. It is suggested that a hydrogen bonded chain runs across the membrane formed by the side chains of residues such as tyrosine and aspartic acid of bR thus facilitating the proton transport [ 9-111. This is supported by the involvement of aspartic acid residues and tyrosine residues, which undergo protonation/deprotonation during the photocycle [ 12,131. Proton transfer between formic acid, phenol and water have been studied, with and without solvent effect [ 141. Solvent effect was found to reverse the stability of the potential profile. Active transport is believed to take place through a process of protonation/ deprotonation of the hydrogen bonded residues. However, the process is trig-
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
54
gered, when the light is absorbed by the chromophore, by the deprotonation of the Schiff base which is formed between the nitrogen of lysine and the retinal chromophore. To understand the nature of proton transport we have recently studied [ 151 the proton affinities of systems which may participate in the hydrogen bonded chain in bR. In this paper we present the proton potential curve in hydrogen bonded systems with Schiff base and other nitrogen compounds as donor/acceptor. Also, we have investigated the effect of an additional neighbouring hydrogen bond on a few of the proton potential curves. COMPUTATIONAL DETAILS
The donor and acceptor systems considered in this investigation are shown schematically in Fig. 1 (the arrow represents the direction of proton approach). Optimised geometries of NHB, CH2=NH and CH,-NH2 were taken from ref. 16. Standard internal parameters were used for CH,=CH-NH, and CH,-CH-NH [ 171. Partial optimisation (bond lengths and bond angles) was carried out on hydrogen bonded systems (CH,=CH-CH=NH*H* *NH,) + and (CH,=CH-CH=NH*H. .HN=CH,) +, and internal parameters for monomers were taken to be the same as in the complex. All calculations were done using the split-valence 4-31G basis set [ 181 and GAUSSIAN 80 series of programs [ 191. The “Berny algorithm” [ 201 was used for optimisation.
147 I N ‘*a, y /
H
Fig. 1. Schematic representation of the donor/acceptor groups considered for proton transfer studies (the arrow indicates the direction of proton approach).
55 RESULTS AND DISCUSSION
Proton transfer curves and proton affinities The proton transfer from A, to A, in the (Al-H* *A,) + hydrogen bond can be studied by its potential energy profile. The AL-A, distance 5 is kept constant and the Al-H distance r is varied from 0.9 A to (R-0.9) A. The energy of the (A,-A, ) + system was obtained for each value of r and the difference in energy was plotted against r to obtain the proton transfer profile. The difference in the proton affinities of the groups A, and AZ makes the profile an asymmetrical double minimum. If the proton affinity of Ai is greater than that of A, the minimum near A, is deeper. The characteristics of such a profile are represented by parameters: Eb, the energy barrier for proton transfer from A, to A,; EL, the energy barrier for proton transfer from A2 to A,; and AE,, the difference in the depths of the minima (the relative stability of the preferred structure). Figure 2 gives the potential profile for the proton transfer from CH2=NH to NH, at values of R equal to 2.7 A and 3.0 A. Table 1 summarises the characteristics of proton transfer in all the (A,-Ha *A,) + systems considered, along with the difference in the proton affinities of A, and Aa.
16-
1
I
1.2
1.1
1.6
1.8
r(i)
Fig. 2. Proton transfer profile in (H,C=NH.v* -NH,)+ at different values of the N-N distance R. Curves 1 and 2 have R equal to 2.7 and 3.0 A respectively.
56 TABLE 1 Characteristics of proton transfer in (A,-H..A,)+ (A,-H.-A,)+
R0
(A)
(H,C=NH*H..NH3)
+
(H,C-NH2.H..NH,) + (HJJ-NH,.H. .NH,=CH,) + (H,C=CH-NH2.H..NH,) + (H,C-CH=NH.He.NH3) +
r0
(A)
hydrogen bonded systems
AE,
Eb
EL
mol-‘)
(kcal mol-‘)
(kcal
(kcal mol-‘)
APA (kcal mol-’ )
2.7 3.0 4.5
1.08 1.06 1.05
3.95 5.21 8.0
5.96 17.57 -
2.01
8.7
2.7 2.7 2.7 2.7
1.075 1.075 1.08 1.06
5.33 1.13 3.26 8.53
6.28 4.7 5.33 9.29
0.95 3.57 3.07 0.76
10.0 1.3 3.0 18.9
In (CH,=NH* *H* *NH,) + as the N-N distance R increases from 2.7 A to 3.0 A. E,, changes from 5.0 kcal mol-’ to 17.57 kcal mol-l and AE, from 4.17 to 6.27 kcal mol-‘. On increasing R to 4.5 A, AE, increases to 8.0 kcal mol-’ and &, shoots beyond 100.0 kcal mol-‘. Similar barrier elevation was observed in the case of proton transfer between two NH3 molecules [ 211. Also, as R is increased from 2.7 to 4.5 A r decreases from 1.08 to 1.05 A. This is due to the decrease in the attractive force exerted by the lone pair of nitrogen on the proton at longer R values. In the case of an extended Schiff base (CH,=CH-CH=NH) forming a hydrogen bond with NH3 and CH,=NH, partial optimisation gave the preferred structure with hydrogen near the extended Schiff base. Optimised values of the parameters appear in Table 2; of these, r is nearly the same (about 1.04 A), but R is slightly different, being 2.815 A for the hydrogen bond with NH3 and 2.804 A for that with CH2=NH. This reflects the difference in the hybridisation on the nitrogen in AS, which changes from sp3 to sp’ upon going from NH3 to CH2=NH. It is seen from Table 1 that AE, and Eb are related to the difference in the proton affinity of A, and A2 (APA). AE, increases as proton affinity increases: while Eb increases with increase in proton affinity, Ek decreases. So with increase in proton affinity the second minimum becomes shallower and finally vanishes thus making the profile an asymmetrical single well. The value of Pis also affected by APA, becoming smaller for larger values of APA. As proton affinity is a measure of the attractive force exerted by the donor/acceptor group on the proton, the group with higher PA attracts the proton more, making r shorter. The hydrogen bonds considered above belong to the group of ionic hydrogen bonds which have been studied extensively by mass spectroscopic [22] and theoretical methods [23]. They have quite high bond strengths. The 4-31G optimised structure of (H3N-H. -NHB) + has an N-N distance equal to 2.716
57 TABLE 2 Optimised geometrical parameters of (H,C=CH-CH=NH CH=NH.H* .NHB) + hydrogen bonded systems
-H - . HN=CJ&) + and (H.&‘&H-
Parameter
(H,C=CH-CH=NH,...HN-CH,)
+
Bond lengths (A) Nl-Cl Cl-C2 C2-C3 Nl-H Cl-H Nl-N (R) Nl-H (r) C-N N-H
1.277 1.441 1.329 0.999 1.072 2.804 1.041 1.259 -
1.279 1.440 1.329 0.999 1.072 2.814 1.041 -
Bond angles (degrees) H-Nl-Cl Nl-Cl-C2 Cl-C2-C3 C2-C3-C4 N-Nl-Cl
121.14 121.18 121.93 120.25 123.83
121.08 121.17 121.92 120.47 122.85
(H,C=CH-CH=NH,.
. .N&) +
1.003
A and the hydrogen bond strength is 32.0 kcal mol-’ [23]. The optimised structure of (H,C=CH-CH=NH.H* *NH,) + has an N-N distance equal to 2.814 A (Table 2) and the hydrogen bond strength is 23.98 kcal mol-‘. Corresponding values in the (H,C=CH-CH=NH*H* *HN=CH,) + hydrogen bond are 2.804 A (Table 2) and 23.14 kcal mol-l. The reduced hydrogen bond strength and the increase in the length are perhaps related to the redistribution of electrons around the proton donor and the acceptor. The hydrogen bond strength depends on the difference in the proton affinities of the donor and the acceptor. The hydrogen bond energies are evaluated for systems with NH3 as the acceptor and the molecules given in Fig. 1 as the donors. Figure 3 shows the variation of hydrogen bond energy with APA. It is observed that as APA increases, hydrogen bond strength decreases. This is in agreement with previous studies [ 241 with other strong hydrogen bonds.
Effect of a neighbouring hydrogen bond The proton transfer curve is influenced by various external factors. This is basically due to the change in the acidity of the groups involved. External ions influence the proton transfer curves drastically [ 51. Neighbouring hydrogen bonds also influence the potential profiles, as was observed in cationic oligomers of water [ 251. Below we present the results of such a study in (CH2=NH.H**NH,)+ and (CH,=NH*H.*OH,)+ systems.
I
I
I
I
6
1
I
I
I
22
14
I
APA(kca1)
Fig. 3. Plot of hydrogen bond strength against proton affinity difference (APA). The proton acceptor is NH3 and the molecules in Fig. 1 are considered as proton donors.
30-
7 z a20s :: w 4
lo-
oI 1.0
I 1.2
I 1.4
I 1.6
1
r(A)
Fig. 4. Effect of a neighbouring hydrogen bond on the proton transfer profile in (H2C=NH.H**OH2) +. Curve 1 shows the profile without any external influence and curve 2 that under the influence of a neighbouring hydrogen bond.
59
Figure 4 shows the results for (CH,=NH*H* *OH2) + system with the N-O distance R kept at 2.7 A. Curve 1 is without any external influence and curve 2 is under the influence of a neighbouring hydrogen bond (inset shows the orientation). The proton affinity of CH2=NH is - 231.1 kcal mol-’ and that of OH, is - 189.0 kcal mol-‘. This large difference makes the potential profile an asymmetrical single minimum, which is nearer to CH,=NH, but when another water molecule is placed at the hydrogen bond distance from 0, the potential profile becomes an asymmetrical double minimum. The second well near 0 has a depth of about 4.0 kcal mol-’ with a barrier for proton transfer of 18.0 kcal mol-‘. Figure 5 shows proton transfer profiles for (CH,=NH*H* *NH,)+ with (curve 2) and without (curve 1) the influence of a neighbouring hydrogen bond. A small difference in proton affinity (about 9.0 kcal mol-’ ) makes the profile an asymmetrical double minimum with AE, equal to 3.95 kcal mol-’ and Eb equal to 5.96 kcal mol-l. Neighbouring hydrogen bond reverses the stability (curve 2; EL = 2.5 kcal mol-’ and A?&= -3.0 kcal mol-‘). This favours a proton transfer from CH,=NH to NH3. Thus a polar environment, although neutral, can profoundly change the nature of a proton potential curve. Therefore the position of a proton cannot be judged solely on the relative pro-
Fig. 5. Effect of a neighbouring hydrogen bond on the proton transfer profile in (H,C=NH-He *NH,) +. Curve 1 shows the profile without any external influence and curve 2 that under the influence of a neighbouring hydrogen bond.
60
ton affinity of participating groups and the effect of an ionic or polar environment is crucial in this matter. SUMMARY
The three factors that influence the proton transfer profile are the orientation of the groups involved, the presence of an external ion and the neighbouring group that can be hydrogen bonded to either of the groups involved in proton transfer. These three factors can bring about a substantial change in the barrier (I$,) or/and the relative stability (A&,). A reduction in the barrier, or a reduction in the relative depths of the minima, or both, or a reversal of the minima, are the major consequences that might result from these external factors. In this paper some of these factors are investigated at a quantitative level in model systems. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
E. Caldin and V. Gold (Eds. ) , Proton Transfer Reactions, Wiley, New York, 1975, and references cited therein. P. Mitchell, Ann. Rev. Biochem., 46 (1977) 996. W. Stoeckenius and R.A. Bogomolni, Ann. Rev. Biochem., 52 (1982) 587 and references cited therein. S. Scheiner and E.A. Hillenbrand, Proc. Natl. Acad. Sci. U.S.A., 82 (1985) 2741. S. Scheiner, P. Redfern and M.M. Szczesniak, J. Phys. Chem., 89 (1985) 262. S. Scheiner, J. Mol. Struct., 177 (1988) 79. D. Oesterhelt and W. Stoeckenius, Nature (London), New Biol., 233 (1971) 149. R.R. Birge, Ann. Rev. Biophys. Bioeng., 10 (1981) 315. H. Merz and G. Zundel, B&hem. Biophys. Res. Commun., 101 (1981) 540. J.F. Nagle and H.J. Morowitz, Proc. Natl. Acad. Sci. U.S.A., 75 (1978) 298. A.K. Dunker and D.A. Marvin, J. Theor. Biol., 72 (1978) 181. M. Engelhard, K. Gerwart, B. Hess, W. Kreutz and F. Siebert, Biochemistry, 24 (1985) 400. G. Dollinger, L. Eisenstein, S.L. Lin, K. Nakanishi and J. Termini, Biochemistry, 25 (1986) 6524. D. Hadzi, J. Mol. Struct., 177 (1988) 1. N. Sreerama and S. Vishveshwara, J. Mol. Struct., 194 (1989) 61. W.A. Lathan, L.A. Curtiss, W.J. Hehre, J.B. Lisle and J.A. Pople, Prog. Phys. Org. Chem., 11 (1974) 175. J.A. Pople and M. Gorden, J. Am. Chem. Sot., 89 (1967) 4253. R. Ditchfield, W.J. Hehre and J.A. Pople, J. Chem. Phys., 54 (1971) 724. J.S. Binkley, R.A. Whiteside, R. Krishnan, R. Seegar, D.J. Defrees, J.B. Schlegel, S. Topiol, L.K. Kahn and J.A. Pople, GAUSSIAN 80, QCPE, 406,198l. H.B. Schlegel, J. Chem. Phys., 77 (1982) 3676. S. Scheiner and L.B. Harding, Chem. Phys. Lett., 79 (1981) 39; J. Am. Chem. Sot., 103 (1981) 2169. P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. P.J. Desmeulles and L.C. Allen, J. Chem. Phys., 72 (1980) 4731. W.R. Davidson, J. Sunner and P. Kebale, J. Am. Chem. Sot., 101 (1979) 1675. S. Scheiner, J. Am. Chem. Sot., 103 (1981) 315.
53 in The Netherlands
AB INITIO STUDIES ON PROTON TRANSFER INVOLVING SCHIFF BASE AND RELATED NITROGEN COMPOUNDS
N. SREERAMA
and SARASWATHI
VISHVBSHWARA
Molecular Biophysics Unit, Indian Institute of Science, Bangalore - 560 012 (India) (Received 6 December
1988)
ABSTRACT Proton transfer across cationic hydrogen bonds involving Schiff base, ammonia and related compounds has been studied at the 4-31G level. Proton transfer characteristics are correlated to the proton affinities of the species involved. Hydrogen bond strengths of these hydrogen bonds are correlated to the difference in the proton affinity of the donor and the acceptor. Influence of a neighbouring hydrogen bond on the proton transfer from Schiff base to ammonia and Schiff base to water is also discussed.
INTRODUCTION
Proton transfer through hydrogen bonds is an important mechanism by which certain chemical and biological processes are carried out [l-3]. The proton potential curve (proton transfer profile) between a donor and an acceptor depends on several factors such as the proton affinities of the donor/acceptor and the nature of the ionic or polar environment around the system. Scheiner et al. have carried out ab initio calculations on various models of hydrogen bonded systems involving Schiff base, ammonia and water in order to investigate the effect of hydrogen bond geometry and external ions on proton potential curves [ 4-61. Bacteria-rhodopsin (bR) is an interesting membrane protein, which acts as a light driven proton pump [ 7,8]. It is suggested that a hydrogen bonded chain runs across the membrane formed by the side chains of residues such as tyrosine and aspartic acid of bR thus facilitating the proton transport [ 9-111. This is supported by the involvement of aspartic acid residues and tyrosine residues, which undergo protonation/deprotonation during the photocycle [ 12,131. Proton transfer between formic acid, phenol and water have been studied, with and without solvent effect [ 141. Solvent effect was found to reverse the stability of the potential profile. Active transport is believed to take place through a process of protonation/ deprotonation of the hydrogen bonded residues. However, the process is trig-
0022-2860/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
54
gered, when the light is absorbed by the chromophore, by the deprotonation of the Schiff base which is formed between the nitrogen of lysine and the retinal chromophore. To understand the nature of proton transport we have recently studied [ 151 the proton affinities of systems which may participate in the hydrogen bonded chain in bR. In this paper we present the proton potential curve in hydrogen bonded systems with Schiff base and other nitrogen compounds as donor/acceptor. Also, we have investigated the effect of an additional neighbouring hydrogen bond on a few of the proton potential curves. COMPUTATIONAL DETAILS
The donor and acceptor systems considered in this investigation are shown schematically in Fig. 1 (the arrow represents the direction of proton approach). Optimised geometries of NHB, CH2=NH and CH,-NH2 were taken from ref. 16. Standard internal parameters were used for CH,=CH-NH, and CH,-CH-NH [ 171. Partial optimisation (bond lengths and bond angles) was carried out on hydrogen bonded systems (CH,=CH-CH=NH*H* *NH,) + and (CH,=CH-CH=NH*H. .HN=CH,) +, and internal parameters for monomers were taken to be the same as in the complex. All calculations were done using the split-valence 4-31G basis set [ 181 and GAUSSIAN 80 series of programs [ 191. The “Berny algorithm” [ 201 was used for optimisation.
147 I N ‘*a, y /
H
Fig. 1. Schematic representation of the donor/acceptor groups considered for proton transfer studies (the arrow indicates the direction of proton approach).
55 RESULTS AND DISCUSSION
Proton transfer curves and proton affinities The proton transfer from A, to A, in the (Al-H* *A,) + hydrogen bond can be studied by its potential energy profile. The AL-A, distance 5 is kept constant and the Al-H distance r is varied from 0.9 A to (R-0.9) A. The energy of the (A,-A, ) + system was obtained for each value of r and the difference in energy was plotted against r to obtain the proton transfer profile. The difference in the proton affinities of the groups A, and AZ makes the profile an asymmetrical double minimum. If the proton affinity of Ai is greater than that of A, the minimum near A, is deeper. The characteristics of such a profile are represented by parameters: Eb, the energy barrier for proton transfer from A, to A,; EL, the energy barrier for proton transfer from A2 to A,; and AE,, the difference in the depths of the minima (the relative stability of the preferred structure). Figure 2 gives the potential profile for the proton transfer from CH2=NH to NH, at values of R equal to 2.7 A and 3.0 A. Table 1 summarises the characteristics of proton transfer in all the (A,-Ha *A,) + systems considered, along with the difference in the proton affinities of A, and Aa.
16-
1
I
1.2
1.1
1.6
1.8
r(i)
Fig. 2. Proton transfer profile in (H,C=NH.v* -NH,)+ at different values of the N-N distance R. Curves 1 and 2 have R equal to 2.7 and 3.0 A respectively.
56 TABLE 1 Characteristics of proton transfer in (A,-H..A,)+ (A,-H.-A,)+
R0
(A)
(H,C=NH*H..NH3)
+
(H,C-NH2.H..NH,) + (HJJ-NH,.H. .NH,=CH,) + (H,C=CH-NH2.H..NH,) + (H,C-CH=NH.He.NH3) +
r0
(A)
hydrogen bonded systems
AE,
Eb
EL
mol-‘)
(kcal mol-‘)
(kcal
(kcal mol-‘)
APA (kcal mol-’ )
2.7 3.0 4.5
1.08 1.06 1.05
3.95 5.21 8.0
5.96 17.57 -
2.01
8.7
2.7 2.7 2.7 2.7
1.075 1.075 1.08 1.06
5.33 1.13 3.26 8.53
6.28 4.7 5.33 9.29
0.95 3.57 3.07 0.76
10.0 1.3 3.0 18.9
In (CH,=NH* *H* *NH,) + as the N-N distance R increases from 2.7 A to 3.0 A. E,, changes from 5.0 kcal mol-’ to 17.57 kcal mol-l and AE, from 4.17 to 6.27 kcal mol-‘. On increasing R to 4.5 A, AE, increases to 8.0 kcal mol-’ and &, shoots beyond 100.0 kcal mol-‘. Similar barrier elevation was observed in the case of proton transfer between two NH3 molecules [ 211. Also, as R is increased from 2.7 to 4.5 A r decreases from 1.08 to 1.05 A. This is due to the decrease in the attractive force exerted by the lone pair of nitrogen on the proton at longer R values. In the case of an extended Schiff base (CH,=CH-CH=NH) forming a hydrogen bond with NH3 and CH,=NH, partial optimisation gave the preferred structure with hydrogen near the extended Schiff base. Optimised values of the parameters appear in Table 2; of these, r is nearly the same (about 1.04 A), but R is slightly different, being 2.815 A for the hydrogen bond with NH3 and 2.804 A for that with CH2=NH. This reflects the difference in the hybridisation on the nitrogen in AS, which changes from sp3 to sp’ upon going from NH3 to CH2=NH. It is seen from Table 1 that AE, and Eb are related to the difference in the proton affinity of A, and A2 (APA). AE, increases as proton affinity increases: while Eb increases with increase in proton affinity, Ek decreases. So with increase in proton affinity the second minimum becomes shallower and finally vanishes thus making the profile an asymmetrical single well. The value of Pis also affected by APA, becoming smaller for larger values of APA. As proton affinity is a measure of the attractive force exerted by the donor/acceptor group on the proton, the group with higher PA attracts the proton more, making r shorter. The hydrogen bonds considered above belong to the group of ionic hydrogen bonds which have been studied extensively by mass spectroscopic [22] and theoretical methods [23]. They have quite high bond strengths. The 4-31G optimised structure of (H3N-H. -NHB) + has an N-N distance equal to 2.716
57 TABLE 2 Optimised geometrical parameters of (H,C=CH-CH=NH CH=NH.H* .NHB) + hydrogen bonded systems
-H - . HN=CJ&) + and (H.&‘&H-
Parameter
(H,C=CH-CH=NH,...HN-CH,)
+
Bond lengths (A) Nl-Cl Cl-C2 C2-C3 Nl-H Cl-H Nl-N (R) Nl-H (r) C-N N-H
1.277 1.441 1.329 0.999 1.072 2.804 1.041 1.259 -
1.279 1.440 1.329 0.999 1.072 2.814 1.041 -
Bond angles (degrees) H-Nl-Cl Nl-Cl-C2 Cl-C2-C3 C2-C3-C4 N-Nl-Cl
121.14 121.18 121.93 120.25 123.83
121.08 121.17 121.92 120.47 122.85
(H,C=CH-CH=NH,.
. .N&) +
1.003
A and the hydrogen bond strength is 32.0 kcal mol-’ [23]. The optimised structure of (H,C=CH-CH=NH.H* *NH,) + has an N-N distance equal to 2.814 A (Table 2) and the hydrogen bond strength is 23.98 kcal mol-‘. Corresponding values in the (H,C=CH-CH=NH*H* *HN=CH,) + hydrogen bond are 2.804 A (Table 2) and 23.14 kcal mol-l. The reduced hydrogen bond strength and the increase in the length are perhaps related to the redistribution of electrons around the proton donor and the acceptor. The hydrogen bond strength depends on the difference in the proton affinities of the donor and the acceptor. The hydrogen bond energies are evaluated for systems with NH3 as the acceptor and the molecules given in Fig. 1 as the donors. Figure 3 shows the variation of hydrogen bond energy with APA. It is observed that as APA increases, hydrogen bond strength decreases. This is in agreement with previous studies [ 241 with other strong hydrogen bonds.
Effect of a neighbouring hydrogen bond The proton transfer curve is influenced by various external factors. This is basically due to the change in the acidity of the groups involved. External ions influence the proton transfer curves drastically [ 51. Neighbouring hydrogen bonds also influence the potential profiles, as was observed in cationic oligomers of water [ 251. Below we present the results of such a study in (CH2=NH.H**NH,)+ and (CH,=NH*H.*OH,)+ systems.
I
I
I
I
6
1
I
I
I
22
14
I
APA(kca1)
Fig. 3. Plot of hydrogen bond strength against proton affinity difference (APA). The proton acceptor is NH3 and the molecules in Fig. 1 are considered as proton donors.
30-
7 z a20s :: w 4
lo-
oI 1.0
I 1.2
I 1.4
I 1.6
1
r(A)
Fig. 4. Effect of a neighbouring hydrogen bond on the proton transfer profile in (H2C=NH.H**OH2) +. Curve 1 shows the profile without any external influence and curve 2 that under the influence of a neighbouring hydrogen bond.
59
Figure 4 shows the results for (CH,=NH*H* *OH2) + system with the N-O distance R kept at 2.7 A. Curve 1 is without any external influence and curve 2 is under the influence of a neighbouring hydrogen bond (inset shows the orientation). The proton affinity of CH2=NH is - 231.1 kcal mol-’ and that of OH, is - 189.0 kcal mol-‘. This large difference makes the potential profile an asymmetrical single minimum, which is nearer to CH,=NH, but when another water molecule is placed at the hydrogen bond distance from 0, the potential profile becomes an asymmetrical double minimum. The second well near 0 has a depth of about 4.0 kcal mol-’ with a barrier for proton transfer of 18.0 kcal mol-‘. Figure 5 shows proton transfer profiles for (CH,=NH*H* *NH,)+ with (curve 2) and without (curve 1) the influence of a neighbouring hydrogen bond. A small difference in proton affinity (about 9.0 kcal mol-’ ) makes the profile an asymmetrical double minimum with AE, equal to 3.95 kcal mol-’ and Eb equal to 5.96 kcal mol-l. Neighbouring hydrogen bond reverses the stability (curve 2; EL = 2.5 kcal mol-’ and A?&= -3.0 kcal mol-‘). This favours a proton transfer from CH,=NH to NH3. Thus a polar environment, although neutral, can profoundly change the nature of a proton potential curve. Therefore the position of a proton cannot be judged solely on the relative pro-
Fig. 5. Effect of a neighbouring hydrogen bond on the proton transfer profile in (H,C=NH-He *NH,) +. Curve 1 shows the profile without any external influence and curve 2 that under the influence of a neighbouring hydrogen bond.
60
ton affinity of participating groups and the effect of an ionic or polar environment is crucial in this matter. SUMMARY
The three factors that influence the proton transfer profile are the orientation of the groups involved, the presence of an external ion and the neighbouring group that can be hydrogen bonded to either of the groups involved in proton transfer. These three factors can bring about a substantial change in the barrier (I$,) or/and the relative stability (A&,). A reduction in the barrier, or a reduction in the relative depths of the minima, or both, or a reversal of the minima, are the major consequences that might result from these external factors. In this paper some of these factors are investigated at a quantitative level in model systems. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
E. Caldin and V. Gold (Eds. ) , Proton Transfer Reactions, Wiley, New York, 1975, and references cited therein. P. Mitchell, Ann. Rev. Biochem., 46 (1977) 996. W. Stoeckenius and R.A. Bogomolni, Ann. Rev. Biochem., 52 (1982) 587 and references cited therein. S. Scheiner and E.A. Hillenbrand, Proc. Natl. Acad. Sci. U.S.A., 82 (1985) 2741. S. Scheiner, P. Redfern and M.M. Szczesniak, J. Phys. Chem., 89 (1985) 262. S. Scheiner, J. Mol. Struct., 177 (1988) 79. D. Oesterhelt and W. Stoeckenius, Nature (London), New Biol., 233 (1971) 149. R.R. Birge, Ann. Rev. Biophys. Bioeng., 10 (1981) 315. H. Merz and G. Zundel, B&hem. Biophys. Res. Commun., 101 (1981) 540. J.F. Nagle and H.J. Morowitz, Proc. Natl. Acad. Sci. U.S.A., 75 (1978) 298. A.K. Dunker and D.A. Marvin, J. Theor. Biol., 72 (1978) 181. M. Engelhard, K. Gerwart, B. Hess, W. Kreutz and F. Siebert, Biochemistry, 24 (1985) 400. G. Dollinger, L. Eisenstein, S.L. Lin, K. Nakanishi and J. Termini, Biochemistry, 25 (1986) 6524. D. Hadzi, J. Mol. Struct., 177 (1988) 1. N. Sreerama and S. Vishveshwara, J. Mol. Struct., 194 (1989) 61. W.A. Lathan, L.A. Curtiss, W.J. Hehre, J.B. Lisle and J.A. Pople, Prog. Phys. Org. Chem., 11 (1974) 175. J.A. Pople and M. Gorden, J. Am. Chem. Sot., 89 (1967) 4253. R. Ditchfield, W.J. Hehre and J.A. Pople, J. Chem. Phys., 54 (1971) 724. J.S. Binkley, R.A. Whiteside, R. Krishnan, R. Seegar, D.J. Defrees, J.B. Schlegel, S. Topiol, L.K. Kahn and J.A. Pople, GAUSSIAN 80, QCPE, 406,198l. H.B. Schlegel, J. Chem. Phys., 77 (1982) 3676. S. Scheiner and L.B. Harding, Chem. Phys. Lett., 79 (1981) 39; J. Am. Chem. Sot., 103 (1981) 2169. P. Kebarle, Ann. Rev. Phys. Chem., 28 (1977) 445. P.J. Desmeulles and L.C. Allen, J. Chem. Phys., 72 (1980) 4731. W.R. Davidson, J. Sunner and P. Kebale, J. Am. Chem. Sot., 101 (1979) 1675. S. Scheiner, J. Am. Chem. Sot., 103 (1981) 315.