J. Phys. Chem. 1082, 86, 3951-3956
3051
Protonation of Glycine: An Ab Initio Self-Consistent Field Study Lance R. Wrlght, Raymond F. Borkman,' School of Chemlstry, Georgia InsHTute of Technokgy, AtLanta, Georgia 30332
and Alan M. Gabrlelll Department of Chemistry, Southern Technlcal Institute, Marietta, Georgia 30060 (Received: January 26, 1982; In Final Form: June 21, 1982)
Ab initio SCF potential energy curves have been calculated for the gas-phase protonation of glycine with a 6-31G Gaussian basis set. Interaction energies for the approach of five potential protonating species (H+,HeH+, H3+,H, and H2)to several sites of both neutral and zwitterionic glycine were computed. The most strongly bound species at each attack site of glycine was the free proton, H+, followed in order by the ions HeH+ and H3+. No energy minima were found for the approach of the neutral species H and H2. The adducts formed between HeH+ or H3+and glycine were shown to be thermodynamically unstable with respect to GlyH+plus He or H2,respectively. Hence, all three of the attacking species H+, HeH+, and H3+are potentially capable of yielding protonated glycine, GlyH+,as the final product. Of the several approach directions considered, attack of the amine group was predicted to be the favored direction in neutral glycine, with calculated binding energies of 222.4, 126.4, and 54.7 kcal/mol for H+, HeH+, and H3+, respectively. For the glycine zwitterion, the greatest interaction with these ions was predicted to occur at the carboxylate group, with calculated interaction energies of 2285,157.1, and 90.4 kcal/mol for a collinear approach to the 0' atom. The ab initio results were further used, together with core ionization potentials, to predict the site and relative energetics of protonation of dipeptide, Gly-Gly, based on the proposed linear relationshipbetween ionization potential and proton affiiity. A method for extending this latter approach to larger peptides and to larger amino acids is suggested.
Introduction The mechanism of protonation of large molecules involved in biological processes is currently of interest due to the belief that protonated species are important intermediates for many biochemical reactions. The experimental determination of proton affinities, however, has previously been limited to molecules in the gas phase, i.e., to compounds having sufficiently high vapor pressures. Naturally, this criterion excludes many interesting compounds, although refinements in experimental techniques now permit some such molecules to be studied. Recent mass spectrometric studies of the simplest amino acid, glycine, have shown that the major product of ionmolecular association reactions is the protonated species, GlyH+.' As a result of these experimental findings, a number of theoretical investigations have attempted to determine the energetics of interaction between the bare proton and various biological molecules including amino Chung et al., for example, have investigated the gas-phase protonation of glycine using the CNDO method and have shown that a stable GlyH+ is expected theoretical1y.l Unfortunately, the problem may be considerable more complicated in that protonation of glycine may occur indirectly via protonating agents other than free H+. That is, interaction of glycine with ions such as Hz+and H3+may also result in stable or metastable adducts. To date, there have been no experimental or theoretical investigations on the protonation of amino acids by such ions. The need for data on amino acid association reactions with various hydrogenic ions is further prompted by a promising new experimental technique recently reported by Lively, Moran, and Powers for the tritiation of proteins? (1) K. Chung,R. M. Hedges,and R. D. Macfarlane,J. Am. Chem. Soc., 98,7623-5 (1976),and references cited therein. (2)J. B. Moffat, J. Theor. B i d , 40,247-58 (1973). (3)P.G. Mezey, J. J. Ladik, and S. Suhai, Theor. Chim. Acta, 51, 323-9 (1979).
0022-3654/82/2006-395 7 $0 1.2510
Whereas traditional tritiation methods have relied on homogeneous reactions in tritium-enriched aqueous solutions, the new approach involved a heterogeneous reaction between solid-phase proteins and gaseous tritium in the presence of an electron beam. The experimental conditions are such that exposure of gaseous T, to the electron beam is expected to produce T2+,T3+,T+, T, and HeT+ (the latter produced by radioactive 0 decay of T,) in the atmosphere above the solid protein sample. Any or all of these species could, in principle, function as a tritiating agent, but at present there is no experimental information to establish which of these species is active in attacking the protein and accomplishing T-H exchange. In an attempt to further elucidate the ion-molecular reactions of glycine, we report in the present work ab initio self-consistent field (SCF) calculations of potential energy surfaces for the approach of each of the species H+, H, H,, H3+,and HeH+ to glycine along certain hypothetical reaction coordinates. Both the neutral and zwitterionic forms of glycine were considered. Also included are some preliminary results on the protonation of the simplest dipeptide, Gly-Gly. The interaction energies determined for glycine were used to test the applicability of a linear relationship between core ionization potential and proton affinity to a m ino acids. This relation has recently been shown to give an excellent correlation for series of N- and 0-containing molecule^.^^^ In the present work, ionization potentials determined from free amino acid calculations and the calculated model proton affinities given herein are used to make predictions on the protonation of peptides. (4)M.0.Lively, T. F. Moran, and J. C. Powers, J . Biol. Chem., 254, 262-4 (1979). (5)D.W.Davis and J. W. Rabalais, J . Am. Chem. SOC.,96,5305-10 (1974). (6)R.L. Martin and D. A. Shirley, J. Am. Chem. SOC.,96,5299-5304 (1974).
0 1982 American Chemlcal Society
3952
The Journal of Physical Chemistry, Vol. 86, No. 20, 1982
Wright et ai.
TABLE I: SCF Energies and Bond Lengths for the Five Protonating Species and SCF Energies for Gly, +Gly-, and Gly-Gly compd
bond length, au
energy, au ~~
H' H
::+
HeH+ GlY Gly Gly-Gly
+
1.40 1.65 1.50
0.00000 -0.49823 -1.12674 -1.27352 - 2.90244 - 282.68841 - 282.63486 -485.88035
Comparison with experimental and theoretical results is also given, although the model calculations presented here are not always directly comparable to experimental proton affinities. The present calculations were greatly facilitated by recent ab initio SCF studies of several amino acids and peptides by Wright and Borkman.'a It is hoped that data from the present calculations together with new experimental information will ultimately result in elucidation of the mechanism(s) involved in protonation and tritiation reactions of amino acids and proteins.
Method of Calculation The molecular orbital calculations were performed on a Control Data Cyber 70 Model 74 computer. The HONDO (Version 5.0) program, written by Dupuis et al.e was used for all calculations to obtain restricted SCF energies. A split-valence 6-31G Gaussian basis setlo was used for all atoms with the exception of helium where a (5s)/[l] basis set" was employed. Previous calculations on glycine and other amino acids have shown that the 6-31G basis set gives SCF energies estimated to be within 0.13% of Hartree-Fock value^.^ Bond lengths and angles for glycine were taken from the standard values given by Pople and Gordon;12the C-0 bond length of the glycine zwitterion was taken to be 1.25 A. The conformations of neutral and zwitterionic glycine in the gas phase have been calculated in several theoretical s t ~ d i e s . ~ J ~ -The ' ~ most stable orientation of glycine predicted by each of these works was a planar arrangement of the heavy atoms with the dihedral angles 4 and)I being f180O. These conformations were used for all protonation calculations. The conformation of the dipeptide Gly-Gly used in the protonation calculations was the planar trans form predicted to be the most stable confiiation in the gas phase.8 Due to the large size of this molecule, a smaller 2slp Gaussian basis set was used. Computational details for Gly-Gly can be found in a previous theoretical study of peptides by Wright and Borkmams (7)L.R. Wright and R. F. Borkman, J.Am. Chem. Soc., 102,6207-10 (1980). (8)L.R.Wright and R. F. Borkman J. Phys. Chem., following article
in this issue. (9)M. Dupuis, J. Rys, and H. F. King, J. Chem. Phys., 64, 111-6 (1976). ~ - -_,_ .
(10)W. J. Hehre, R. Ditchfield, and J. A. Pople, J. Chem. Phys., 56, 2257-61 (1972). (11)S . Huzinaga, J. Chem. Phys., 42, 1293-1302 (1965). (12)J. A. Pople and M. J. Gordon, J. Am. Chem. SOC.,89,4253-61 (1967). (13)Y.C.Tse, M. D. Newton, S. Vishveshwara, and J. A. Pople, J. Am. Chem. SOC.,100,4329-31 (1978). (14)S. Vishveshwara and J. A. Pople, J.Am. Chem. Soc., 99,2422-6 (1977). (15)H. L. Sellers and L. Schafer, J. Am. Chem. Soc., 100, 7728-9 (1978).
'
H . . . . . r.. . , . X
/HA H
H . . . . .r . . . . . X
H-H
..... .....X
Figure 1. Depiction of internuclear distances ( I )between the protonating species and the attack sites (X).
Figure 2. Directions of approach for the protonating species to neutral glycine.
SCF energies and molecular bond lengths for the protonating species are presented in Table I. Also included are the SCF energies calculated for the most stable conformations of neutral and zwitterionic glycine and the neutral dipeptide Gly-Gly. These energies were used to determine dissociation limits for the various interactions presented herein. Calculations on the small molecules NH3 and CH3NH2, using the 6-31G basis set, yielded proton affinities of 217 and 232 kcal/mol, about 5 % above the accepted experimental values.6 Thus, the calculated proton affinities of glycine were expected to agree with experimental values to within 5-lo%, and this was subsequently verified for the case of the N protonation of glycine (see Discussion section).
Results Potential energy surfaces were calculated for the approach of three ionic (H+,HeH+, and H3+)and two neutral (H and H2) species to neutral glycine (Gly) in the gas phase. The protonation of the glycine zwitterion (+Gly-) was also considered, with potential energy curves calculated for interaction with the ionic species H+, HeH+, and H3+only. All interaction energies were calculated as a function of the internuclear distances (r) between the protonating agents and the various glycine attack sites (X), as illustrated in Figure 1. Interaction energies of neutral glycine were determined for the four directions of approach depicted in Figure 2. In this f i e , directions of approach A, B, and D represent attack at the N, C",and 0' atoms, respectively, in the y,z plane of the heavy atoms of glycine; direction of approach C is a perpendicular approach to the C". The molecular protonating agents were oriented such that steric hinderance between the species and glycine was minimzied. This resulted in the major axis of the diatomics lying along the direction of approach. In A, B, and D, H3+ was oriented in the x,z plane, perpendicular to the plane of the heavy atoms of glycine; in C, the H3+species was situated in the x,y plane. Figures 3-7 present the SCF energy vs. internuclear distance for the interaction of H+, HeH+, H3+,H, and H2, respectively, with neutral glycine. The four curves in each
The Journal of Physlcal Chemistry, Vol. 86,No. 20, 1982 3953
Protonation of Glycine
-282.40
n
1
H'
B
n+
NH, CH,
C 0
H'
CH,
H'
c-0
A
-28280
?
+
H
NH,
H
o
n
CH, CH,
-
c-o
0
I
> c3
-28280
z
W
1
-28300
-
-28320
-
t c7
-1
I
-283.00
A
B
c n
e,
W
E
-
h
w Z
w
10
30
20
40
60
50
70
90
80
INTERNUCLUR DISTANCE (Bohr)
t
30
20
10
50
40
70
60
80
-
90
Figure 8. Potential energy curves for the interaction of H with neutral glycine. Letters correspond to those directions of approach depicted in Figure 2.
i
INTERNUCLEAR DISTANCE (Bohr)
-
Figwe 3. Potential energy curves for the interactionof H+ with neutral glycine. Letters correspond to those directions of approach depicted in Figure 2.
L"""""""""
1
-28340
0,
+ L
o
1
'
1
'
1
~
1
'
l
'
1
'
1
'
/
'
1
'
I
l
i
\ HeH' HcH' HeH* HeH'
A
E C
D n 0)
J
-285.40
I W
NH,
CH, CH,
C-0
t
-28180
10
* 0
E
a
30
20
40
50
60
70
90
80
INTERNUCLEAR DISTANCE (Bohr)
Figure 7. Potential energy curves for the interaction of H2with neutral glycine. Letters correspond to those directions of approach depicted in Figure 2.
-28560
1
t
-285 80
F t
i H \,
H q N
+ I
I
H r
-28360 n
'
~
'
l
~
l
'
l
-
'
l
i
A
H;
NH,
B
n:
CH, CH, C-0
c 0
al
~
H: H;
~
l
I
-28380
-
-28400
-
l
w
-
TABLE 11: Equilibrium Interaction Energies ( D e ) and Internuclear Distances ( r e )for the Attack of H', HeH', and H,' along the Directions of Approach Depicted in Figure 2
I T De kcall mol
HeH+
9
10
20
SO
40
50
60
70
80
90
>Y
x
Figure 8. Directions of approach for the protonating species to the glycine zwitterion.
0 E
z
~
:
>W
l
l3'
f!
-5
'
-
INTERNUCLEAR DISTANCE (Bohr)
F W 5. Potential energy m e s for the interactionof H+ , with neutral glycine. Letters correspond to those directions of approach deplcted in Figure 2.
figure, which were smoothly drawn through an average of twelve calculated points,correspond to the four directions of approach described in Figure 2. The internal dimensions of glycine and of the attacking species were held constant at each internuclear distance along the direction of approach. Potential energy minima are seen to exist for the approach of all ionic species to Gly (Figures 3-5).
direction of approach
A B C
D
re, ii
De, kcall mol
H,+
De kcall re, ii mol re, ii 9
222.4 1.01 126.4 1.13 54.7 163.3 2.33 76.8 2.82 35.5 73.2 1.35
170.9
0.95
10.0
1.38 3.20
1.78
101.8 1.08 44.2
1.33
No minima, however, occurred in the approach curves for the neutral species H and Hz (Figures 6 and 7). Interaction energies (De,in kcal/mol) and equilibrium internuclear distances (re, in A) for the adducts formed between H+, HeH+, and H3+at the four attack sites of Gly were determined from the minima in their respective energy vs. distance curves, and are given in Table 11. The
3954
Wright et ai.
The Journal of Physical Chemistty, Vol. 86, No. 20, 1982 1
-
-282 40
A' -28360
B
-
E!
u
H: H:
C' D'
H:
E'
H:
H;
0CO' CH, CH,
C=0: C=O;
0
h
I
v
+ -28380 c3 W
Z
W
0
-28400
-28280
5. -283 00
t, )
,
,
I
1
,
,
I
30
20
,
40
!
,
1
50
1
1
1
l
70
60
,
I
90
80
INTERNUCLEAR DISTANCE (Bohr)
-
>
I
t
'
I
1 6' t
t
E w
6
'
I
'
I
'
I
'
'
\
'\
I1
-28560
50
60
70
80
90
Flgure 11. Potential energy curves for the interaction of H,+ with the glycine zwitterion. Letters correspond to those directions of approach depicted in Figure 8.
Flgure 9. Potential energy curves for the interaction of H+ wRh the glycine zwitterbn. Letters correspond to those directlons of approach depicted in Figure 8.
&
40
E'
10
-28540
30
20
INTERNUCLEAR DISTANCE (Bohr)
I
U
10
1
-E!
-
E
I v
8
'
1
'
d
Hen'
HeH*
CH,
D E'
He#
C-0 C-0:
Hen.
1
1
C'
CH,
..
i
a'
C'
TABLE 111: Equilibrium Interaction Energies ( D e )and Internuclear Distances ( r e ) for the Attack of H', HeH', and H,' along the Directions of Approach Depicted in Figure 8
i
HeH+
H+ De kcali mol
re, A
187.3 172.5 49.2 218.1 228.5
1.50 124.0 1.62 2.37 84.4 2.76 1.30 0.92 141.6 0.98 0.91 157.1 0.97
1
direction of approach
A' B C'
D E'
De, kcall mol
H,'
De, kcali re, A mol re, A
82.2 1.84 42.3 3.13 75.1 1.08 90.4 1.06
J
A'
t -28580
E' 10
20
50
40
50
60
70
80
INTERNUCLEAR DISTANCE (Bohr)
90
-
i
Flgure 10. Potential energy curves for the interaction of HeH+ with the glycine zwltterion. Letters correspond to those directions of approach depicted in Figure 8.
most strongly bound species at each attack site was H+, followed in order by HeH+ and H3+. The most favorable direction of approach for all three ionic species was that along A of Figure 2, i.e., at tetrahedral attack at the amine nitrogen. Interaction energies of zwitterionic glycine were determined for the directions of approach illustrated in Figure 8. B', C', and D' are identical with B, C, and D in Figure 2. A' represents an approach which bisects the OCO- bond angle. E' is analogous to D', each being attack at 0' approaching along the C-0 bond vector. Presented in Figures 9-11 are the potential energy curves calculated for the approach of H+, HeH+, and H3+ to +Gly-, with all internal coordinates frozen during each approach. As there were no interaction minima between the neutral species H and H2 with Gly, these potential surfaces were not computed for +Gly-. Each figure contains fives curves corresponding to the five directions of approach depicted in Figure 8. As with Gly, the greatest interaction occurred between the bare proton and each of the attack sites of +Gly-. The most favorable direction of approach was to the 0' atoms-directions of approach D' and E'-with the latter providing a more stable adduct. Equilibrium internuclear distances and energies computed for the interaction of the three ionic species with +Gig are summarized in Table 111. The interaction energy of Gly-Gly was determined with free H+ only at a single attack site, the nitrogen of the
peptide bond. Two directions of approach were considered: one perpendicular to the peptide plane and the second a backside attack of the nitrogen, collinear to the N-H bond. Due to proximity of the 0' atom, the latter approach showed a considerable interaction between this atom and the proton, not unlike that which resulted from protonation along the B and B' directions of approach of Gly and +Gly-, respectively (see Figures 4 and 10). In the present work, protonation of the peptide bond was predicted to occur at the 0', in agreement with several experimental studiesS2Interaction energies for the dipeptide as a whole, however, were calculated to be about 30 kcal/mol greater at the N terminal than at the peptide bond. 0 protonation, which can occur at either the peptide bond or the C terminal, was calculated to be favored at the former site by about 20 kcal/mol. Since a smaller basis set was used for the peptide calculations, caution must be exercised in making quantitative comparisons to the amino acid results.
Discussion Of all the interactions studied between neutral and zwitterionic glycine and the various potential protonating agents, the most stable adducts were invariably formed with the bare proton. Along every direction of approach, a stable GlyH+ (proton binding energies of 73-222 kcal/ mol) was predicted to result. With HeH+ and H3+,metastable adducts were predicted to exist only when approaching the nitrogen or oxygen attack sites, but not the carbon site. The exception was a small binding (10 kcal/mol) of HeH+ to C" along direction of approach C of neutral glycine. No minima were found in the potential curves for the interaction of the neutral p-otonating species H or H2 with glycine. Attack of the ionic species along directions of approach B and B' of neutral and zwitterionic glycine, respectively, resulted in minima in the potential surfaces at relatively
The Journal of Physical Chemistry, Vol. 86, No. 20, 1982 3955
Protonation of Glycine
TABLE IV: Total Energy Differences (eV) for the Formation of GlyH+ from the Interaction of Neutral Glvcine with the Ions H'. HeH'. and H,' direction of approacha
A C
D
Gly
+ I-
GlyH+ + J
10
-
8-
I = H', J=-
I = HeH', J = He
I = H3+, H = H,
- 9.64 -3.17 - 7.41
-8.49 - 2.01 -6.25
+ 0.82
-5.65
-3.42
Directions of approach to neutral glycine are depicted in Figure 2.
64-
2-
0-
TABLE V : Total Energy Differences (eV) for the Formation of GlyH' from the Interaction of the Glycine Zwitterion with the Ions H', HeH', and H,' direction of approacha C'
D'
E'
+Gly- + I
I = H+, J=-2.13 -9.46 -9.91
-
-Gly + HeH'
GlyH+ + J
I = HeH', J = He
I = H3+, J = H,
- 0.98
+1.86
-8.30
-5.47 - 5.92
- 8.7 5
-GlyHcH' 3
a Directions of approach to the glycine zwitterion are depicted in Figure 8.
GlyH'
O t
large internuclear distances compared to the other directions of approach. Since this distance was measured from the protonating species to the attack site (in this case, C"), the location of these potential wells seems to have been influenced by additional structural features. These minima all occurred as the protonating species passed within 1.2 A of the oxygen atom of the neighboring carbonyl group, at a distance of about 2.7 A from the primary attack site. This secondary interaction distance compares favorably to an average 1.0 A found for attack along direction of approach D. Hence, the calculated potential w e b in these cases are largely due to interaction with the 0' atom. As previously stated, the most stable product formed by the attack of the protonating species to glycine was GlyH+;along certain directions of approach, the metastable ions GlyHeH+ and GlyH3+were also predicted to exist. These latter produds, however, are not thermodynamically stable with respect to dissociation to GlyH+ He or to GlyH+ + Ha, respectively. Figure 12 depicts the exothermic pathways from the initial reaction between neutral glycine and the ionic protonating species and the final product GlyH+ for approach to the nitrogen atom (direction of approach A of Figure 2). Similar diagrams can be drawn for each of the other directions of approach depicted in Figure 2 for neutral glycine and in Figure 8 for the glycine zwitterion. Given in Tables IV and V are the total energy differences for the formation of GlyH+ from the interaction of neutral and zwitterionic glycine with the ions H+, HeH+, and H3+. Thus, the protonated form of glycine (GlyH+) can likely be formed from both the free proton and the other ionic protonating agents. Due to the low vapor pressure of glycine, the experimental determination of the proton affinity has been difficult. Recently, however, Meot-Ner, Hunter, and Field, using a pulsed high-pressure mass spectrometer, determined the gas-phase proton affinity of glycine to be 208.2 kcal/mol;16 this is in good agreement with the theoretical value obtained in the present work for N protonation (222.4 kcal/mol). In the glycine zwitterion, protonation was favored along directions of approach to the carboxylate group. Here
+
Giy
+
.
q
+
He-
+
H,
H;
GlyH'
Figure 12. Total energy differences for the formation of GlyH' as a result of N protonation of neutral glycine.
there are two oxygens capable of accepting the protonating species H+, HeH+, and H3+. In each case, protonation was calculated to be preferred at 0; rather than 01' by an average of 14 kcal/mol (see Figures 9-11, directions of approach D' and E', and also Table 111). This is probably due in part to the stabilization of the amino acid by the formation of an intramolecular hydrogen bond between 01' and the amine group, resulting in a lower electron compared to 02/. It is interesting to note density on 01' that these results are consistent with predictions based on the linear relationship between proton affinities (PA) and inner-shell ionization potentials (IP)proposed by Davis and Rabalais5and, independently, by Martin and Shirley? For oxygen- and nitrogen-containingmolecules, each group proposed the relationship: PA = -IP,,,, + constant (x = 0, N) (1) where IP,,,x, is the ionization potential of the 1s oxygen or nitrogen electron, respectively. The la0 ionization potentials of the 0; and 0; atoms of the glycine zwitterion are 561.1 and 559.8 eV, respectively;" using eq 1, one would predict the protonation of 04 to be more favorable than that of 01' as indicated by the calculations in the present work. Similar ideas can also be applied to the simplest dipeptide, Gly-Gly, where there are five potential nitrogen and oxygen protonation sites. Of these, two have already been considered in the free glycine calculations, namely the N-terminal nitrogen and the carbonyl oxygen of the
(16) M. Meot-Ner, E.P. Hunter,and F. H. Field, J. Am. Chem. Soc.,
101, 686-9 (1979).
(17) L.R.Wright, Ph.D. Thesis, Georgia Institute of Technology,1981.
3956
J. Phys. Chem. 1982, 86,3956-3962
C-terminal of the neutral molecule. The peptide linkage of the dipeptide, however, has considerable double bond character owing to the resonance structures.
Thus,as a first approximation, the carbonyl of the peptide bond may be considered to be similar to that of the amino acid zwitterion. From previous calculations on Gly-Gly, the order of ionization of 1s electrons was1' 0: N:
IPC terminal IPpeptide
'IPpeptide 'IPN
bond
bond
terminal
Therefore, the proton affmity of the oxygen at the peptide bond is predicted to be greater than that of the carboxylate group. Conversely, protonation at the nitrogen of the peptide bond is predicted to be less favorable than that at the N terminal. Hence, given the assumption above and the calculated proton affinities of the nitrogen of neutral glycine and the oxygen of the glycine zwitterion, the interaction energy of a proton with the peptide bond would be expected to be greater at the oxygen. This conclusion, arrived at through the use of eq 1and the present work on the amino acid glycine, is consistent with experimental fmdings that 0 protonation is favored during protonation of the peptide linkage2 and the limited calculations on Gly-Gly presented above.
A similar analysis of the zwitterionic dipeptide cannot be performed here as there is only one nitrogen capable of protonation and no trend can be established. However, in a polypeptide where the central bonds are not significantly influenced by the charged terminals, the conclusions drawn above may be applicable. In a previous theoretical investigation,' it was found that the inductive effects of the side chains in alanine, serine, cysteine, and threonine influenced the proton affinity relative to glycine. Electron donating groups (e.g., -CHJ tended to increase the interaction energy of a proton with an attack site while electron-withdrawing groups (e.g., -OH) had the opposite effect. From the relationship given in eq 1,therefore, the ionization potential of, for example, the lsNof the peptide bond of Gly-Ala should be calculated (or determined) to be less than that of Gly-Gly. Results from a previous theoretical study predict this to be the c8se.l' It must be stressed that quantitative results for peptides cannot be made on the limited amount of data presented here, but only qualitative trends of protonation based on the results of theoretical studies of amino acids. The use of eq 1has resulted in at least a limited number of predictions which have been shown to be accurate. Theoretical calculations may therefore be useful in determining both proton affinities and preferred sites of attack in molecules containing oxygen and nitrogen atoms, such as amino acids and peptides. This approach may be especially important in cases where proton affinities are difficult to determine experimentally.
Ab Inltlo Self-Conslstent Field Studies of the Peptldes Gly-Gly, Gly-Ala, Ala-Gly, and GIy-Gly-Gly Lance R. Wrlght and Raymond F. Borkman" School of Chemlsby, &wg& Institute of Technoey, Atlanta, GsOrgla 30332 ( R w W d : January 26, 1982; In Flnal Form: June 21, M82)
Ab initio self-consistentfield calculations have been performed on the three smallest dipeptides: glycylglycine, glycylalanine, and alanylglycine. SCF energies are reported for the neutral and zwitterionic forms of the molecules by using a 2slp Gaussian basis set. The quality of this basis set was tested by calculating a conformationsurface for the model compound N-formylglycylamideand comparing this map with those from previous calculations in the literature. The energy surfaces were found to be parallel to within a few kcal/mol. The barrier to rotation about the peptide bond was computed for each of the dipeptides studied; these values ranged from 22.9 f 0.4 kcal/mol for the neutral species to 23.9 f 1.8 kcal/mol for the zwitterionic forms. All of the neutral dipeptides, and also the zwitterions +Gly-Ala-and +Ala-Gly-were found to be most stable with the peptide bond in the trans orientation. The cis conformation, however, was calculated to be preferred in the +Gly-Gly-zwitterion. In addition to determining the energy minimum associated with rotation about the peptide bond, rotation barriers were determined for the other torsion angles of neutral Gly-Gly. Six metastable conformations of Gly-Gly were found which were less than 4 kcal/mol above the most stable conformation. Dipole moments and ionization potentials were also computed for each of the dipeptides; the average values for these properties were calculated to be 2.8 f 0.1 D and 10.7 f 0.1 eV for the neutral molecules and 26.0 f 1.4D and 5.2 f 0.2 eV for the zwitterionic species, respectively. Finally, SCF energies were calculated for the neutral and zwitterionic forms of the tripeptide Gly-Gly-Gly in the extended trans conformation.
Introduction The conformations of peptides play a fundamental role in determining the structures and hence the biological properties of proteins.' As a result, numerous theoretical (1) G.N.Ramechandran and V.Sasisekjaran,Adv. Protein Chem., 23, 283-438 (1968). 0022-3654182120863956$0 1.2510
investigations have been performed in an effort to characterize the peptide bond unit. In vacuo calculations allow one to determine conformation maps of molecules free of bulk medium effects and can be used to further the understanding of the structure of proteins. In many previous studies, "model peptides" of t h e type RCONHCH,CONHR, where R is usually a hydrogen or 0 1982 American Chemical Society
