Int. J. Peptide Protein Res. 28, 1986, 637--648

Backbone side chain interactions in peptides. VI

Crystal structures of four tBuCO-Pro-X-NHMe dipeptides with X = His, His', PF6 -, His( 7-Me) and His( n-Me)

A. AUBRY' , M. VLASSI' and M. MARRAUD3

Laboratory of Mineralogy and Crystallography, UA-CNRS 809 University of Nancy I, Nancy, France, Department of Molecular Physical Chemistry and Crystallography,

University o f Louvain, Louvain, Belgium and Laboratory of Macromolecular Physical Chemistry, UA-CNRS 494, ENSIC-INPL, Nancy, France

Received 10 February, accepted for publication 9 May 1986

The crystal structures of 4 dipeptides with the general formula tBuC0-Pro-X- NHMe (X = His 1, His(.r-Me) 17, His(n-Me) In, and His', PF6- 1') were solved by X-ray diffraction experiments. The first 3 derivatives accommodate a PI turn conformation with the classical i -k 3 -+ i hydrogen bond. Moreover, 1 and 17 contain an additional intramolecular interaction between the His-NH and His- N" sites. The protonated dipeptide li assumes a completely different confor- mation in which the CO(tBu) carbonyl exhibits a strong intramolecular hydrogen bond with His+-N"H. The pKa of the His residue allows its existence in both the neutral and protonated states at physiological pH, and the crystal structures of 1 and 1' are the first examples of histidine-containing peptides investigated in the neutral ana protonated states. They give some information on the possible His-imidazole interaction modes as a function of the electrical state. Key words: p-turn; crystal structure; histidine; hydrogen bond; peptide conformation; X-

ray diffraction

The histidine residue is frequently encountered in the functional sites of proteins and enzymes, and its imidazole ring is often involved in inter and intramolecular contacts. The multiple tautomeric neutral and protonated states of the ring (1, 2) confer different possible interaction modes on the His residue (3). As part of a general program devoted to tBuCO-X-Y-NHMe dipeptides (4, S ) , we prepared some Pro-His derivatives and obtained single crystals of 4

Abbreviations follow the recommendations of IUPAC- IUB Commission on Biochemical Nomenclature (6, 14).

derivatives containing the neutral His residue, the protonated His' residue associated with the PF6- anion, and the N" and N7 methylated homologues: 1, tBuCO-Pro-His-NHMe; In, tBuCO-Pro-His(n-Me)-NHMe; 17, tBuCO-Pro- His(7-Me)-NHMe; and l', tBuCO-Pro-His'- NHMe, PF6-. This provided the opportunity of' obtaining precise information on the possible interaction modes of histidine by solving the crystal structures of these 4 dipeptides by X- ray diffraction experiments. The results presented in this paper are discussed with reference to those concerning the confor- mational properties in the solute state of the

631

A . Aubry et al.

same dipeptides investigated by 'H NMR and IR spectroscopy ( 3 ) . Some preliminary results have been presented at the 8th American Peptide Symposium (7).

EXPERIMENTAL PROCEDURES

The synthesis of the Pro-His dipeptides is described in the preceding paper of this series (3). Single crystals of 1, In and 17 were ob- tained by slow evaporation of ethyl acetate solutions. The protonated dipeptide 1+ was prepared by mixing equimolecular water solutions of 1 with diethyloxonium hexafluoro- phosphate (Aldrich), followed by lyophilisation, and crystallization in CHC13 containing 10% MeCN.

Data were collected at room temperature on an Enraf Nonius CAD 4 automatic diffracto- meter, with a graphite monochromator and CuKa radiation with 0 < 70" (8 - 28 scanning mode). Intensity data were corrected for decay when three standard reflections decreased by more than 5%, and corrected for Lorentz and polarization effects. The absorption was dis- regarded (p R < I), Cell dimensions obtained by refinement from a set of 25 high angle reflections are indicated in Table 1, together with other experimental parameters.

The structures were solved by direct

methods by means of the computer program MULTAN-80 (8), and refined by a full matrix least-squares procedure (9). Atom scattering factors used were those listed in the Inter- national Tables for X-Ray Crystallography (10). E maps revealed all the nonhydrogen atoms and two equally occupied positions, called A and B, for the PF6- anion in 1'. The hydrogen atom of 1, In and 17 appeared in different map, but those of 1' could not be located because of the PF6- disorder. Refined parameters were calculated by using anisotropic temperature factors for the nonhydrogen atoms and fixed isotropic thermal factors for the hydrogen atoms.

To allow comparison with the data from the literature, and following the recommendations of Taylor & Kennard ( l l ) , NH hydrogen atoms were replaced at 1.03 a from N in the observed NH direction (Table 2).

RESULTS

Molecular dimensions The precision of a crystal structure depends upon the number of significant reflections, the amount of thermal atomic vibrations, and the existence of atomic disorder. The crystal structures of the neutral dipeptides 1, 17 and In were the most accurate, with low thermal

TABLE 1 Oystallographic data for tBuCO-Pro-His-NHMe 1, tBuCO-Pro-His (7-Me)-NHMe 17, tBuCO-Pro-His (n-Me)-NHMe

In, and tBuCO-Pro-His+-NHMe, PF; 1+

1 l r In l+

Space group p2, p2,2,21 p212121 p212121 2 2 4 4 4 Dimensions a, A 9.979 (1) 7.880 (1) 8.867 (1) 9.200 (1)

b, a 9.770 (1) 15.037 (4) 13.363 (2) 16.747 (4) c, A 11.055 (2) 16.249 (4) 16.611'(2) 17.595 (4) P, deg 108.28 (2) - - -

- - Density calculated 1.18 1.25 Independent reflections 1846 2128 2176 2622 Unique reflectionsa 1512 1995 2021 1562 R 0.040 0.039 0.055 0.15

1 Iw 1.1473 u2 (F) + u2 (F) -!- a* (F) -k 0.044 0.046 0.062 -

0.00365 F2 0.0021 FZ 0.00147 F2

R w

'1 > 1.50 (I)

638

Fractional coordinates with standard deviations, and equivalent thermal parameters (A’) for tBuCO-Pro-His-NHMe, 1, and tBuCO-Pro-His’-NHMe, PF;, I +

1 I + Atom

x Y 2 Beq x Y 2 Beq

0.1579 (5)

0.1409 (6) 0.1324 (4) 0.2726 (4) 0.3998 (3) 0.2613 (3) 0.1403 (4) 0.2295 (5) 0.3526 (5) 0.4046 (4) 0.4717 (4) 0.6095 (3) 0.3759 (3) 0.4344 (3) 0.5389 (3) 0.6233 (3) 0.5268 (3) 0.6129 (6) 0.2295 (3) 0.1849 (3) 0.1813 (4)

0.0593 (3) 0.0525 (3) 0.2 7 2 8a 0.4619a 0.0207a

-0.0281 (4)

-0.0166 (4)

0.0240 (5) -0.1751 (5) -0.1533 (7) -0.1324 (4) -0.2102 (4) -0.1515 (4) -0.3410 (4) -0.4452 (4) -0.5795 (5) -0.5541 (4) -0.4069 (4) -0.3399 (4) -0.3606 (4) -0.2635 (0) -0.1906 (4) -0.0703 (4) -0.0168 (4) -0.0214 (4)

0.0972 (6) -0.1434 (5) -0.0490 (4)

0.0895 (4) 0.0118 (4)

0.1266 (4) -0.0977 (4)

-0.2448a -0.07 1 8a

0.22 17a

0.3827 (5) 0.3134 (5) 0.5441 (4) 0.4047 (3) 0.3856 (3) 0.4088 (2) 0.3425 (2) 0.3404 (4) 0.3578 (5) 0.2937 (5) 0.3311 (3) 0.2362 (3) 0.2438 (2) 0.1430 (2) 0.0527 (3) 0.1124 (3) 0.0566 (3) 0.2217 (3) 0.2845 (5)

-0.0626 (3) -0.0321 (3) -0.0333 (4)

0.0203 (3) 0.0022 (3) 0.0008 (3) 0.1384a 0.2634a 0.0212a

4.9 4.4 5.3 3.1 2.6 3.5 2.4 3.6 4.1 3.8 2.8 2.7 4.0 2.3 2.4 2.9 4.4 3.3 4.6 2.7 2.4 3.2 2.9 2.6 3 .O

-0.4347 (29) -0.2804 (28) -0.4202 (23) -0.3283 (20) -0.2001 (18) -0.2083 (12) -0.0800 (15) -0.0478 (24)

0.0836 (24) 0.1632 (21) 0.0322 (16)

-0.0217 (18) -0.1144 (13)

0.0351 (13) 0.0005 (15) 0.1426 (19) 0.2528 (13) 0.1323 (14) 0.2618 (25)

-0.0850 (17) -0.2428 (17)

0.3702 (19) -0.4244 (21) -0.2752 (15) -0.4779 (16)

b b b

0.0178 (16) 0.0071 (14)

-0.1098 (13) -0.0345 (11) -0.0528 (9) -0.0407 (6) -0.0863 (8) -0.1201 (15) -0.1804 (14) -0.1407 (13) -0.1104 (10) -0.1775 (9) -0.2248 (6) -0.1769 (7) -0.2368 (8) -0.2689 (10) -0.2296 (8) -0.3417 (8) -0.3776 (12) -0.1994 (11) -0.1803 (11) -0.2142 (12) -0.1195 (14) -0.1199 (9) -0.1734 (12)

b b b

0.9987 (15) 1.1260 (13) 1.0560 (14) 1.0441 (10) 0.9944 (10) 0.9246 (7) 1.0194 (7) 1.0977 (12) 1.0832 (11) 1.0175 (10) 0.9676 (10) 0.9135 (10) 0.9348 (7) 0.8418 (7) 0.7894 (10) 0.7532 (10) 0.7535 (10) 0.7214 (9) 0.6827 (12) 0.7218 (12) 0.7485 (12) 0.7180 (15) 0.8087 (11) 0.8005 (8) 0.7597 (12)

b b b

9.6 7.9 7 .O 5 .O 3.6 4.3 3.4 6.7 6.9 5.6 3.8 3.7 4.6 3.2 3.4 4.3 8 .O 4.9

W 6.7 6

ii 4.7 Cr 0

5.0 g 6.4 E. a 5.4 ; 4.5 6.7 F:

z CD

e, - 2. 5‘ 2 w

\o

aThe hydrogen atom was moved in the observed N-H direction until N-H bond length was equal to 1.03 A (Ref. 11). bHydrogen atom not located on E map differences.

TABLE 2b Fractional coordinates with standard deviations and equivalent thermal parameters (A2) for tBuCO-Pro-His(r-Me)-NHMe 17, and tBuCO-Pro-His(n4fe)

NHMe, In .9

17 In G z x Y 2 Beq x Y 2 Beq m

o\ P 0

9

r I-)

0.5460 (5) 0.8398 (5) 0.7675 (5) 0.7328 (3) 0.7648 (3) 0.6429 (2) 0.9235 (2) 1.0893 (3) 1.1992 (3) 1.1287 (3) 0.9368 (3) 0.8624 (3) 0.8015 (3) 0.8776 (2) 0.8155 (3) 0.6336 (3) 0.5736 (3) 0.5473 (3) 0.3708 (3) 0.9358 (3) 0.9621 (3) 0.9643 (3) 1.0280 (3) 1,0012 (3) 1.0074 (3)

1.0271 (7) 0.9229 0.5981

-0.2298 (2) -0.2628 (2) -0.2502 (2) -0.2140 (1) -0.1129 (1) -0.0608 (1) -0.0785 (1) -0.1215 (2) -0.0505 (2)

0.0357 (2) 0.0192 (1) 0.0681 (1) 0.1427 (1) 0.0257 (1) 0.0611 (1) 0.0336 (2) 0.0535 (2)

-0.0099 (2) -0.0343 (3)

0.0366 (2) -0.0611 (1) -0.1067 (2) -0.1961 (2) -0.1182 (1) -0.1930 (1)

-0.2667 (3) -0.0383 -0.0250

0.8334.(3) 0.7892 (2) 0.9401 (2) 0.8531 (2) 0.8525 (1) 0.8488 (1) 0.8553 (1) 0.8661 (2) 0.9047 (2) 0.8712 (2) 0.8624 (1) 0.7888 (1) 0.7963 (1) 0.7162 (1) 0.6388 (1) 0.6182 (1) 0.5515 (1) 0.6761 (1) 0.6642 (2) 0.5677 (1) 0.5552 (1) 0.4826 (1) 0.5848 (2) 0.6194 (1) 0.5023 (1)

0.4448 (3) 0.7111 0.7328

5.9 4.9 4.8 3.2 2.4 3 .O 2.5 3.6 3.8 3.5 2.6 2.5 3.6 2.5 2.6 3.1 4.9 3.6 4.5 3.0 2.6 3.5 3.6 3.3 3.7

6.2

0.7498 (5) 0.9524 (5) 0.8454 (6) 0.8116 (4) 0.6952 (4) 0.7141 (3) 0.5686 (3) 0.5083 (4) 0.3377 (5) 0.3275 (5) 0.4536 (4) 0.5048 (4) 0.4466 (3) 0.6139 (3) 0.6687 (4) 0.8375 (4) 0.9170 (3) 0.8871 (4) 1.0401 (6) 0.6366 (4) 0.4773 (4) 0.4218 (5) 0.2308 (5) 0.3528 (3) 0.2673 (4) 0.3496 (6 )

0.6478 0.8119

-0.2298 (3) -0.1712 (5) -0.0562 (4) -0.1376 (3) -0.1030 (3) -0.1207 (2) -0.0536 (2) -0.0351 (3) -0.0284 (4)

0.0177 (4)

0.0276 (3) 0.0145 (2) 0.0960 (2) 0.1605 (3) 0.1428 (3) 0.2086 (3) 0.0509 (3) 0.0200 (7) 0.2707 (3) 0.2882 (3) 0.3063 (3) 0.2959 (3) 0.2819 (2) 0.3120 (3) 0.2667 (5)

0.1064

-0.0337 (3)

-0.0040

1.1083 (3) 1.0192 (3) 1.1247 (3) 1.0646 (2) 1.0015 (2) 0.9298 (1) 1.0242 (2) 1.1048 (2) 1.0918 (3) 1.0084 (3) 0.9617 (2) 0.8900 (2) 0.8240 (2) 0.9033 (2) 0.8396 (2) 0.8237 (2) 0.7969 (2) 0.8383 (3) 0.8243 (3) 0.8562 (2) 0.8844 (2) 0.9577 (2) 0.8829 (3) 0.8345 (2) 0.9571 (2) 0.7485 (3)

0.9620 0.8552

R 4.8 p-.

6 .O 5.5 4.0 3.8 4.6 3.8 4.4 6.0 5.6 3.9 3.8 5.6 3.6 3.8 4.9 7 .O 5.8 8.9 4.1 3.9 4.7 4.9 4.3 5.2 6.7

Backbone side chain interactions

parameters, even for the proline cycle (Table 2) which is known for its conformational flexi- bility (12). Conversely, the large thermal coefficients for 1' (Table 2a), and the atomic disorder of the PF6- anion (Table 2c), giving two distorted octahedrons (P-F bond length of 1.60( 17)A, F-P-F bond angle of 90( 15)"), resulted in a much less accurate crystal structure determination. However, the precision was sufficient to determine the peptide molecular conformation.

The geometry of the peptide groups was very similar to that already reported for homo- logous dipeptides (9, but that of the histidine residues deserves some comments. In most crystal structures, His is found either in the N'H tautomeric neutral form or in the pro- tonated form (1). The comparison of the average dimensions of these two forms (Fig. la, b) indicates that protonation exerted only slight changes in the imidazole geometry, with a C;-N,7 bond shortened by 0.02 A and a small variation by nearly 3" for four of the intra- cyclic bond angles.

N"- and N'-substituted residues are much less frequent, since Boc-His(n-CH2 OBz1)-OH is the only crystal structure solved so far (13). One notes that the geometry of His(.r-Me) in 1.r (Fig. Id) was as expected quite similar to that of the more common His(.r-H) residues (Fig. la). Conversely, His(n-Me) (Fig. lc)

3- \>

(C 1 ( d ) c FIGURE 1 Dimensions of the His(T-H), a, His', b, His("-Me), c, and His(.r-Me), d, residues. Bond lengths (A) and bond angles (deg) are average values taken from the literature for His(T-H) and His+, and the experimental values for His(a-Me) and His(T-Me). The 2 subscript indexing histidine has been omitted for clarity.

TABLE 2c Fractional coordinates wirh standard deviations and equivalent thermal parameters (A') for the two positions A

and B of the PF, 'anion of tBuC0-Pro-His+-NHMe, PF, '

P (A) 0.2589 (10) -0.0371 (5) 0.2941 (5) 3.0 P (B) 0.2684 (10) -0.0255 (5) 0.2664 (5) 2.9 F, (A) 0.4309 (20) -0.0057 (11) 0.2864 (12) 4.4 F, (A) 0.1033 (21) -0.0596 (12) 0.2756 (13) 4.7 F 3 (A) 0.2273 (33) -0.0295 (16) 0.2173 (16) 8.6 F, (A) 0.3006 (45) -0.0945 (21) 0.3436 (20) 10.5 F, (A) 0.2959 (26) -0.1157 (13) 0.2503 (13) 6.2 F, (A) 0.2321 (23) 0.0502 (11) 0.3232 (10) 4.4 F, (w 0.2136 (25) 0.0248 (12) 0.3522 (12) 5 . 5

F, (B) 0.3893 (41) -0.0595 (22) 0.3379 (21) 11.2 F, (B) 0.3720 (42) 0.0220 (22) 0.2470 (20) 12.1 F, (B) 0.1185 (37) -0.0177 (20) 0.2352 (20) 10.4 F, (B) 0.1639 (56) -0.0878 (28) 0.3310 (27) 15.0

F, (B) 0.3171 (22) -0.0906 (11) 0.2115 (12) 4.8

64 1

A. Aubry et al.

differed mainly by the C:-N; (lengthened by 0.04a), CZ-N: (shortened by O.OSA), and Cy-C: (shortened by 0.02 a) bond lengths, and by the bond angles at C l , C: and N;. This probably illustrates the different hybridization states for the imidazole rings of His(n-Me) and His('r-Me) (Fig. 2).

Molecular con formations Fig. 3 shows stereoscopic views of the mol- ecular conformations; the torsional angles (14) are listed in Table 3. All amide bonds are nearly trans planar groups (a1 z 180"), and the Pro pyrrolidine ring assumed the classical CY-exo(1, 17, ln) or Cy-endo (1') confor- mations (1 2).

The molecules 1, 17 and l n containing unionized His residues were folded by the so- called i + 3 + i hydrogen bond (Table 4), typical of a fi turn conformation (1 5). This is of the PI type as revealed by the and q2 torsional angles (Table 4). Another intramole- cular interaction involving the His-N2 H and His-NT sites occurred in 1 and 17, with a rather infrequent disposition of the His C:-Cf bond (16, 17) corresponding to xi s 6 0 " (rotamer

111). This interaction was obviously absent from In, and the His Cf-Cf bond assumed the more frequent disposition corresponding to the so- called rotamer I(16, 17).

The molecular conformation of the ionized dipeptide 1' was quite different from the preceding one. The His' residues accommodated an extended conformation (Fig. Ib), with a short atomic contact between the imidazolium fl and the ChO,(tBu) sites (Table 4). The two amide NH bonds were not in contact with a carbonyl group as is usually found, but with the PF, anion, with several N. . .F distances rangin from 3.0 to 3.3A (Table 4). The His' C:-C2 bond again assumed the rotamer 1 orientation, and the His' Cf-Cx bond was rotated by nearly 180" with reference to 1 (Table 3).

1

Molecular packing The molecular packing of the molecules is represented in Fig. 4, and the short intermole- cular contacts are given in Table 4. The imidazole rings of 1 were hydrogen bonded by Ni-H. . .% interactions, N; being the common accepting site for N;H (short contact) and His- N2H (larger contact at the upper limit for

FIGURE 2 Representation of the main contributing mesomeric structures for the imidazole ring of His(?-Me), a, and His(7- Me), b.

642

Backbone side chain interactions

9

hydrogen bonding). The imidazole rings of In and 1+ were also engaged in intermolecular interactions with amide functions, acting as an accepting site for His-N2H in lr, and as a donating site to His+-Cz'Oz in 1'. No short contact was found for 17.

DISCUSSION

The basic or protonated state of the His imidazole ring has a direct influence on the molecular conformations of Pro-His dipeptides. The basic dipeptide 1 accommodates a 01 turn conformation in which His-NT is engaged in a double interaction with His-N2 H (weak intra-

molecular hydrogen bond) and a neighboring imidazole NZH bond (Fig. 4a). The latter inter- action is obviously absent from 17 (Fig. 4b), allowing a shorter intermolecular His-N2 H to His-N; contact (Table 4) with a slight rotation of the His C:-Cg bond (Table 3) .

Protonation of the imidazole ring in I' results in a completely different molecular conformation, with an intramolecular His+-N2 H to CfoO0 contact (Fig. 2d). To our knowledge, this is the first example of a non-0-folded RCO- Pro-X-NHR' dipeptide in the solid state, indicating that the imidazolium ring is a stronger proton donating site than the C-terminal NH bond (5, 18-22).

643

A. Aubry et al.

FIGURE 3 Stereoviews of the crystal molecular conformation for tBuCO-Pro-His-NHMe, a, tBuCo-Pro-His(~-Me)-NHMe, b, tBuCO-Pro-His(n-Me)-NHMe, c, and tBuCO-Pro-His+-NHMe, d. The intramolecular hydrogen bonds are indicated by a thin line.

It must be noted that the crystal molecular structures assumed by 1 (Fig. 3a) and 1' (Fig. 3d) are essentially retained in the solute state (3). The only difference concerns 1' for the orientation of the His' Cg-C! bond, with a rotational angle xi = - 74" (rotamer I) in the crystal against x i -60" (rotamer 111) in the solute state. In fact, molecular models reveal

that both dispositions are compatible with the His'-NtH to CA Oo (tBu) interaction, a1 tho ugh the latter is the most stable conformation in solution. The molecular packing forces in the crystal are probably responsible for this local conformational transition.

The PI turn accommodated by 1 is also to be noted with reference to the fact that most

644

Backbone side chain interactions

TABLE 3 Conformational angles (deg)a for tBuCO-PTo-His-NHMe 1, tBuCO-Pro-His(7-Me)-NHMe IT, tBuCO-Pro-His

fn-Me)-NHMe In, and tBuC0-Pro-His+-NHMe, PF; 1'

Atoms Angle 1 IT I n I +

Cb -N , -C:C', N, -CyC; -N, C', -N, -Cy-C; N, -C:-C: -N, CzCb -Nl -Cy CY-C; -N, -C: CY-C;-N,-CY N , C: -Ct -CT

Ct-CT-Cf -N , CTCf -Nl -Cy Cf -N , -Cy-Cf N, -C;-C$ -CT C:-Cf-CT-N:

cy-c~-c:-c:

c:c$c:-c:

-63.2 -22.5 -70.4 -19.6

-178.4 176.8

-176.5 -31.3

39.5 -32.0

12.6 11.6 59.9

-86.1 94.6

-63.0 -32.8 -90.8

7.4 -174.3 -179.9

176.4 -23.8

33.8 -30.5

16.1 4.8

58.8 -48.6 136.4

-59.1 -30.7

-118.3 29.8

-172.7 -178.3

177.8 -31.1

39.7 -32.9

14.1 10.5

-46.0 -72.8 103.1

-64 147

-131 159 173 177 178 36

-40 32

- 10 -16 -74

69 -118

aThe standard deviations are 0.4" for I , 0.3" for IT, 0.4" for In, and 2" for 1+.

TABLE 4 N-H. . . A hydrogen bond distances (A) with standard deviations

data set N. . .A

t BuCO-Pro-His-NHMe N,-H.. .OOa

N:-H.. .qb N, -H. . .Nq a

t BuCO-Pro-His(T-Me)-NHMe N,-H. . .O, a N, -H. . .N: a

t BuCO-Pro-His(n-Me)-NHMe N,-H.. .O, a

N, -H. . .N:

tBuCO-ProHis+-NHMe, PF, q - H . . .O, a N:-H.. .O, N,-H.. .F, (A) N,-H.. .F, (A) N,-H . . . F, (B) N,-H.. .F, (B) N,-H.. .F, (A) N,-H.. ( A )

x, Y, x, Y, =

x, Y, z 112 + x, 112 - y, 2 - 2

x, Y, 2 -1 + x , y , z

112 - x, - y, 112 + 2 112 - x, - y, 112 + z 112 - x, - y, 112 + 7.

-112 + x , - 1 1 / 2 - y , l - z -1/2+x,-11/2-yy,1--z -112 + x, - 112 - y, 1 - z

2.945 (5) 3.218 (3) 2.872 (5)

3.004 ( 3 ) 2.847 (2)

3.150 (5) 2.956 (4)

2.62 (2) 2.65 ( 2 ) 3.03 (2) 3.23 (2) 3.20 (4) 2.91 (4) 3.16 (2) 3.22 (3)

aIntramolecular hydrogen bond. Intermolecular hydrogen bond. 645

A. Aubry et al.

RCO-Pro-X-NHR' dipeptides are 011-folded in the solid state ( 5 ) . We have shown that the 011 turn is a consequence of the molecular packing forces because it allows the X-N2H bond stronger intermolecular interactions ( 5 ) . The 01 turn is found when the X-N2 H bond is engaged in an intramolecular interaction with the side chain of X, as for X = Ser (18), Thr (19) and Asp(OMe) (22). This is also the case for X = His and His(.r-Me). However, this intraresidue contact is not a necessary condition since In assumes a 01 turn conformation (a minor conformer in the solute state (3)) with an intermolecular interaction between the His- N2 H and His-N; sites.

CONCLUSION

The tBuCO-Pro-X-NHMe dipeptides with X = His, His(.r-Me) and His (n-Me) are 01 folded in the solid state by a classical i + 3 + i hydrogen bond. The first two derivatives contain an additional His-NH to His-N* intramolecular interaction closing a six-membered cycle, and forcing the His C"-Cp bond in the rather infrequent rotamer 111 orientation.

The protonated tBuCO-Pro-His+-NHMe dipeptide associated with the PF6--anion assumes an open conformation in which the CO- (tBu) carbonyl is intramolecularly hydrogen- bonded to His+-N*H. These two molecular

646

Backbone side chain interactions

. ~- ,

FIGURE 4 Stereoviews of the crystal cells for the same dipeptides as in Fig. 3 showing the molecular packing. The hydrogen bonds are indicated by a thin line and the PF, anion (d) is represented in the A disposition.

647

A. Aubry et al.

conformations illustrate the influence of the 10. International Tables for X-Ray Crystajlography basic or protonated histidine residue, presenting (1974) Vol. IV, Kynoch Press, Birmingham, different possible interaction modes for the England imidazole or imidazolium side group. The Same 11. Taylor, R. & Kennard, 0. (1983) Acts crystal-

log. , Sect. B 39, 133-138 conformational transition is found in the solute 12. Nair, C.M.K. &Vijayan, M. (1981) J. Indian Inst. state and deserves special attention considering sci,, Ser, c63, 81-103

near the physiological pH. J. Chem. SOC., Perkin Trans. 1, 3045-3048 that the pKa Of the His imidazole ring is quite 13. Brown, T,, Jones, J.H. & Wallis, J.D. (1982)

1.

2.

3.

4.

5.

6.

7.

8.

9.

REFERENCES

Ramani, R. & Boyd, R.J. (1981) Can. J. Bio- chem. 59,3232-3236 Tanokura, M. (1983) Biochim. Biophys. Acta

Boussard, G., Marraud, M. & Aubry, A. (1986) Int. J. Peptide Protein Res. See paper no V of this series (in press) Boussard, G. & Marraud, M. (1985) J. Am. Chem.

Aubry, A., Cung, M.T. & Marraud, M. (1985) J. Am. Chem. SOC. 107,7640-7647 IUPAC-IUB Commission on Biochemical Nomen- clature (1984) Biochem. J. 219,345-373 Aubry, A., Boussard, G. & Marraud, M. (1983) in Peptides: Structure and Function (Hruby, V.J. & Rich, D.H., eds.), pp. 817-820, Pierce Chemical Co., Rockford, IL Main, P., Fiske, S.J., Hall, S.E., Lessinger, L., Germain, G., Declerc, J.P. & Woolfson, M.M. (1980) MULTAN 80, Universities of York, England and Louvain, Belgium Sheldrick, G.M. (1976) Programs for Oystal Structure Determination, University of Cam- bridge, England

742,576-585

SOC. 107,1825-1828

14. IUPAC-IUB Commission on Biochemical Nomen- clature (1970) Biochemistry 9,3471-3479

15. Rose, G.D., Gierasch, L. & Smith, J.A. (1985) Adv. Protein Chem. 37, l -109

16. Janin, J., Wodak, S. , Levitt, M. & Maigret, B. (1978) J. Mol. Biol. 125, 357-386

17. Benedetti, E., Morelli, G., NBmethy, G. & Scheraga, H.A. (1978) In?. J. Peptide Protein Res. 22, 1-15

18. Aubry, A., Ghermani, N. & Marraud, M. (1984) Int. J. Peptide Protein Res. 23, 113-122

19. Aubry, A. & Marraud, M. (1985) Acta Crystal- logr., Sect. C41,65-61

20. Ramaprasad, S . (1980) J. Indian Inst. Sci., Ser.

21. Prasad, B.V.V., Balaram, H. & Balaram, P. (1982) Biopolymers 21, 1261 -1273

22. Mcharfi, M., Aubry, A., Boussard, G. & Marraud, M. (1 986) Eur. J. Biochem., in press

C 62,83-98

Address:

M. Marraud

1 Rue Grandville 54042 Nancy France

LCPM-ENSIC

648