C-alpha-methyl, C-alpha-phenylglycine peptides: A structural study

Letters in Peptide Science 5: 223-225, 1998. KLUWER/ESCOM 9 1998 Kluwer Academic Publishers. Printed in the Netherlands.

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LIPS 230

C ~ - M e t h y l , C'~-phenylglycine peptides: A s t r u c t u r a l study Eric Mossel a, Fernando Formaggiob, Giovanni Valleb, Marco Crismab, Claudio Toniolob,*, Mitsunobu Doi c, Toshimasa Ishida c, Quirinus B. Broxtermana & Johan Kamphuisd DSMResearch, Organic Chemistry and Biotechnology Section, PO. Box 18, 6160 MD Geleen, The Netherlands b BiopolymerResearch Centre, CNR, Department of Organic Chemistry, University of Padova, Via Marzolo 1, 1-35131 Padova, Italy c Osaka University of Pharmaceutical Sciences, Osaka 569-11, Japan d DSM Fine Chemicals, 6401 JH Heerlen, The Netherlands a

Received 1 May 1997; Accepted 2 July 1997

Key words:/3-bend; disubstituted glycines; fully extended conformation; 310-helix; C~-methylated a-amino acids

Summary In order to obtain further information on the role played by phenyl ring position in the Ca-methylated a-amino acid side chain on peptide preferred conformation, the crystal-state structural preferences of C~-methyl, C ~phenylglycine peptides have been determined by X-ray diffraction. This study shows that either the fully extended conformation or the fl-bend/310-helical structures are adopted by peptides characterized by this Ca-methylated, fl-branched, aromatic a-amino acid.

Introduction Recent years have seen impressive advances in the design of peptides with predetermined secondary structures and physical properties. These studies have laid the groundwork for tailoring peptides capable of recognizing other bioorganic molecules in a sequencedependent manner. To this aim the conformationally constrained C~-methylated a-amino acids are among the most widely used structural units. In this connection we have already shown that peptides based on (aMe)Phe (Ca-methyl phenylalanine) [1,2] and (otMe)Hph (Ca-methyl homo-phenylalanine) [2,3] preferentially adopt fl-turn and 310-helical structures, and that turn and helix handedness is critically biased by the position of side-chain branching. In order to complete the picture of the role played by the phenyl ring position in the Ca-methylated a-amino acid side chain on peptide preferred conformation, we have synthesized a number of derivatives and peptides to the pentamer level, including homo-peptides, based on * To whom correspondence should be addressed.

(aMe)Phg (ca-methyl, C~-phenylglycine) (E. Mossel, F. Formaggio, M. Crisma, C. Toniolo, Q.B. Broxterman and J. Kamphuis, to be published). In the present article we describe the results of a crystalstate structural analysis performed on two (aMe)Phg derivatives, two tripeptides, and two pentapeptides by X-ray diffraction (see Scheme 1).

I"~C

(Cl"12)n

--HNXCO

-

n = 1 (cdVle)Phe n = 2 (u.Me)l-Iph

H3C X --HN"

/CH--CI" ~ (cdVle)Val "CO-

Scheme 1. The C~-methylated ~x-amino acids discussed in this work.

The fit of the minimum-energy conformer of the aspartame analogue H-L-Asp-L-(otMe)Phg-OMe in the model of the active site of the sweet receptor has been reported [4].

224

O0

NT

:

Figure 1. X-raydiffractionstructuresof the aminoacid derivativesAC-L-(c~Me)Phg-NHBzl1 and BZ-L-(ctMe)Phg-NHMe2. The intramolecutar H-bondsare indicatedby dashed lines.Numberingof the heteroatomsis also reported.

OT

OT

02

03

N3 02 t~2

o, I I

O1

N2

,/ H1 "

3

4

Figure2. X-raydiffractionstructuresof the tripeptidesZ-L-(otMe)Phg-(L-Ala)2-OMe3 and Z-L-(otMe)Phg-(D-Ala)2-OMe4. The intramolecular H-bondsare indicatedby dashed lines. Numberingof the heteroatomsis also reported. Results

The following six X-ray diffraction structures of (otMe)Phg derivatives and peptides were solved: AcL-(otMe)Phg-NHBzl (Ac, acetyl; NHBzl, benzylamino) 1; Bz-L-(ctMe)Phg-NHMe (Bz, benzoyl; NHMe, methylamino)2; Z-L-(otMe)Phg-(L-Ala)2OMe (Z, benzyloxycarbonyl; OMe, methoxy) 3; Z-L(c~Me)Phg-(D-Ala)2-OMe 4; Ac-(Aib)2-L-(~Me)Phg(Aib)z-OtBu (Aib, a-aminoisobutyric acid; OtBu, tert-butoxy) 5; andpBrBz-(Aib)z-L-(otMe)Phg-(Aib)zOtBu (pBrBz, para-bromobenzoyl) 6. The (otMe)Phg derivatives 1 and 2 (Figure 1) adopt the fully extended conformation (~o, 7z ~ 180 ~ 180 ~ stabilized by an N1-H...O1=C'1 intramolecular H-bond giving rise to a five-membered cyclic

structure (C5) [5]. The same structure is adopted by the (aMe)Phg residue in tripeptide 4 (Figure 2). In all of the three C5 forms, the r (N-C~-C p) bond angle is remarkably smaller than the tetrahedral value (~103-105~ Both D-AIa residues in tripeptide 4 are semi-extended. The -L-(otMe)Phg-L-Ala- sequence of tripeptide 3 is folded in a type I/~-bend conformation [6], characterized by a weak intramolecular H-bond N3.- -O0, 3.170(5),A (Figure 2). The C-terminal L-AIa residue is left-handed helical [7]. Pentapeptides 5 and 6 are both 310-helical [8]. While the structure of the NU-acetylated peptide 5 is a regular left-handed helix (Figure 3), that of the NC~-para-bromobenzoylated peptide 6 is a distorted right-handed helix (Figure 3). In this latter peptide the third/~-bend, -L-(otMe)PhgAib-, is type I. In both pentapeptides the helical screw

225

T

N4

04

05

CJ

N4

~

N5

5 OT

05

Figure 3. X-ray diffraction structures of the pentapeptides Ac-(Aib)2-L-(otMe)Phg-(Aib)2-OtBu 5 and pBrBz-(Aib)2-L-(~Me)Phg-(Aib)2OtBu 6. The intramolecular H-bonds are indicated by dashed lines. Numbering of the heteroatoms is also reported.

sense of the C-terminal Aib residue is reversed compared to that of the preceding residues [7]. In peptides 3, 5, and 6 the conformation of the (aMe)Phg residue is helical (mean absolute values for ~0, qz: 57.5 ~ 30.5 ~ and the r bond angle is slightly larger than the tetrahedral value (110-112~

lated counterparts. A comparison between the two C~-methylated, 13-branched a-amino acids (aMe)Val and (aMe)Phg seems to indicate that the aliphatic residue has a more pronounced tendency to fold into bend/helical structures and to adopt the normal relationship between a-carbon chirality and bend/helix handedness [1,9].

Discussion and conclusions References Our crystal-state structural analysis strongly supports the view that there are two accessible conformations for the (otMe)Phg residue (a fully extended and a folded/helical conformation). It seems that the latter would prevail as peptide main-chain length is increased. The semi-extended conformation, which would have been required for the formation of the type II /%bend in the heterochiral -L-(aMe)Phg-DAla- sequence of tripeptide 4, is not adopted by this C~-methylated amino acid. The limited structural data available to date do not allow us to assess the relationship between (aMe)Phg a-carbon chirality and turn/helix handedness. To this aim attempts to grow single crystals from the (aMe)Phg homo-peptides are currently in progress. In summary, the results already published [13] and those described here allow us to conclude that the three Ca-methylated, aromatic a-amino acids (aMe)Phg, (aMe)Phe, and (aMe)Hph are/3-bend and helix-inducers, much stronger than their unmethy-

1. Toniolo, C., Crisma, M., Formaggio, E, Valle, G., Cavicchioni, G., Prrcigoux, G., Aubry, A. and Kamphuis, J., Biopolymers, 33 (1993) 1061. 2. Toniolo, C., Crisma, M., Formaggio, E, Polese, A., Doi, M., lshida, T., Mossel, E., Broxterman, Q.B. and Kamphuis, J., Pept. Sci., 40 (1996) 523. 3. Doi, M., Ishida, T., Polese, A., Formaggio, E, Crisma, M., Toniolo, C., Broxterman, Q.B. and Kamphuis, J., Int. J. Pept. Protein Res., 47 (1996) 491. 4. Mossel, E., Formaggio, F., Crisma, M., Toniolo, C., Broxterman, Q.B., Boesten, W.H.J., Kamphuis, J., Quaedflieg, EJ.L.M. and Temussi, P., Tetrahedron Asymmetry, 8 (1997) 1305. 5. Toniolo, C. and Benedetti, E., In Balaram, P. and Ramaseshan, S. (Eds.) Molecular Conformation and Biological Interactions, Indian Academy of Sciences, Bangalore, 1991, pp. 511-521. 6. Venkatachalam, C.M., Biopolymers, 6 (1968) 1425. 7. Toniolo, C. and Benedetti, E., Macromolecules, 24 (1991) 4004. 8. Toniolo, C. and Benedetti, E., Trends Biochem. Sci., 16 (1991) 350. 9. Polese, A., Formaggio, E, Crisma, M., Valle, G., Toniolo, C., Bonora, G.M., Broxterman, Q.B. and Kamphuis, J., Chem. Eur. J., 2 (1996) 1104.