X-ray Crystallographic Characterization Of Designed Peptides Containing Heterochiral And Homochiral Diproline Segments And Database Analysis
Understanding the relation between amino acid sequences and protein structures is one of the most important problems in modern molecular biology. However, due to the complexities in the protein structure, this task is really daunting. Hence, understanding the structural features of proteins and the rules of folding is central to the design of novel and more effective biomaterials. With the inception of the de novo design of synthetic mimetics for protein structural elements, the study of designed peptides is a subject of intense current research. The de novo design of polypeptide structures provides insights into the factors that govern the folding of peptides and proteins. The rational design of synthetic peptide models for secondary structural motifs in proteins depends on the ability to control the polypeptide chain stereochemistry. An approach, which seems to be useful, is the introduction of constrained genetically coded amino acids like Proline or the introduction of non-protein constrained amino acids like Aib which are capable of restricting the range of available backbone conformations of the polypeptide chain. The use of such residues would then permit the design of well defined and intended structural motifs like the β-turns which serve as chain reversal areas of the polypeptide chain. Templates incorporating multiple repeats of such conformationally constrained residues would in turn further enhance the choice of conformational parameters for the polypeptide chain towards folding. Crystal structure determination of the oligopeptides by X-ray diffraction gives insight into the specific conformational states, modes of aggregation, hydrogen bond interactions and solvation of peptides. Precise structural analysis and good characterization of geometrical parameters and stereochemical details of these molecules provide valuable inputs for peptide design and are indispensable for exploring strategies to design peptide sequences which serve as synthetic mimics for folding motifs in proteins. Many of the above points have been investigated in this thesis which incorporates study of designed peptides containing heterochiral and homochiral diproline segments followed by protein database analysis. This thesis reports results of x-ray crystallographic studies of twenty two (22) oligopeptides containing heterochiral or homochiral diproline segments. Apart from the crystal data, protein database analysis has also been carried out to investigate what actually is found in nature. Given in brackets are the compound names used in the thesis for the peptides solved. 1) Piv-DPro-LPro-NHMe ( DPPN ) [C16H27N3O3 ] 2) Piv-DPro-LPro-LVal-OMe ( DPPV ) [C21H35N3O5 . 0.09 H2O] 3) Piv-DPro-LPro-LPhe-OMe ( DPPF ) [C25H35N3O5 . H2O] 4) Piv-DPro-LPro-DAla-OMe ( DPPDA ) [C19H31N3O5] 5) Piv-LPro-DPro-LAla-OMe ( PDPA ) [C19H31N3O5] 6) Piv-DPro-LPro-LVal-NHMe ( DPPVN ) [C21H36N4O4 . H2O] 7) Piv-DPro-LPro-LLeu-NHMe ( DPPLN ) [C22H38N4O4 . 0.34H2O] 8) Piv-DPro-LPro-LPhe-NHMe ( DPPFN ) [C25H36N4O4 . H2O] 9) Piv-DPro-LPro-Aib-NHMe ( DPPUN ) [C20H34N4O4] 10) Piv-DPro-LPro-DAla-NHMe ( DPPDAN ) [C19H32N4O4] 11) Piv-DPro-LPro-DVal-NHMe ( DPPDVN ) [C21H36N4O4 .1.43 H2O] 12) Piv-DPro-LPro-DLeu-NHMe ( DPPDLN ) [C22H38N4O4 . H2O] 13) Piv-LPro-DPro-LAla-NHMe ( PDPAN ) [C19H32N4O4] 14) Piv-LPro-DPro-LVal-NHMe ( PDPVN ) [C21H36N4O4] 15) Piv-LPro-DPro-LLeu-NHMe ( PDPLN ) [C22H38N4O4 . H2O] 16) Piv-LPro-DPro-LVal-OMe ( PDPVO ) [C21H35N3O5 . H2O] 17) Racemic mixture of Piv-DPro-LPro-DVal-NHMe + Piv-LPro-DPro-LVal-NHMe ( PPVVN ) [C21H36N4O4 . 0.74H2O] 18) Racemic mixture of Piv-DPro-LPro-DLeu-NHMe + Piv-LPro-DPro-LLeu-NHMe ( PPLLN ) [C22H38N4O4 . H2O] 19) Racemic mixture of Piv-DPro-LPro-DPhe-NHMe + Piv-LPro-DPro-LPhe-NHMe ( PPFFN ) [C25H36N4O4 . 2 H2O] 20) Piv-LPro-LPro-LPhe-OMe ( PPFO ) [C25H35N3O5 . 0.5 H2O] 21) Piv-LPro-LPro-LVal-NHMe ( PPVN ) [C21H36N4O4 . H2O] 22) Piv-LPro-LPro-Aib-NHMe ( PPUN ) [C20H34N4O4. H2O] Results from the X-ray crystallographic analysis of the above peptides provided substantial information regarding role of diproline templates on the folding of the polypeptide chain. The thesis is divided into the following eight chapters : Chapter 1 gives a general introduction to the stereochemistry of polypeptide chains and the secondary structure classification: helices, β-sheets and β-turns. This section also provides a brief overview of the use of non standard and D-amino acids into peptide design. Discussions on DProline, puckering states of the Proline ring, diproline segments and racemic mixtures of peptides are also presented. A brief discussion on X-ray diffraction and solution to the phase problem is also given. Chapter 2 describes the structural characterization in crystals of the five following designed peptides: Piv-DPro-LPro-NHMe (DPPN), Piv-DPro-LPro-Xxx-OMe [Xxx = LVal (DPPV); LPhe (DPPF); DAla (DPPDA)] and Piv-LPro-DPro-LAla-OMe (PDPA) containing the heterochiral diproline segment with an aim towards understanding the directive influence of short range interaction on polypeptide folding. Except PDPA, in all the structures, a type II’ β-turn was observed at the DPro-LPro segment with the formation of a 4→1 intramolecular hydrogen bond between the atoms of the polypeptide backbone. In PDPA, the expected type II β-turn occurred at the LPro-DPro segment. Thus, the DPro-LPro segment preferably adopts a type II’ β-turn conformation when present at the C-terminus which is mimicked by the methyl ester group. The use of pivalyol group at the N-terminus is to ensure the trans geometry of the peptide bond between pivalyol and the first Proline. Crystal parameters DPPN: C16H27N3O3; P21; a = 10.785(1) Å, b = 15.037(1) Å, c = 11.335(1) Å; β = 109.96(1)°; Z = 4; R = 0.0388, wR2 = 0.1047. DPPV: C21H35N3O5 . 0.09 H2O; P212121; a =10.676(1) Å, b = 16.608(1) Å, c = 39.887(1) Å, Z = 12; R = 0.0688, wR2 = 0.1701. DPPF: C25H35N3O5 . H2O; P21; a = 9.538(1) Å, b = 10.367(1) Å, c = 13.102(1) Å; β = 93.04(1) °; Z = 2; R = 0.0504, wR2 = 0.1455. DPPDA: C19H31N3O5; P21; a = 11.269(1) Å, b = 9.945(1) Å, c = 18.550(2) Å; β = 97.46(1)°; Z = 4; R = 0.0563, wR2 = 0.1249. PDPA: C19H31N3O5; P212121; a = 9.043(1) Å, b = 10.183(2) Å, c = 23.371(1) Å; Z = 4; R = 0.0753, wR2 = 0.1603. Chapter 3 describes the crystal structures of the four following designed peptides containing the heterochiral diproline segment followed by a L-residue or an achiral amino acid residue like Aib : Piv-DPro-LPro-Xxx-NHMe [Xxx = LVal (DPPVN); LLeu (DPPLN); LPhe (DPPFN) and Aib (DPPUN)]. In the first three peptides the DPro-LPro segennt adopts a type II’ β-turn conformation with the formation of a type I β-turn at the LPro-Xxx segment. The peptide backbone overall therefore adopts a consecutive β-turn structure. When the L-amino acids at the C-terminus are replaced by the achiral amino acid Aib, the overall folded structure adopted by the peptide backbone still remains unchanged with the formation of a consecutive β-turn. All the structures are stabilized by two intramolecular 4→1 hydrogen bonds between the C=O group and the nitrogen atom of the polypeptide backbone. Crystal parameters DPPVN: C21H36N4O4 . H2O; P21; a = 9.386(1) Å, b = 12.112(1) Å, c = 10.736(1) Å; β = 99.53(1) °; Z = 2; R = 0.0528, wR2 = 0.1337. DPPLN: C22H38N4O4 . 0.34H2O; P21; a =9.231(1) Å, b = 17.558(1) Å, c = 15.563(1) Å; β = 91.94(1) °; Z = 4; R = 0.0555, wR2 = 0.1422. DPPFN: C25H36N4O4 . H2O; P212121; a = 10.473(1) Å, b = 15.980(1) Å, c = 15.994(1) Å; Z = 4; R = 0.0620, wR2 = 0.1826. DPPUN: C20H34N4O4; P212121; a = 10.571(2) Å, b = 11.063(1) Å, c = 18.536(1) Å; Z = 4; R = 0.0578, wR2 = 0.1256. Chapter 4 describes the crystal structures of the seven designed peptides containing heterochiral diproline segment. Three of these contain sequences of the type DPro-LPro-DXxx [DXxx = DAla (DPPDAN); DVal (DPPDVN); DLeu (DPPDLN)] and three contains the enantiomeric peptides of the ones that are mentioned earlier in sequences of the type LPro-DPro-LXxx [LXxx = LAla (PDPAN); LVal (PDPVN); LLeu (PDPLN)]. In order to investigate the effect of the C-terminal protecting group, a final peptide Piv-LPro-DPro-LVal-OMe (PDPVO) was crystallographically characterized. All the peptides containing the DXxx residues adopted different backbone conformations. For DAla, a structure simultaneously having a β-turn and an α-turn was obtained which is the first example in designed peptides of an isolated α-turn. In the case of DVal, an open / extended structure devoid of any intramolecular hydrogen bonding was obtained whereas for DLeu, type II β-turn occurred at the LPro-DLeu segment instead of the expected type II’ turn at the DPro-LPro segment. In the case of enantiomeric peptides, all the three peptides adopted folded structures with exact mirror image conformation being generated for LAla and nearly identical mirror image conformation in the case of LLeu. The enantiomeric peptide of DVal which contained LVal residue following the diproline segment also adopted a folded conformation with the formation of type II β-turn at the LPro-DPro segment as expected. X-ray crystallographic characterization of PDPVO resulted in the peptide adopting an overall extended / open structure. Thus, the chirality of the C-terminal residue seems to have a profound effect on the conformation of the heterochiral diproline segments. The role of the C-terminal protecting group cannot also be undermined. Crystal parameters DPPDAN: C19H32N4O4; P1; a = 5.964(1) Å, b = 9.354(1) Å, c = 9.961(1) Å; α = 75.44(1), β = 78.90(1) °, γ = 77.04(1); Z = 1; R = 0.0728, wR2 = 0.1528. DPPDVN : C21H36N4O4 .1.43 H2O; P212121; a = 8.744(8) Å, b = 11.609(1) Å, c = 23.577(2) Å; Z = 4; R = 0.0625, wR2 = 0.1856. DPPDLN : C22H38N4O4 . H2O; P212121; a = 10.531(3) Å, b = 11.659(3) Å, c = 20.425(6) Å; Z = 4; R = 0.0444, wR2 = 0.1239. PDPAN: C19H32N4O4; P1; a = 5.964(1) Å, b = 9.354(2) Å, c = 9.961(2) Å; α = 75.44(1), β = 78.90(1) °, γ = 77.04(1); Z = 1; R = 0.0745, wR2 = 0.1572. PDPVN : C21H36N4O4; P212121; a = 9.743(1) Å, b = 11.423(1) Å, c = 21.664(3) Å; Z = 4; R = 0.0803, wR2 = 0.1899. PDPLN : C22H38N4O4 . H2O; P212121; a = 10.462(4) Å, b = 11.572(4) Å, c = 20.262(7) Å; Z = 4; R = 0.0968, wR2 = 0.2418. PDPVO : C21H35N3O5 . H2O; P212121; a = 8.784(4) Å, b = 11.587(5) Å, c = 23.328(1) Å; Z = 4; R = 0.0888, wR2 = 0.1465. Chapter 5 describes the crystal structures of the three designed peptides containing racemic mixtures [Racemic mixture of Piv-DPro-LPro-DVal-NHMe + Piv-LPro-DPro-LVal-NHMe (PPVVN); Racemic mixture of Piv-DPro-LPro-DLeu-NHMe + Piv-LPro-DPro-LLeu-NHMe (PPLLN); Racemic mixture of Piv-DPro-LPro-DPhe-NHMe + Piv-LPro-DPro-LPhe-NHMe (PPFFN)] having the heterochiral diproline segment in their sequences and three peptides having a homochiral diproline segment [Piv-LPro-LPro-LPhe-OMe (PPFO); Piv-LPro-LPro-LVal-NHMe (PPVN); Piv-LPro-LPro-Aib-NHMe (PPUN)]. The inability of the pure enantiomers to crystallize in the case of Phe (chapter 4) invoked the use of peptide racemates for obtaining a crystal state conformation for the said compound. In all the cases, the L-enantiomer of Xxx crystallized in the asymmetric unit. A type II β-turn was obtained in the case of PPVVN at the LPro-DPro segment and a type II’ β-turn was obtained for PPLLN at the DPro-LLeu segment. in the case of Phe, an open structure devoid of any intermolecular hydrogen bonding an very similar to DPPDVN (chapter 4) was obtained. In the case of homochiral diproline segment containing peptides, PPFO crystallized with two molecules in the asymmetric unit, both of which adopted a type VIA1 hydrogen bonded β-turn conformation with a cis peptide bond between the diproline segment. In the case of Valine (PPVN) however, a structure devoid of any intramolecular hydrogen bonding was obtained. In the final peptide PPUN, a type II β-turn conformation is observed at the LPro-Aib segment. The analysis revealed that the hydration of the peptide can cause dramatic changes in its backbone conformation. In homochiral LPro-LPro sequences, the tendency to form hydrogen bonded turns competes with the formation of semi-extended polyproline structures. The results also emphasize the subtle role of sequence effects in modulating the conformations of short, constrained peptide segments. The possibility of trapping distinct conformational segments of the diproline segments in crystals by generating racemic centro-symmetric crystals in which packing effects may be appreciably different from those observed in the crystals of individual pure enantiomeric peptides has been clearly exploited in this chapter to obtain a crystal in the case of Phe. These results suggest that the energetic differences between these states is small. Conformational choice can therefore be readily influenced by environmental and sequence effects. Crystal parameters PPVVN: C21H36N4O4 . 0.74H2O; C2/c; a = 36.667(17) Å, b = 10.092(5) Å, c = 13.846(6) Å; β = 107.27(1) °; Z = 8; R = 0.1317, wR2 = 0.3141. PPLLN: C22H38N4O4 . H2O; P21/c; a = 10.555(1) Å, b = 11.687(1) Å, c = 20.108(2) Å; β = 95.47(1) °; Z = 4; R = 0.0761, wR2 = 0.2034. PPFFN: C25H36N4O4 . 2 H2O; P21/c; a = 8.883(5) Å, b = 18.811(10) Å, c = 16.033(9) Å; β = 96.28(1) °; Z = 4; R = 0.1218, wR2 = 0.2848. PPFO : C25H35N3O5 . 0.5 H2O; P212121; a = 10.199(1) Å, b = 20.702(2) Å, c = 23.970(2) Å; Z = 8; R = 0.0716, wR2 = 0.1901. PPVN : C21H36N4O4 . H2O; P212121; a = 9.454(1) Å, b = 11.119(1) Å, c = 23.021(2) Å; Z = 4; R = 0.0551, wR2 = 0.1587. PPUN: C20H34N4O4. H2O; P21; a = 6.276(1) Å, b = 14.011(2) Å, c = 12.888(1) Å; β = 96.80(1) °; Z = 2; R = 0.0475, wR2 = 0.1322. Chapter 6 describes the pyrrolidine ring puckering states of the Proline residue present in diproline segments in the peptides solved in this thesis, the Cambridge structural database (CSD) [only acyclic diproline containing peptides have been taken into account] and in a non-redundant dataset of proteins in the Protein Data Bank (PDB). The five membered pyrrolidine ring of Proline can be best characterized in terms of the following five endocyclic torsion angles χ1, χ2, χ3,χ4 and θ. Using various values of these endocyclic torsion angles the following puckering states were identified :  Cγ-exo (A)  Cγ-endo (B)  Cβ-exo (C)  Cβ-endo (D)  Twisted Cγ-exo-Cβ-endo (E)  Twisted Cγ-endo-Cβ-exo (F)  Planar (G)  Cα-distorted (H)  Twisted Cβ-exo-Cα-endo (I)  Cδ-endo (K)  N-distorted (L)  Twisted Cδ-endo- Cγ-exo (N). In the case of peptides solved in this thesis for heterochiral diproline segments, the Cγ-exo / Cβ-exo (AC) combination turns out to more preferred than the other combinations. The Cγ-endo / Cγ-endo (BB) state is the second most populated state. The overall investigation of Proline rings in peptides show that the states Cγ-exo (A), Cβ-exo (C) and Twisted Cγ-endo-Cβ-exo (F) are the most preferred states of occurrence of the pyrrolidine ring conformation. In the case of proteins, the overall percentage distribution of various combinations indicates that the AA (Cγ-exo / Cγ-exo), AE (Cγ-exo / Twisted Cγ-exo-Cβ-endo) and FF (Twisted Cγ-endo-Cβ-exo / Twisted Cγ-endo-Cβ-exo) categories are the most preferred combinations. For Proline rings in proteins, the states Cγ-exo (A), Twisted Cγ-exo-Cβ-endo (E) and Twisted Cγ-endo-Cβ-exo (F) are the most preferred states of occurrence of the pyrrolidine ring conformation. Chapter 7 describes the analysis of diproline segments in a non-redundant dataset of proteins In this chapter, the possible conformational states for the diproline segment (LPro-LPro) found in proteins taken from a non-redundant dataset has been investigated an identified with an emphasis on the cis and trans states for the peptide bond between the diproline segment. The occurrence of diproline segments in type VIA1 turns (cis Pro-Pro peptide bond) and other regular secondary structures like type III β-turns and α-helices has been studied. This has been followed up by the amino acid distribution flanking the diproline segment and the conformation adopted by Xaa-Pro and Yaa-Pro segments in proteins. It is observed that for cis Pro-pro peptide bond, the conformation adopted by the first Proline lies in PII region whereas the second Proline inevitably adopts a conformation in the Bridge region, leading to the formation of the type VIA1 β-turn structure. But in the trans case, the conformation adopted by the first Proline is overwhelmingly populated in the PII (Polyproline) and right-handed α-helical region. For position i+2, the major conformation adopted by Proline is P II and α with a substantial amount of occurrences in Bridge and the C7 (γ-turn) region. The analysis also reveals that the cis-cis configuration of the peptide bond is very rare when considering the diproline segment. With a cis-trans peptide linkage, PII-PII conformation is the most stable and favoured conformation for the Pro-Pro segment in proteins. With trans peptide bond linkage between the Proline residues, α- α and PII-Bridge conformations are equally likely for the diproline segment. The population in trans-cis and cis-trans states are comparable indicating that the energy differences between these states is small. However, trans-trans is the most populated state with a percentage occurrence of 85.43%. The analysis and comparison of conformational states for the Xaa-Pro-Yaa sequence reveals that the Xaa-Pro peptide bond exists preferably as the trans conformer rather than the cis conformer. The same is valid for Pro-Yaa segment, with the cis conformer being populated to even lesser extent. The data shows that α- α, PII-α, PII-PII and extended-PII are the most populated states for Xaa-Pro and Pro-Yaa segments as compared to PII-PII and PII-α and states observed for the Pro-Pro segment. Chapter 8 describes the analysis of single and multiple β-turns in a non-redundant dataset of proteins. The analysis on β-turns in proteins has shed a new light into the propensity values for amino acid residues at various positions of β-turns which in certain cases have undergone appreciable change in values than previously observed. One of the other notable feature of the analysis is the fact that the data displays a higher occurrence of unprimed β-turns of type I and type II as compared to their primed counterparts of type I’ and type II’ as previously observed. In fact, the results show that type I β-turn is the highest occurring turn both in isolated as well as in consecutive β-turn examples. The analysis of multiple β-turns in proteins has revealed many new categories like the (I,I+1,I+3), (I,I+2,I+3) and combination of turns which can be used for the design of the loops, especially in the case of β-hairpins. Among the multiple turns, double turns occur more frequently than the other consecutive turns like triple and quadruple turns. It is also important to note that the number of examples of a hydrogen bonded turn being followed by a hydrogen bonded turn is very less with type IV turn preceding a primed turn in most of the cases. Thus, the data available from consecutive β-turn analysis and the type-dependent amino acid positional preferences and propensities derived from the present study may be useful for modeling various single and consecutive turns, especially in designing loop regions of β-hairpins.
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