Designed Peptides with Homochiral and Heterochiral Diproline Templates as Conformational Constraints

Abstract

Diproline segments have been advanced as templates for nucleation of folded structure in designed peptides. The conformational space available to homochiral and heterochiral diproline segments has been probed by crystallographic and NMR studies on model peptides containing L-Pro-L-Pro and D-Pro-L-Pro units. Four distinct classes of model peptides have been investigated: a) isolated D-Pro-L-Pro segments which form type II' beta-turn; b) D-Pro-L-Pro-L-Xxx sequences which form type II'-I (betaII'-I, consecutive beta-turns) turns; c) D-Pro-L-Pro-D-Xxx sequences; d) L-Pro-L-Pro-L-Xxx sequences. A total of 17 peptide crystal structures containing diproline segments are reported. Peptides of the type Piv-D-Pro-L-Pro-L-Xxx-NHMe are conformationally homogeneous, adopting consecutive beta-turn conformations. Peptides in the series Piv-D-Pro-L-Pro-D-Xxx-NHMe and Piv-L-Pro-L-Pro-L-Xxx-NHMe, display a heterogeneity of structures in crystals. A type VIa beta-turn conformation is characterized in Piv-L-Pro-L-Pro-L-Phe-OMe (18), while an example of a 5-->1 hydrogen bonded alpha-turn is observed in crystals of Piv-D-Pro-L-Pro-D-Ala-NHMe (11). An analysis of pyrrolidine conformations suggests a preferred proline puckering geometry is favored only in the case of heterochiral diproline segments. Solution NMR studies, reveal a strong conformational influence of the C-terminal Xxx residues on the structures of diproline segments. In L-Pro-L-Pro-L-Xxx sequences, the Xxx residues strongly determine the population of Pro-Pro cis conformers, with an overwhelming population of the trans form in L-Xxx=L-Ala (19).

Natural abundant solid state NMR studies in designed tripeptides for differentiation of multiple conformers

“…In solution, the C a 's has an accidental overlap, 28 whereas they are well separated in solids inspite of their larger linewidths ($35 Hz). The most interesting difference is in peptide (1) where doubling of resonance lines are observed in the solid state.…”

“…Solution state NMR of peptide (1) also showed doubling of resonances. 28 A detailed analysis in solution showed that these doubling was due to the formation of cis and trans isomers across the di-Proline segment in the ratio 3:2, respectively. X-ray study, however, shows the presence of only one cis conformer; therefore, the SSNMR result showing a doubling of resonances is intriguing.…”

“…The turn types in peptide (1) were type (VI) and type (II) with the cis and trans geometry, respectively, as reported earlier. 28 In peptide (2) with D Pro substitution, the turn type becomes type II 0 and conformation across di-Proline switches to solely trans. Last, in peptide (3), addition of one more amide group-NHMe, at the C-terminus, adds one more hydrogen bond and leads to the formation of double b turn.…”

“…28 All intermediates and final peptides were characterized by solution 1 H NMR at 400 or 500 MHz spectrometer in CDCl 3 . Purification was done on silica-gel column chromatography followed by HPLC (C18, 5-10 ll) using methanol-water gradients.…”

“…Hence study of similar systems gain importance. In a recent report from our group, 28 extensive study was undertaken on short peptide sequences containing homo-/ hetero di-Proline segments to probe the allowed conformational space using solution NMR studies along with single crystal X-ray analysis. N nuclei at natural abundance and are compared with solution NMR and X-ray single crystal data.…”