C O M M U N I C A T I O N S
the peptide helix inversion was well programmed by the nature of
the metal center. Since L1-M(ClO4)2 complexes have a much
longer helicity inversion lifetimes than the corresponding L2
complexes (t1/2 for M ) Co: 25 s, t1/2 for M ) Ni: 4.7 h), the two
helical peptide chains further communicated to promote helix
inversion of the other peptide chain in the L2-metal complex
systems.
As observed with protein folding,10a the rotation of a F1-ATPase
motor,10b and other biological machinery systems, the present
metallo-peptide complex has the time-controlled helicity induction
around a metal center, anion-triggered helicity inversion, and
helicity transfer to peptide units. Such molecular-based time
programming systems provide new insight on how natural biopoly-
mers produce highly organized supramolecular structures and
dynamically sophisticated functions.
Figure 3. Stereoviews and schematic illustrations of crystal structures of
[Zn(L2)(H2O)2](ClO4)2‚(CH3CN)0.5‚(H2O)10 (a) and [Zn(L2)(NO3)](NO3)‚
(CH3CN)‚(CHCl3)5 (b). The minor component in [Zn(L2)(H2O)2]2+, most
hydrogen atoms and solvent molecules are omitted for simplification.
Acknowledgment. We thank to Ms. Matsumi Doe of the
analytical center at Osaka City University for obtaining COSY and
NOESY spectra of Zn(II) complexes. This work was partially
supported by a Grant-in-Aid for Scientific Research from JSPS,
Japan (18350032 and 18655024 to H.M.).
Supporting Information Available: Synthetic procedures of L2,
and crystal structures and NOESY spectra of Zn(II) complexes. CD
and ESI-MS spectra of metal complexes. This material is available free
Figure 4. Relaxation traces (270 nm) of the L2-M(ClO4)2 complex by
mixing 50 equiv of Bu4NNO3 in CH3CN at room temperature measured by
stopped-flow CD apparatus [M ) Zn (a), Co (b), Ni (c)]; [L2] ) 0.3 ×
10-3 mol dm-3, [M(ClO4)2‚6H2O] ) 0.6 × 10-3 mol dm-3, [Bu4NNO3] )
15 × 10-3 mol dm-3; 0.5 mm cuvette. Relationship between half-lifetime
(t1/2) of helicity inversion and water exchange lifetime (τ/s) on each metal
cation (d).
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helical structures and a ∆ form Zn(II) center with one bidentate
NO3- anion (Figure 3b). In these complexes, the pentapeptide chains
were stabilized by a hydrophobic interaction and weak CH-π
1
interactions between the NCH3 and ∆Phe(2) moieties.7 The H
NMR spectra of the L2-Zn(ClO4)2 and L2-Zn(NO3)2 complexes
exhibited C2-symmetric patterns, indicating that intramolecular
hydrogen bonds formed by two types of NH groups (Figures S6
and S8). Marked NOESY cross-peaks between NiH and Ni+1
H
signals in the segment between ∆Phe(4) and Aib(5) (Figures S7
and S9) support the idea that both complexes have similar 310
peptide helical structures observed in the crystal state.
The time course of the peptide helix inversion was analyzed using
stopped-flow CD measurements (Figure 4a-c). When 50 equiv of
NO3- anion was added to a CH3CN solution of the L2-Co(ClO4)2
complex, two positive CD signals at 270 and 510 nm were rapidly
inverted to negative signals in same time scale. The half-lifetime
values (t1/2) were estimated by exponential decay curve fitting as
1.5 s for peptide helix inversion and 1.8 s for Co(II) complex
helicity inversion. Thus, inversion of the peptide helices im-
mediately followed the inversion of Co(II) complex helicity, as
illustrated in Figure 1. The L2-Ni(ClO4)2 complex exhibited
positive CD signals at ∼270 and ∼1000 nm, both of which were
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between the NiH and Ni+1H signals in the segment from Aib(3) to Aib(5)
(Figure S4), three types of NH groups were involved in intramolecular
hydrogen bonding to build up the 310 peptide helices.
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-
slowly inverted upon addition of 50 equiv of NO3 anion (half-
(8) Helicity inversion of the L2-Cu(ClO4)2 complex occurred too fast to be
lifetime of 44 min for inversion of both peptide and complex
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half-lifetime of 49 ms for peptide inversion. The estimated half-
lifetime (log t1/2) of these metallo-peptide complexes determined
at 270 nm show a linear correlation with the water exchange lifetime
of aqueous metal cations (Figure 4d).8,9 Thus, the time course of
detected by our measurement (t1/2 ) <10 ms).
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