Organic Letters
Letter
Figure 4. (a) ROESY and VT NMR of 5 and 6. (b) Conformation of 5 (green) and 6 (blue) with its hydrogen bonding (red dash). (c) Overlay with 1
(pink), 5 (green), and 6 (blue).
hydrogen bonding occurred between D-Phe1-NH and Pro5-C
O (Figure 2b). These findings matched the solution
conformation and the crystal structure of compound 1 and
can be verified by the overlay of the solution structures of 1 and 3
(Figure 2 c). The macrocycle 4, which featured amidine at the D-
Ala residue, showed a similar conformation to 1 and 3 (Figure
2c). The incorporation of an unprotonated amidine did not
perturb the conformation of 1 and maintained the hydrogen
bonding pattern. This suggests that the amidine in its
unprotonated state has similar hydrogen bond acceptor capacity
as its carbonyl counterpart.
signals of D-Phe1-NH to amidine-NH1 and CαH-Pro5 and
amidine-NH1 in compound 6 were also observed.
The conformations of 5 and 6 given by the ROESY-restrained
search (Figure 4b) showed similarity to macrocycle 1, as seen in
the overlay (Figure 4c). Slight deviation of the hydrogen
bonding pattern from compound 1 was seen in compound 5 as
D-Phe1-NH was solvent-exposed and not involved in hydrogen
bonding. Additionally, no hydrogen bonding to D-Ala4-NH was
observed, and a possible hydrogen bond between amidine-NH2
and D-Ala4-CO was seen in compound 5. The conformation
of 6 showed only D-Phe1-CO and D-Ala4-NH participating in
hydrogen bonding to form a type-II β-turn. This observation
deviates from what the VT NMR showed for residue D-Phe1-
NH (−0.33 ppb/K). The 20 minimized energy conformations
for 5 and 6 showed retention of the β-turn portion of the
showed some flexibility, which explains the observed NH
temperature coefficient. Alternatively, this discrepancy may also
be due to the deshielding effects caused by the adjacent aromatic
ring.22
Disruption of intramolecular hydrogen bonding in cyclic
peptides has been explored in Snyder’s work on roseotoxin B,5
where hydrogen-bonded amides were substituted to esters, and
in Robinson’s work where amides undergoing hydrogen
bonding were replaced with N-methyl amide.23 The alteration
of hydrogen bonds was done at the hydrogen bond donor site. In
the case of the amidine, hydrogen bonding was disrupted by
altering the hydrogen bond acceptor site upon protonation. This
provides a complementary method to interrogate the conforma-
tional consequences of intramolecular hydrogen bonding. For
macrocycles 5 and 6, protonation of the amidine blocked off the
lone pair that originally participated in an intramolecular
hydrogen bond, switching the amidine role from the hydrogen
Upon protonating compound 3, the 1H NMR spectrum of 3
and 5 showed significant changes in chemical shifts, especially
for the amidine-NH (Figure 3). The amidine proton of 3 (∼6.1
ppm) shifted downfield, and two peaks (∼8.6 ppm and ∼9.5
ppm) corresponding to the two protons of amidine of 5 were
observed.
The VT NMR of compound 5 suggests a strong hydrogen
bonding at amidine-NH1 (−2.39 ppb/K). Weaker hydrogen
bonds were observed for D-Ala4-NH (−3.76 ppb/K) and
amidine-NH2 (4.02 ppb/K). This hydrogen bonding pattern
differed from compound 1 as no hydrogen bonding was
observed for D-Phe1-NH in compound 5 (Figure 4a). In
compound 6, temperature coefficients of D-Phe1-NH (−0.33
ppb/K) and D-Ala4-NH (−0.95 ppb/K) indicated that these
two residues were undergoing hydrogen bonding, while Gly3-
NH (−7.59 ppb/K) did not. Additionally, hydrogen bonding
appeared to occur for the two amidine protons (NH1 = −1.43
ppb/K and NH2 = −2.69 ppb/K). Similar NOE couplings were
observed for 1, 5, and 6 where NOE signals of both Gly3-NH to
CαH-Pro2 and D-Phe1-NH to CαH-Pro5 were observed. In
compound 5, additional NOE signals of D-Ala4-NH to amidine-
NH1 and CαH-Pro5 and amidine-NH2 were observed. NOE
C
Org. Lett. XXXX, XXX, XXX−XXX