wavelengths and stabilized in more polar solvents. These two components bring
about different CPL signs, and the change of their relative contribution to the overall
CPL spectrum at different temperatures thus explains the g lum profile variations. In
this case g lum and g abs (measured on the first Cotton effect) are of the same order of
magnitude (|10
À3 |).
Later, Sisido’s group synthesized two similar polypeptides, namely pyr-3 and
pyr-4 (Scheme 12.1) [14]. In these examples, high g lum values of |1.6 Â 10
À2 | were
detected associated with an excimer situation; no CPL being otherwise allied with
monomer emission. The sign of the CPL was related to the handedness of the helix,
which in turn depended on the length of the peptidic spacer: negative for pyr-3 and
positive for pyr-4. In these cases, the handedness is the same in both ground and
excimer states, as indicated by the sign of the ECD band at 349 nm (negative for pyr3 and positive for pyr-4). For pyr-3 and pyr-4, the ECD and CPL were also studied
in DMF (6 Â 10
À5 M) and tetrahydrofuran (THF) at +20 and À40
C [15]. As a first
observation, the authors noticed that the g lum values were constant over the whole
emission band, which is consistent with only a single excimer configuration being
present in each polypeptide. As a second observation, the signs of g lum were opposite
for pyr-3 and pyr-4 suggesting configurations with opposite screw sense in the two
cases. For pyr-3 specifically, a decrease of g lum value, from |1.2 Â 10
À2 | to |
0.4 Â 10
À2 |, was observed in DMF upon lowering the temperature from +20 to
À40
C; a similar behavior occurring in THF although less pronounced. This
suggests that a major conformational change occurs in the À40 to +20
C range.
More recently, other pyrene-decorated peptides bearing 1, 2, 3, or 4 pyrene
moieties, pyr-5–pyr-8, were prepared and investigated by the group of Imai
(Fig. 12.3) [17]. In CHCl 3 (10
À4 M), a low g lum value of |1.9 Â 10
À4 | was observed
for pyr-5 associated with the monomer fluorescence, while the dipeptide pyr-6
showed the highest dissymmetry factor, |1.1 Â 10
À2
|, allied with excimer emission.
This latter g lum value is 240-fold higher than the corresponding g abs (~|10
À5 |), while
in the former case, in which no excimer occurs, the g abs and g lum are of the same
order of magnitude.
Furthermore, Imai and collaborators prepared several other peptides bearing two
pyrene moieties spaced by chains with different number (n) of methylene spacers,
allowing the authors to study the relationship between chiroptical properties and
spacer lengths (pyr-9–pyr-16, Fig. 12.3) [16]. The sign of CPL associated with
excimer emission underwent a sign inversion going from n ¼ 1 to n ¼ 2 (pyr-9 and
pyr-10), showing a sort of odd–even effect, and from n ¼ 6 to n ¼ 7 (pyr-14 and
pyr-15, see Fig. 12.3). On the other hand, the first Cotton effect, as shown in the
ECD spectrum, did not show any sign inversion, revealing a completely different
geometry of the ground and excited excimer states, confirming again the complementary nature of ECD and CPL when excimer states are at play. The maximum g lum
was recorded for n ¼ 3 (pyr-11, g lum ¼ 0.8 Â 10
À2
). In all instances, g lum values
were 1 to 2 orders of magnitude higher than the corresponding g abs factors (|10
À3 |–|
10
À4 |). With pyr-12 (n ¼ 4), CPL sign inversion was also observed going from
relatively apolar chlorinated solvents, such as CHCl 3 /CH 2 Cl 2 , to polar ones
278
F. Zinna et al.
about different CPL signs, and the change of their relative contribution to the overall
CPL spectrum at different temperatures thus explains the g lum profile variations. In
this case g lum and g abs (measured on the first Cotton effect) are of the same order of
magnitude (|10
À3 |).
Later, Sisido’s group synthesized two similar polypeptides, namely pyr-3 and
pyr-4 (Scheme 12.1) [14]. In these examples, high g lum values of |1.6 Â 10
À2 | were
detected associated with an excimer situation; no CPL being otherwise allied with
monomer emission. The sign of the CPL was related to the handedness of the helix,
which in turn depended on the length of the peptidic spacer: negative for pyr-3 and
positive for pyr-4. In these cases, the handedness is the same in both ground and
excimer states, as indicated by the sign of the ECD band at 349 nm (negative for pyr3 and positive for pyr-4). For pyr-3 and pyr-4, the ECD and CPL were also studied
in DMF (6 Â 10
À5 M) and tetrahydrofuran (THF) at +20 and À40
C [15]. As a first
observation, the authors noticed that the g lum values were constant over the whole
emission band, which is consistent with only a single excimer configuration being
present in each polypeptide. As a second observation, the signs of g lum were opposite
for pyr-3 and pyr-4 suggesting configurations with opposite screw sense in the two
cases. For pyr-3 specifically, a decrease of g lum value, from |1.2 Â 10
À2 | to |
0.4 Â 10
À2 |, was observed in DMF upon lowering the temperature from +20 to
À40
C; a similar behavior occurring in THF although less pronounced. This
suggests that a major conformational change occurs in the À40 to +20
C range.
More recently, other pyrene-decorated peptides bearing 1, 2, 3, or 4 pyrene
moieties, pyr-5–pyr-8, were prepared and investigated by the group of Imai
(Fig. 12.3) [17]. In CHCl 3 (10
À4 M), a low g lum value of |1.9 Â 10
À4 | was observed
for pyr-5 associated with the monomer fluorescence, while the dipeptide pyr-6
showed the highest dissymmetry factor, |1.1 Â 10
À2
|, allied with excimer emission.
This latter g lum value is 240-fold higher than the corresponding g abs (~|10
À5 |), while
in the former case, in which no excimer occurs, the g abs and g lum are of the same
order of magnitude.
Furthermore, Imai and collaborators prepared several other peptides bearing two
pyrene moieties spaced by chains with different number (n) of methylene spacers,
allowing the authors to study the relationship between chiroptical properties and
spacer lengths (pyr-9–pyr-16, Fig. 12.3) [16]. The sign of CPL associated with
excimer emission underwent a sign inversion going from n ¼ 1 to n ¼ 2 (pyr-9 and
pyr-10), showing a sort of odd–even effect, and from n ¼ 6 to n ¼ 7 (pyr-14 and
pyr-15, see Fig. 12.3). On the other hand, the first Cotton effect, as shown in the
ECD spectrum, did not show any sign inversion, revealing a completely different
geometry of the ground and excited excimer states, confirming again the complementary nature of ECD and CPL when excimer states are at play. The maximum g lum
was recorded for n ¼ 3 (pyr-11, g lum ¼ 0.8 Â 10
À2
). In all instances, g lum values
were 1 to 2 orders of magnitude higher than the corresponding g abs factors (|10
À3 |–|
10
À4 |). With pyr-12 (n ¼ 4), CPL sign inversion was also observed going from
relatively apolar chlorinated solvents, such as CHCl 3 /CH 2 Cl 2 , to polar ones
278
F. Zinna et al.