allowing the evolution from one helical sense to the other according to the valence
of the metal ion. The transmission of the changes in the pendants to the skeleton is
controlled by the structure and conformation of the complexes and their interactions
with the surrounding pendants.
Crucial information on the helical senses and morphologies of the HPMCs was
obtained from the AFM images of poly-(R)-2/Ba
2+ and poly-(R)-2/Ag
+ on highly
oriented pyrolytic graphite (HOPG). Results were in full agreement with those
previously obtained from CD, confirming the presence of the two enantiomeric
axial chiralities.
So, with Ba
2+ , the single chains were parallel packed, presenting a 3:1 righthanded (clockwise, P) pendant disposition, with the periodic oblique stripes forming
60.0
angles and a helical pitch of 3.23 nm (Fig. 6). By contrast, in the case of Ag
+
,
the parallel chains showed a 3:1 left-handed (counterclockwise, M ) helix with the
periodic oblique stripes forming 60.8
angles and a helical pitch of 3.21 nm.
Molecular mechanics (MM) calculations (MMFF94) on the secondary structure
of poly-(R)-2 confirmed that a 3/1 right-handed helix was the result when the (R)MPA pendants adopted sp conformations (Fig. 6). When the pendants adopted ap
conformations, the calculations yielded a 3/1 left-handed helix of analogous energy.
The angles formed by the oblique stripes and the helical pitches matched the
experimental values in both cases.
The values obtained from the external “crests” of the single chains implied
internal angles for the polyene backbone close to +75
and À75
for sp and ap
conformations, respectively, and explained the P and M chirality of the
corresponding backbones.
In both cases, poly-(R)-2/Ba
2+ and poly-(R)-2/Ag
+
, the “external” and “internal”
helices (generated by the pendants and by the backbones, respectively) presented
the same helical sense, as expected for cis-cisoid PPAs.
Knowledge of the secondary structure also allowed correlation of the CD spectra
and the type of helicity. For example, in the case of poly-(R)-2/Ba
2+ , a positive
Cotton effect at the vinylic region corresponds to a clockwise sense at the conjugated double bonds.
4 Chiral Amplification by Metal Ion Complexation
Chiral amplification is an interesting phenomenon in helical polymers [2, 19]. It has
been observed in the following three cases:
1. Induction of a predominant helicity in polymers containing achiral repeating
units, caused by coordination with adequate chiral substances.
2. In the known “sergeants and soldiers” effect, where a chiral unit “orders” a
number of achiral repeating units.
3. In the case of copolymers composed of a mixture of (R)- and (S)-enantiomeric
repeating units with a small enantiomeric excess (ee) that originates a predominantly one-handed helical conformation (“majority rule”).
Helical Polymer–Metal Complexes: The Role of Metal Ions on the Helicity and. . .
131
of the metal ion. The transmission of the changes in the pendants to the skeleton is
controlled by the structure and conformation of the complexes and their interactions
with the surrounding pendants.
Crucial information on the helical senses and morphologies of the HPMCs was
obtained from the AFM images of poly-(R)-2/Ba
2+ and poly-(R)-2/Ag
+ on highly
oriented pyrolytic graphite (HOPG). Results were in full agreement with those
previously obtained from CD, confirming the presence of the two enantiomeric
axial chiralities.
So, with Ba
2+ , the single chains were parallel packed, presenting a 3:1 righthanded (clockwise, P) pendant disposition, with the periodic oblique stripes forming
60.0
angles and a helical pitch of 3.23 nm (Fig. 6). By contrast, in the case of Ag
+
,
the parallel chains showed a 3:1 left-handed (counterclockwise, M ) helix with the
periodic oblique stripes forming 60.8
angles and a helical pitch of 3.21 nm.
Molecular mechanics (MM) calculations (MMFF94) on the secondary structure
of poly-(R)-2 confirmed that a 3/1 right-handed helix was the result when the (R)MPA pendants adopted sp conformations (Fig. 6). When the pendants adopted ap
conformations, the calculations yielded a 3/1 left-handed helix of analogous energy.
The angles formed by the oblique stripes and the helical pitches matched the
experimental values in both cases.
The values obtained from the external “crests” of the single chains implied
internal angles for the polyene backbone close to +75
and À75
for sp and ap
conformations, respectively, and explained the P and M chirality of the
corresponding backbones.
In both cases, poly-(R)-2/Ba
2+ and poly-(R)-2/Ag
+
, the “external” and “internal”
helices (generated by the pendants and by the backbones, respectively) presented
the same helical sense, as expected for cis-cisoid PPAs.
Knowledge of the secondary structure also allowed correlation of the CD spectra
and the type of helicity. For example, in the case of poly-(R)-2/Ba
2+ , a positive
Cotton effect at the vinylic region corresponds to a clockwise sense at the conjugated double bonds.
4 Chiral Amplification by Metal Ion Complexation
Chiral amplification is an interesting phenomenon in helical polymers [2, 19]. It has
been observed in the following three cases:
1. Induction of a predominant helicity in polymers containing achiral repeating
units, caused by coordination with adequate chiral substances.
2. In the known “sergeants and soldiers” effect, where a chiral unit “orders” a
number of achiral repeating units.
3. In the case of copolymers composed of a mixture of (R)- and (S)-enantiomeric
repeating units with a small enantiomeric excess (ee) that originates a predominantly one-handed helical conformation (“majority rule”).
Helical Polymer–Metal Complexes: The Role of Metal Ions on the Helicity and. . .
131