7 Biologically Relevant Molecules Studied in Low Temperature Inert Matrices
199
the imino–oxo isomers. on the whole, all the five lower energy forms of cytosine
could be experimentally observed.
7.5 Phenolic Compounds
Phenolic compounds have unique properties as antioxidants and free radical scavengers. Plant phenols, in particular, have attracted the attention of scientists interested
in the elucidation of many aspects of their biological activity [83, 84]. the phenol
moiety is also a fundamental building block of many other aromatic biomolecules,
such as the aromatic amino acids tyrosine and phenylalanine and neurotransmitters
(dopamine, among others). the parent phenol itself has long been receiving biomedical applications as disinfectant [85], and it is a precursor in the synthesis of aspirin.
Nowadays, phenol derivatives like triclosan (or 5-chloro-2-(2,4-dichlorophenoxy)
phenol) constitute major broad-spectrum antibacterial and antifungal agents found
in a wide variety of consumer goods, like detergents, dish-washing liquids, soaps,
deodorants, cosmetics, lotions, anti-microbial creams and toothpastes [86–90].
Phenols can also be extremely toxic. Approximately 1 g of parent phenol is
enough to cause human death. during the Second World War, phenol injections
were given to thousands of people in concentration camps by the Nazis (especially
at Auschwitz-Birkenau), in what constitutes one of the darkest periods of the history
of mankind.
Structurally, phenol is a simple molecule existing in a unique stable geometry in
the electronic ground state, where the oh group stays in the plane of the aromatic
ring. Substitutions in the ring may introduce asymmetry and lead to complex
conformational landscapes. Isoeugenol (2-methoxy-4-(prop-1-enyl)phenol), for
example, which is one of the most important plant phenols, has two geometric isomers, E and Z, differing in the relative position of the substituents around the C = C
double bond of the propenyl moiety (Fig. 7.22). For each of these species, there are
several conformers resulting from internal rotation about the single bonds connecting the –Ch = ChCh 3 , –oh and –oCh 3 groups to the aromatic ring. According to
theoretical predictions, the E-type species with the intramolecular O−H···O hydrogen bond and with the anti arrangement of the propenyl substituent (E1 and E2 in
Fig. 7.22) are the most stable forms of the isoeugenol molecule [91].
upon isolation of isoeugenol in a cryomatrix, the four E1, E2, Z1 and Z2 forms
of the compound were observed experimentally [91]. due to the low energy barrier
between the two Z forms (~ 2.8 kJ mol
−1
) annealing of the matrices at 28 K led to
observation of the Z1 → Z2 conversion. On the other hand, the E2 → E1 conformational conversion could not be observed experimentally by matrix annealing, since
the energy barrier associated with this conversion is relatively high (> 10 kJ mol
−1
),
thus precluding the conformational cooling.
very interestingly, irradiation of matrix-isolated isoeugenol (both in argon and
xenon matrices) in the 310–308 nm range was found to selectively convert the
E1 form into the Z1 species (Fig. 7.23). Further irradiation at λ = 305 nm induced
199
the imino–oxo isomers. on the whole, all the five lower energy forms of cytosine
could be experimentally observed.
7.5 Phenolic Compounds
Phenolic compounds have unique properties as antioxidants and free radical scavengers. Plant phenols, in particular, have attracted the attention of scientists interested
in the elucidation of many aspects of their biological activity [83, 84]. the phenol
moiety is also a fundamental building block of many other aromatic biomolecules,
such as the aromatic amino acids tyrosine and phenylalanine and neurotransmitters
(dopamine, among others). the parent phenol itself has long been receiving biomedical applications as disinfectant [85], and it is a precursor in the synthesis of aspirin.
Nowadays, phenol derivatives like triclosan (or 5-chloro-2-(2,4-dichlorophenoxy)
phenol) constitute major broad-spectrum antibacterial and antifungal agents found
in a wide variety of consumer goods, like detergents, dish-washing liquids, soaps,
deodorants, cosmetics, lotions, anti-microbial creams and toothpastes [86–90].
Phenols can also be extremely toxic. Approximately 1 g of parent phenol is
enough to cause human death. during the Second World War, phenol injections
were given to thousands of people in concentration camps by the Nazis (especially
at Auschwitz-Birkenau), in what constitutes one of the darkest periods of the history
of mankind.
Structurally, phenol is a simple molecule existing in a unique stable geometry in
the electronic ground state, where the oh group stays in the plane of the aromatic
ring. Substitutions in the ring may introduce asymmetry and lead to complex
conformational landscapes. Isoeugenol (2-methoxy-4-(prop-1-enyl)phenol), for
example, which is one of the most important plant phenols, has two geometric isomers, E and Z, differing in the relative position of the substituents around the C = C
double bond of the propenyl moiety (Fig. 7.22). For each of these species, there are
several conformers resulting from internal rotation about the single bonds connecting the –Ch = ChCh 3 , –oh and –oCh 3 groups to the aromatic ring. According to
theoretical predictions, the E-type species with the intramolecular O−H···O hydrogen bond and with the anti arrangement of the propenyl substituent (E1 and E2 in
Fig. 7.22) are the most stable forms of the isoeugenol molecule [91].
upon isolation of isoeugenol in a cryomatrix, the four E1, E2, Z1 and Z2 forms
of the compound were observed experimentally [91]. due to the low energy barrier
between the two Z forms (~ 2.8 kJ mol
−1
) annealing of the matrices at 28 K led to
observation of the Z1 → Z2 conversion. On the other hand, the E2 → E1 conformational conversion could not be observed experimentally by matrix annealing, since
the energy barrier associated with this conversion is relatively high (> 10 kJ mol
−1
),
thus precluding the conformational cooling.
very interestingly, irradiation of matrix-isolated isoeugenol (both in argon and
xenon matrices) in the 310–308 nm range was found to selectively convert the
E1 form into the Z1 species (Fig. 7.23). Further irradiation at λ = 305 nm induced