7 Biologically Relevant Molecules Studied in Low Temperature Inert Matrices
191
In tryptophan, besides intramolecular hydrogen-bonding interactions, specific
intramolecular electrostatic interactions also significantly contribute to determine
the relative stability of the different conformers, with stabilizing interactions of
this type involving the pyrrole ring being considerably more important that those
involving the phenyl ring of the molecule [33]. Such trend seems to have biochemical relevance, since it has also been noticed for tryptophan residues in proteins [39].
Both serine and phenylalanine were also investigated by matrix isolation infrared spectroscopy in relation to their photochemistry [36–38]. When submitted to
uv irradiation (λ > ~ 200 nm), serine decomposes by two different channels in the
S 1 excited state surface: decarboxylation, leading to production of Co 2 and ethanolamine, and decarbonylation, with formation of Co, h 2 o and acetamide (Fig. 7.12).
very interestingly, the two observed photoprocesses were found to be dependent
on the conformation assumed by the reactant molecule, with conformers bearing
o–h A ···N or o–h A ···o = hydrogen bonds undergoing decarboxylation and those
possessing an o–h C ···N h-bond decarbonylating [36, 37]. this was one of the first
observations of conformationally dependent photochemical processes, a nowadays
very active field of research. the results also suggest that the transition states’ geometries of the carboxylic group for the two processes in the S 1 surface should
resemble the cis and trans arrangements, respectively. For matrix-isolated phenylalanine, the decarboxylation, leading to production of Co 2 and phenethylamine, was
the sole observed photochemical pathway [38].
7.4 Nucleic Acid Bases
Among the nucleic acid bases, the most investigated one has been uracil, due to its
simpler structure. In solution, at ph = 7, uracil exists as a mixture of two tautomers,
amide and imide (Fig. 7.13). on the other hand, in crystalline phase [40], as well
Serine
Ethanolamine
Carbon Dioxide
Acetamide
C arbon Monoxide
Water
hν
cis COOH
conformers
+
+
+
hν
trans COOH
conformers
C
H
H
H 2 N
C
H
H
H 2 N
CO 2
C
CH 3
O
H 2 N
C
O
H 2 N
CO
H 2 O
CH 2 OH
C
C OOH
H
H 2 N
C
C OOH
H
H 2 N
CH 2 OH
Fig. 7.12 unimolecular photodecomposition channels observed in the uv (λ > 200 nm) photochemistry of matrix-isolated serine. (Adapted from [37] with permission from Elsevier)
191
In tryptophan, besides intramolecular hydrogen-bonding interactions, specific
intramolecular electrostatic interactions also significantly contribute to determine
the relative stability of the different conformers, with stabilizing interactions of
this type involving the pyrrole ring being considerably more important that those
involving the phenyl ring of the molecule [33]. Such trend seems to have biochemical relevance, since it has also been noticed for tryptophan residues in proteins [39].
Both serine and phenylalanine were also investigated by matrix isolation infrared spectroscopy in relation to their photochemistry [36–38]. When submitted to
uv irradiation (λ > ~ 200 nm), serine decomposes by two different channels in the
S 1 excited state surface: decarboxylation, leading to production of Co 2 and ethanolamine, and decarbonylation, with formation of Co, h 2 o and acetamide (Fig. 7.12).
very interestingly, the two observed photoprocesses were found to be dependent
on the conformation assumed by the reactant molecule, with conformers bearing
o–h A ···N or o–h A ···o = hydrogen bonds undergoing decarboxylation and those
possessing an o–h C ···N h-bond decarbonylating [36, 37]. this was one of the first
observations of conformationally dependent photochemical processes, a nowadays
very active field of research. the results also suggest that the transition states’ geometries of the carboxylic group for the two processes in the S 1 surface should
resemble the cis and trans arrangements, respectively. For matrix-isolated phenylalanine, the decarboxylation, leading to production of Co 2 and phenethylamine, was
the sole observed photochemical pathway [38].
7.4 Nucleic Acid Bases
Among the nucleic acid bases, the most investigated one has been uracil, due to its
simpler structure. In solution, at ph = 7, uracil exists as a mixture of two tautomers,
amide and imide (Fig. 7.13). on the other hand, in crystalline phase [40], as well
Serine
Ethanolamine
Carbon Dioxide
Acetamide
C arbon Monoxide
Water
hν
cis COOH
conformers
+
+
+
hν
trans COOH
conformers
C
H
H
H 2 N
C
H
H
H 2 N
CO 2
C
CH 3
O
H 2 N
C
O
H 2 N
CO
H 2 O
CH 2 OH
C
C OOH
H
H 2 N
C
C OOH
H
H 2 N
CH 2 OH
Fig. 7.12 unimolecular photodecomposition channels observed in the uv (λ > 200 nm) photochemistry of matrix-isolated serine. (Adapted from [37] with permission from Elsevier)