Photostability of amino acids: photodissociation dynamics of phenylalanine chromophores

Abstract

The theoretical prediction of H atom elimination on the excited state of phenol, imidazole and indole, the respective chromophores for the amino acids tyrosine, histidine and tryptophan, and the confirmation of theoretical prediction by experimental observations have a great impact on the explanation of photostability of amino acids upon irradiation with UV photons. On the other hand, no theoretical prediction of the excited state photodissociation dynamics has been made on the other aromatic amino acid, phenylalanine. In this work, photodissociation dynamics for various phenylalanine chromophores, including, phenylethylamine, N-methyl-phenylethylamine, and N-acetyl phenylalanine methyl ester was investigated in a molecular beam at 248 and 193 nm using multimass ion imaging techniques. The major dissociation channel for these compounds is the C-C bond cleavage. However, the photofragment translational energy distribution of phenylethylamine contains two components. The slow component corresponds to the dissociation on the ground state surface after internal conversion, and the fast component represents the dissociation from an excited state with a large exit barrier. The competition between the dissociation on the ground state and on the excited state changes as the size of chromophores increases. Internal conversion to the ground state prior to dissociation becomes the major nonradiative process for large chromophores. This study reveals the size-dependent photostability for these amino acid chromophores.

Copper‐Mediated Polymerization without External Deoxygenation or Oxygen Scavengers

“…However, photomediated methods can be incompatible with specific enzymes and proteins as their secondary structure can be disrupted through irradiation . In addition, utilizing light as an external stimuli may limit the monomer pool as strongly absorbing monomers, including chromophores, would be incompatible with these techniques . Additionally, external reducing agents and enzymes can be costly, interfere with the monomer structure, be temperature dependent, or alter the pH of the polymerization mixture, thus significantly increasing the complexity of a given system .…”

Copper‐Mediated Polymerization without External Deoxygenation or Oxygen Scavengers

“…However, photomediated methods can be incompatible with specific enzymes and proteins as their secondary structure can be disrupted through irradiation 26, 27, 28. In addition, utilizing light as an external stimuli may limit the monomer pool as strongly absorbing monomers, including chromophores, would be incompatible with these techniques 29, 30. Additionally, external reducing agents and enzymes can be costly, interfere with the monomer structure, be temperature dependent, or alter the pH of the polymerization mixture, thus significantly increasing the complexity of a given system 31, 32.…”

“…[26][27][28] In addition, utilizing light as an external stimuli may limit the monomer pool as strongly absorbing monomers,i ncluding chromophores, would be incompatible with these techniques. [29,30] Additionally,e xternal reducing agents and enzymes can be costly, interfere with the monomer structure,b et emperature dependent, or alter the pH of the polymerization mixture,t hus significantly increasing the complexity of agiven system. [31,32] Further limitations of the reported methods include the risk of generating additional chains through side products [19] and the incompatibility with aw ide range of monomers,t emperatures,and solvents.…”

UV Photoinduced Dynamics of Conformer-Resolved Aromatic Peptides

“…73 The excitation of phenol with a side chain reveals interesting side-chain size-dependent dissociation properties. 73,74 At 248 nm (5.17 eV), the photoexcitation of p-methylphenol, pethylphenol, and p-(2-aminoethyl) phenol shows that the decay of the excited phenyl ring is very different between these three molecules. For both p-methylphenol and p-ethylphenol, the major relaxation channel is through the coupling between the ππ* and πσ* states leading to H atom elimination from a repulsive state.…”

“…Photoexcitation of alkylbenzenes at 248 nm (5 eV) leads to fragmentation, the main fragment coming from the C  -C  bond rupture leading to the benzyl radical C6H5CH2 and its counterpart. 74 The translational energy distribution of the fragments were measured. Two distinct components have been observed.…”