PHOTOCHEMICAL REACTION BETWEEN PROTEIN AND NUCLEIC ACIDS: PHOTOADDITION OF TRYPTOPHAN TO PYRIMIDINE BASES1

Observation of intra- and intermolecular vibrational coherences of the aqueous tryptophan radical induced by photodetachment

The study of the photodetachment of amino acids in aqueous solution is pertinent to the understanding of elementary processes that follow the interaction of ionizing radiation with biological matter. In the case of tryptophan, the tryptophan radical that is produced by electron ejection also plays an important role in numerous redox reactions in biology, although studies of its ultrafast molecular dynamics are limited. Here, we employ femtosecond optical pump-probe spectroscopy to elucidate the ultrafast structural rearrangement dynamics that accompany the photodetachment of the aqueous tryptophan anion by intense, ∼5-fs laser pulses. The observed vibrational wave packet dynamics, in conjunction with density functional theory calculations, identify the vibrational modes of the tryptophan radical, which participate in structural rearrangement upon photodetachment. Aside from intramolecular vibrational modes, our results also point to the involvement of intermolecular modes that drive solvent reorganization about the N-H moiety of the indole sidechain. Our study offers new insight into the ultrafast molecular dynamics of ionized biomolecules and suggests that the present experimental approach can be extended to investigate the photoionization- or photodetachment-induced structural dynamics of larger biomolecules.

The effect of tryptophan on UV-induced DNA photodamage

Trp-DNA adducts resulting from UV irradiation of pyrimidine bases and nucleotides in the presence of tryptophan (Trp) have been the subject of previous research. However, the relative yield of the adducts compared with the UV screening effect of Trp has not been previously considered. To determine whether Trp-DNA adduct formation or absorption "screening" by Trp is the predominant process when DNA solutions are irradiated with UV light in the presence of Trp, we irradiated Trp-containing DNA oligonucleotide solutions with UVC light and incubated aliquots of those solutions with molecular beacons (MBs) to detect the damage. We observed a rapid decay of fluorescence of the MBs for pure DNA solutions, thereby indicating damage. However, in the presence of Trp, the fluorescence decay is prolonged, with time constants that increase exponentially with Trp concentration. The results are discussed in terms of a beneficial in vivo cellular protection rather than harmful adduct formation and suggest a net sacrificial absorption of UV light by Trp which actually protects the DNA from UV damage.

Recent aspects of the photochemistry of nucleic acids and related model compounds

This survey focuses on recent developments in the far ultraviolet photochemistry of nucleic acids and related model compounds. The photoproducts discussed are the cyclobutidipyrimidines, the pyrimidine-pyrimidone adducts, the purine-pyrimidine adducts and the addition products of amino acids to pyrimidine bases. The specific aspects of the high-intensity laser photochemistry of nucleic acid components are also briefly reviewed.

The influence of 1-(N-L-tryptophan)-1-deoxy-D-fructose [Fru-Trp] and its N-nitrosated analogue [NO-Fru-Trp] on the viability and intracellular synthetic activity (DNA, RNA, and protein synthesis) of HeLa S3-carcinoma cells

Exposing HeLa S3 cells at 37 degrees C to varied concentrations of, respectively, Fru-Trp (0.1 microM - 1 mM), NO-Fru-Trp (0.1 microM - 1 mM), and NaNO2 (0.6 microM - 6 mM) for varied periods of time (1 - 36 hr) does neither affect their viability (trypan blue dye exclusion test) nor capability to synthesize RNA or protein but is of considerable influence on DNA synthesis in the case of NO-Fru-Trp and NaNO2, but not in the case of Fru-Trp which continues to be ineffective. None of the three compounds tested is of significant influence on cell number. Both NO-Fru-Trp and NaNO2 stimulate DNA synthesis: a maximum of activity [( 3H] thymidine incorporation) exists at the 24 hr time point of incubation, with NO-Fru-Trp, for instance, generating a 2.5-fold increase (over control) at 1 mM concentration in the medium while NaNO2, at comparable concentration, increases DNA synthesis by a factor of 1.6 over control. The increase in DNA synthesis is not due to stimulatory influences on (semi-conservative) DNA replication but represents DNA repair. This was verified by keeping the cells under conditions that prevent normal (semi-conservative) replication but permit repair ("unscheduled DNA synthesis"). Two major routes are suggested by which NO-Fru-Trp could impart DNA damage and, thus, assume mutagenic properties.

Mutagenic activity of a nitrosated early Maillard product: DNA synthesis (DNA repair) induced in HeLa S3 carcinoma cells by nitrosated 1-(N-L-tryptophan)-1-deoxy-D-fructose

HeLa S3 cells in suspension were incubated at 37 degrees C with various concentrations of the Amadori compound 1-(N-L-tryptophan)-1-deoxy-D-fructose (Trp-Fru), of its nitrosated analogue NO-Trp-Fru and of sodium nitrite, for varying periods of time, and were assayed for viability (trypan blue exclusion test) and for intracellular DNA, RNA and protein synthesis. None of the compounds tested had any effect on cell viability, or on RNA and protein synthesis apart perhaps from a slightly inhibitory action. While Trp-Fru remained ineffective also as far as intracellular DNA synthesis was concerned, both NO-Trp-Fru and NaNO2 had a major effect on DNA synthesis. With NaNO2, stimulation of DNA synthesis occurred at concentrations above 1 mM in the growth medium, but with NO-Trp-Fru synthesis increased at concentrations below 1 microM. The excess DNA synthesis (i.e. synthesis above control activity) observed with NO-Trp-Fru and also with NaNO2 was due to DNA repair. This was verified by keeping the cells under conditions that prevented normal semi-conservative replication but permitted DNA repair ('unscheduled DNA synthesis'). Two major routes are suggested by which NO-Trp-Fru could damage DNA.