A photodissociation spectrometer, containing a spray ionization source and a temperature-variable multipole ion trap, has been constructed to examine the structure and reactivity of gas phase biological molecular ions at various temperatures. Ultraviolet (UV) and infrared (IR) photodissociation spectra of protonated alanyltryptophan (Ala-TrpH+) and tryptophanylglycine (Trp-GlyH+) have been measured. In UV spectra, the S1-S0 band origin of Ala-TrpH+ exhibits a significant red shift with respect to those of protonated tryptophan (TrpH+) and Trp-GlyH+. This red shift is ascribed to the stabilization of the excited state due to the strong interaction between the NH3+ group and indole ring. We also discuss the temperature effect on the structure and reactivity for these peptides. In addition to the UV photodissociation spectra of the dipeptides, IR spectra of the complex of Ala-TrpH+ with methanol are measured. IR photodissociation spectra of solvated ions show that Ala-TrpH+-methanol has the closed structure, which is consistent with the large spectral shift in UV spectrum of bare dipeptide.

Enantiomer-selective ultraviolet photolysis of temperature-controlled protonated tryptophan (TrpH +) on the chiral crown ether, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6TA), was examined using a tandem mass spectrometer containing a temperature-variable 22-pole ion trap. The spectra of D-TrpH +-(+)-18C6TA at 9-320 K showed that the loss of NH2CH2COOH due to Cα-Cβ bond cleavage decreased gradually with increasing temperature. The spectrum at room temperature was similar to that of L-TrpH +-(+)-18C6TA, which showed no temperature dependence on photolysis. The chiral-specific photolysis of cold D-TrpH +-(+)-18C6TA was attributed to the structures involving the chiral-dependent interactions of the Cα-H group of TrpH + with the oxygen atoms of (+)-18C6TA.

Photodissociation spectroscopy of solvated clusters of protonated tryptophan (TrpH(+)) and dipeptides containing tryptophan (Val-TrpH(+), Ala-TrpH(+), and Gly-TrpH(+)) has been carried out at low temperature to investigate the protonation and solvation effects on the electronic spectrum. For the protonated dipeptides, the S(1)-S(0) transition exhibits a substantial red shift due to the stronger interaction between the NH(3)(+) group and the indole pi ring. The S(1)-S(0) spectra of TrpH(+)(CH(3)OH)(n) clusters exhibit a drastic change with the number of methanol molecules. This behavior is interpreted in terms of the decrease in the interaction between the pi pi* and the repulsive pi sigma* states. Ala-TrpH(+) and Gly-TrpH(+) exhibit an extensive spectral change with addition of two methanol molecules. This change is ascribed to a conformational change, which is induced by the insertion of solvent molecule in between the NH(3)(+) group and the indole pi ring.

No abstract

The formation and relaxation dynamics of NH4(CH3OH)m(NH3)n clusters produced by photolysis of ammonia-methanol mixed clusters has been observed by a time-resolved pump-probe method with femtosecond pulse lasers. From the detailed analysis of the time evolutions of the protonated cluster ions, NH4(+)(CH3OH)m(NH3)n, the kinetic model has been constructed, which consists of sequential three-step reaction: ultrafast hydrogen-atom transfer producing the radical pair (NH4-NH2)*, the relaxation process of radical-pair clusters, and dissociation of the solvated NH4 clusters. The initial hydrogen transfer hardly occurs between ammonia and methanol, implying the unfavorable formation of radical pair, (CH3OH2-NH2)*. The remarkable dependence of the time constants in each step on the number and composition of solvents has been explained by the following factors: hydrogen delocalization within the clusters, the internal conversion of the excited-state radical pair, and the stabilization of NH4 by solvation. The dependence of the time profiles on the probe wavelength is attributed to the different ionization efficiency of the NH4(CH3OH)m(NH3)n clusters.