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Acidic peptides have been studied by negative ion postsource decay (PSD) matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The peptides contained from 5 to 16 residues and were chosen on the basis of their patterns of the acidic residues. Using typical MALDI sample preparation techniques employing an acidic matrix, gastrin I (1-14), and epidermal mitosis inhibiting pentapeptide yielded much larger deprotonated ion signals, [M - H]-, than protonated ions, [M + H]+. This may be due to their absence of basic residues, coupled with their arrays of acidic residues. The PSD fragmentation of the peptide negative ions showed that an array of acidic residues, as in gastrin I (1-14), yielded simple spectra containing mainly backbone cleavage ions from the C-terminus. Hirudin (54-65), which contains two sets of two consecutive Glu residues, and fibrinopeptide A and fibrinopeptide B, with isolated acidic residues, also showed backbone cleavages as common fragment ions. In addition, the two sets of isolated consecutive amino acid residues in Cys(Bzl)84-CD4 (81-92) and hirudin (54-56) yielded internal ions from the cleavages at the (O=C)-NH bond between the acidic residues. Also observed were ions with unique side chain losses, such as the loss of C6H4O from a tyrosine residue and SCH2C6H5 and CH2C6H5 from a benzylated cysteine residue. Compared to the positive mode, the negative-ion PSD yielded fewer fragments which usually involved only one type of backbone cleavage (e.g., [Yn - H2O]-). These simple spectra aided interpretation. Overall, the acidic peptides studied yielded negative ion PSD spectra that were useful for peptide sequencing.

Dissociation of the protonated molecules yielded primarily a,' and b,' fragment ions and low-mass ions characteristic of individual amino acid residues. In contrast, the distinguishing feature of the anion spectra is a complete, or almost complete, series of y; ions. In addition, b ; , c;, x;, and [y. -181-fragment ions are found in the anion spectra. Anion formation from substance P is particularly interesting, because this peptide contains no acidic carboxyl groups to act as a charge site. In general, peptide cations produced predominantly fragment ions from the N-terminus, while anion dissociation gave primarily C-terminal fragment ions. The results suggest that cation and anion PSD yield complementary information on peptide primary structures.Since the introduction of matrix-assisted laser desorptiod ionization (MALDI), time-of-flight (TOF) mass spectrometry has gained in popularity as the most MALDIcompatible mass analyzer of large biomolecules and .polymers. This is due to the pulsed nature of TOF and MALDI, as weil as the high mass range and sensitivity that can be achieved. TOF has become even more attractive since post-source decay (PSD) was first reported by Spengler, Kaufmann and co-~orkers.'-~ PSD refers to fragmentation that occurs in the first field-free region between the ion source and the reflectron. The resulting product ions retain the same velocity as their parent ions and thus would appear at the same m/z values in a linear TOF spectrum; however, because the product ions do not have the same kinetic energies as their precursors, they can be distinguished using a reflectron analyzer. By varying the reflectron voltage, ions of different kinetic energies (and masses) can be subsequently transmitted through the reflectron and reach the detector. A complete PSD spectrum covering a wide mass range is achieved by combining a series of mass spectral segments obtained at different reflectron voltages. Alternatively, Cotter and co-workers4 have recently developed a curved-field reflectron that focuses the PSD product ions at a single reflectron voltage.To date, most PSD studies have involved peptide dissociation. For example, Spengler et al.'. reported the spectra of several peptides to demonstrate the technique, while Martin and co-workers' compared PSD with collision-induced dissociation (CID). In addition, peptide PSD was used to demonstrate the utility of the curved-field reflectron design.' Several studies have also varied the timescale of PSD. Product ion formation in PSD involves a combination of unimolecular dissociation resulting from excess energy ' imparted to the parent ions during the ionization process and bimolecular collisional activation occurring during ion extraction through the dense laser plume of neutrals. Brown and L,ennon6 studied these two effects using variable extraction delay experiments; longer extraction delays minimize the contribution of collisional * Author for correspondence. activation because the neutral plume is significantly pumped away before ion extraction. In th...

Single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] has been used to calculate the ground state potential energy surfaces for He–F2, Ne–F2, and Ar–F2 van der Waals complexes. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions (aug-cc-pVTZ+bf). Single point calculations for approximate minima have also been performed with a larger quadruple zeta basis set (aug-cc-pVQZ+bf). For He–F2 and Ar–F2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–F2 the CCSD(T) interaction energies of the two configurations are virtually the same. The linear configuration of each complex has been found to be much more sensitive than the T-shaped one to the changes of the F–F bond length with the interaction becoming weaker when the F–F bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. High-order correlation corrections have been found to play an important role in determining the relative stability of the linear and T-shaped configurations. The harmonic approximation zero-point vibrational energy for He–F2 exceeds the depth of both wells. For Ne–F2 the zero-point vibrational energy is greater for the linear configuration and, because of that, the complex has a T-shaped ground vibrational state. When the zero-point vibrational energy is taken into account for the Ar–F2 complex the linear and the T-shaped configurations are found to have nearly identical energies.

‘Post source decay’ analysis was utilized in structural confirmation of first, second and third generation polyurethane dendritic wedges as well as first generation porphyrin dendrimers. The samples were prepared by using α‐cyano‐4‐hydroxycinnamic acid and acetone as the matrix and the solvent, respectively. The cleavages take place at both inner and outer layers of the dendritic compounds. Sodium ion adducts from the cleavage at the ester or amide bond (or a combination of both) are observed. The results obtained are in good agreement with the proposed structures of all analyzed compounds. Copyright © 1999 John Wiley & Sons, Ltd.