Dynamics of tin nuclei in alkyltin(IV)–deoxyribonucleic acid condensates by variable-temperature tin-119 Mössbauer spectroscopy

Abstract

The dynamics of tin nuclei in the condensates SnR2(DNA monomer)2 and SnR3(DNA monomer)(R = Me or Et), freeze-dried, has been investigated by variable-temperature 119Sn Mössbauer spectroscopy. Linear functions In (At/A77.3)(T), In farel,abs(T) and 〈x2〉(T)(At= total area under the resonant peaks, fa the relative and the absolute estimates of Lamb-Mössbauer factors, and 〈x2〈 the mean-square displacements of the Mössbauer nucleus extracted from farel and faabs respectively) have been found at T 77.3 K, which indicate harmonic motions and the lack of phase transitions. The latter is also suggested by the temperature-invariant hyperfine parameters, isomer shift, nuclear quadrupole splitting (ΔE) and peak widths. From the slopes of the functions In At(T) and In farel(T), the dynamics of tin in alkyltin(IV)–DNA condensates is found to be analogous to that in organotin(IV) salts and complexes, on the assumption of effective vibrating masses, corresponding to molecular groups. The coincidence between farel,abs, as well as the related 〈x2〉, data, indicates that the negative charge on the DNA backbone phosphodiester groups is fully neutralized by alkyltin(IV) cations in SnR2(DNA monomer)2(R = Me or Et) as well as in SnEt3(DNA monomer), while only partially in SnMe3(DNA monomer) and in SnMe2(DNA monomer)2 obtained by standard procedures for DNA condensation. From the magnitude of the functions, as well as of the Debye temperatures, on fingerprint criteria, SnIVR2 moieties are assumed to bridge phosphodiester groups in toroidal condensates through interstrand bonding, while SnIVR3 would be appended to the double helix. Motions would involve SnR2(mononucleotide)2 and SnR3(mononucleotide) units as the effective vibrating masses. Two tin co-ordination sites occur for SnIVR2 moieties at the DNA surface, both trans-octahedral, and a single trigonal-bipyramidal site for SnIVR3, the organometal moieties being co-ordinated by phosphodiester and water oxygen atoms, according to ΔE rationalization by point-charge model structure simulations, as well as to Mössbauer–Zeeman spectra of the SnIVEt2– and SnIVEt3–DNA condensates.