ANALYSIS OF NUCLEOTIDES AND OLIGONUCLEOTIDES IMMOBILIZED AS SELF-ASSEMBLED MONOLAYERS BY STATIC SECONDARY-ION MASS-SPECTROMETRY

Nucleic acic constituents can be bound to a metal surface in the form of self-assembled monolayers. Binding is achieved either through ionic interactions with a self-assembled 2-aminoethanethiol monolayer or by direct covalent binding of a dithiophosphate oligonncleotide to a met al surface through a sulfur-metal bond. Nucleotides, polynucleotides ( both normal and a dithiophosphate analog) and double-stranded DNA have all been bound to surfaces. When the surfaces are interrogated using static secondary ion mass spectrometry (SIMS), the surface-bound nucle ic acid constituents are observed in the form of the characteristic pr otonated nucleic acid base ions (BH2+). While a silver foil substrate was found to provide the highest absolute signal, vapor-deposited gold yields the best signal-to-noise ratio for ionically bound deoxyguanos ine monophosphate. Under comparable conditions, a Cs+ projectile produ ces a 10-fold increase in the secondary ion signal relative to a Ga+ p rojectile. The experiment has been extended to a triple-quadrupole ins trument where tandem mass spectrometric experiments on ionically immob ilized dGMP showed the characteristic loss of ammonia from the release d BH2+ ion. When a 'biomimetic' surface formed by ionically immobilizi ng double-stranded DNA is exposed to a solution containing ethidium br omide, ions corresponding to the non-covalent adduct are readily detec table using SIMS. This adduct and the nucleic acid constituents can be monitored at levels below 10 fmol.