Effective sequence-specific recognition of duplex DNA is possible by triplex formation with natural oligonucleotides via Hoogsteen H-bonding. However, triplex formation is in practice limited to pyrimidine oligonucleotides binding duplex A-T or G-C base-pair DNA sequences specifically at homopurine sites in the major groove as T-A-T and C+.G-C triplets. Here we report the successful modeling of novel unnatural nucleosides that recognize the T-A DNA base pair by Hoogsteen interaction. Since the DNA triplex can be considered to assume an A-type or B-type conformation, these novel Hoogsteen nucleotides are tested within model A-type and B-type conformation triplex structures. A triplet consisting of the T-A base pair and one of the novel Hoogsteen nucleotides replaces the central T.A-T triplet in the triplex using the same deoxyribose-phosphodiester and base-deoxyribose dihedral angle configuration. The entire triplex is energy minimized and the presence of any structural or energetic perturbations due to the central triplet is assessed with respect to the unmodified energy-minimized (T.A-T)11 proposed starting structures. Incorporation of these novel triplets into both A-type and B-type natural tiplex structures provokes minimal change in the configuration of the central and adjacent triplets. The plan is to produce a series of Hoogsteen-like bases that preferentially bind the T-A major groove in either an A-type or B-type conformation. Selective recognition of the T-A major groove with respect to the G-C major groove, which presents similar keto and amine placement, is also assessed with configurational preference. Evaluation of the triplex solution structure by using these unnatural bases as binding conformational probes is a prerequisite to the further design of triplet forming bases.