Table 2.

Nucleobase modifications and their effects on PNA backbone.

S. No.	Modified Nucleobase	Structure	Properties	References
1.	2,6-Diamino purine		Increased affinity and selectivity for thymine	(Haaima et al., 1997)
2.	Pseudoiso-cytosine		Mimics the C+ recognition pattern for triplex formation irrespective of surrounding pH	(Haaima et al., 1997)
3.	2-Amino purine		Can hydrogen bond with uracil and thymine in the reverse Watson-Crick mode and being inherently fluorescent, can be used to study the kinetics of the hybridization process with complementary nucleic acids	(Gangamani and Kumar, 1997)
4.	Thiazole		Forms PNA probe that fluoresced upon hybridization	(Köhler and Seitz, 2003, Jarikote et al., 2005)
5.	Hypoxanthine		Form Watson-Crick base pairs with adenine, cytosine, thymine, and uracil and achieve multimutant specificity	(Sanders et al., 2013)
6.	Thiouracil		Can invade dsDNA in antigene applications	(Haaima et al., 1997)
7.	N4-benzoyl-cytosine		candidate for a G-C pseudo-complementary base pair	(Olsen et al., 2011)
8.	6-Thioguanine		Decrease in Tm of 8.5 °C due to PNA:DNA heteroduplex	(Henrik et al., 1999)
9.	G-clamp		Enhanced duplex stability	(Rajeev et al., 2002, Ortega et al., 2007)
10.	P-base		In polypurine tracts of double helical RNA, able to isolate pyrimidine interruptions	(Li et al., 2010, Rozners, 2012)
11.	E-base		In polypurine tracts of double helical RNA, able to isolate pyrimidine interruptions	(Li et al., 2010, Rozners, 2012)
12.	5(acridin-9-ylamino)uracil		Hydrolytically labile modification	(Matarazzo et al., 2013)
13.	Thio-pseudo isocytosine		Enhanced RNA duplexes recognition	(Devi et al., 2014)
13.	2-amino pyridine		Selective triplex formation with RNA duplexes (Adenosine to inosine editing)	(Annoni et al., 2016)
14.	Mono-m-(guanidinoethoxy)phenyl]pyrrolocytosine		Enhanced binding affinity for RNA and an exceptionally high fluorescence quantum yield.	(Wojciechowski and Hudson, 2009)