Maltose Nomenclature

Maltose has a 1,4 glycosidic linkage between two molecules of α-D-Glucopyranose.  

My textbook suggests the final name of maltose as 



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> 4-O-(α-D-Glucopyranosyl)-D-glucopyranose
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* I wanted to know what does "4-O-" stands for ?


I dont think so it provides information about the atoms which are linked and moreover feel that [Wikipedia's](https://en.wikipedia.org/wiki/Oligosaccharide_nomenclature) nomenclature (not IUPAC) : 



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> α-D-Glucopyranosyl-(1→4)-β-D-glucopyranose
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more detailized . 




Maltose is 4-*O*-α-D-glucopyranosyl-D-glucose. The "O" in "4-O" indicates the attachment of atom of the substituent. The substituent in this case is "-α-D-glucopyranosyl". It is attached to the *oxygen* atom that is attached to carbon 4 in the base molecule. The base molecule in this case is "-D-glucose". "4-α-D-glucopyranosyl-D-glucose" could be interpreted as a different structure where the α-D-glucopyranosyl substituent is bound directly to *carbon* atom number 4.


The "base molecule" of "-D-glucose" still has an unsubstituted anomeric OH group. Thus, in aqueous solution, maltose will be an equilibrium mixture of at least these three isomers:


1. 4-*O*-α-D-glucopyranosyl-α-D-glucopyranose
2. 4-*O*-α-D-glucopyranosyl-β-D-glucopyranose
3. 4-*O*-α-D-glucopyranosyl-D-glucose (open chain on the right-most "glucose" moiety in the name)


Thus, it doesn't really make sense to disambiguate α vs. β vs. open-chain forms of the "right-most" glucose ring in maltose, unless you need to refer to a particular conformation of maltose that matters for a certain reaction or condition. However, the "left" ring is locked (in maltose) to an α-D-glucopyranosyl conformation because it does not have an anomeric OH group and rapid equilibration in water does not take place.


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The latter name that you describe as Wikipedia-not-IUPAC is actually the preferred name variant. From IUPAC Nomenclature of Organic Chemistry (Preferred names 2013), the “Blue Book”:



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> **P-102.7.1.2** Disaccharides with a free hemiacetal group
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> Disaccharides which can be regarded as formed by elimination of one molecule of water from one glycosidic (anomeric) hydroxy group and one alcoholic hydroxy group, are named as glycosylglycoses. Locants and anomeric descriptors must be cited in the full name.
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> There are two established methods for citing locants:
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> (1) in parentheses between the components with an arrow going from the locant of the glycosyl component to that of the glycose component;
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> (2) at the front of the glycosyl component.
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> Example:
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> [![Fig.1 Maltose structure](https://i.stack.imgur.com/4ewPe.png)](https://i.stack.imgur.com/4ewPe.png)
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> (1) α-ᴅ-glucopyranosyl-(1→4)-β-ᴅ-glucopyranose  
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>  (2) 4-*O*-α-ᴅ-glucopyranosyl-β-ᴅ-glucopyranose  
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>     β-maltose (trivial name; not β-ᴅ-maltose)
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Here it's “preferred” just as in “listed first”; but really explicitly *preferred* (on the other hand, not in a strict sense, like PINs in the Blue Book) in IUPAC Carbohydrate nomenclature (Recommendations 1996) itself (which the portion above was based on, but not completely literally):



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> **[2-Carb-36.3](https://www.qmul.ac.uk/sbcs/iupac/2carb/36.html#363). Disaccharides with a free hemiacetal group**
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> (…)
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> There are two established methods in use for citing locants: either in parentheses between the glycosyl and glycose terms, or in front of the glycosyl prefix, as in the names of glycosides. The former (preferred) method is derived from that used to designate residues in oligosaccharides (see [2-Carb-37](https://www.qmul.ac.uk/sbcs/iupac/2carb/37.html) and [2-Carb-38](https://www.qmul.ac.uk/sbcs/iupac/2carb/38.html)).
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> **Note.** The latter method is that used by Chemical Abstracts Service for disaccharides.
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> The *O*-locants used for the former method in the previous recommendations [1] are omitted here.
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The bottom note refers to previous Carbohydrate nomenclature (1969), that included the *O*- also in the “arrow based name”,



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> Example:  
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>  α-Lactose: *O*-β-ᴅ-galactopyranosyl-(1→4)-α-ᴅ-glucopyranose
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As already mentioned in Curt F.'s answer, the *O*- prefix is to specify that the glycosyl (carbohydrate residue, with formal free valence after anomeric $\ce{OH}$ group removal) is bound to the oxygen rather than on the carbon atom (which is automatically assumed in the “arrow style” names), just as for carbohydrate general *O*-substitution, e.g.



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> 5-*O*-methyl-ᴅ-galactitol
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or



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> 2,3,4,6-tetra-*O*-methyl-β-ᴅ-glucopyranose
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